Merge pull request #1185 from OrkhanAlikhanov/layout-system

Reworked layout system

Merge pull request #1185 from OrkhanAlikhanov/layout-system
Reworked layout system
a89797ae · Daniel Jonathan · GitHub · 091d4306 · 3721e66e · a89797ae
Commit a89797ae authored Nov 01, 2018 by Daniel Jonathan Committed by GitHub Nov 01, 2018
6 changed files
--- a/Material.xcodeproj/project.pbxproj
+++ b/Material.xcodeproj/project.pbxproj
@@ -174,6 +174,9 @@
 		9D054A6520D175AC00D0528D /* Material+UIButton.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D054A6320D175AC00D0528D /* Material+UIButton.swift */; };
 		9D054A6620D175AC00D0528D /* Material+UILabel.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D054A6420D175AC00D0528D /* Material+UILabel.swift */; };
 		9D39A81B20FE8ED100BA8FA1 /* ViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D39A81A20FE8ED100BA8FA1 /* ViewController.swift */; };
+		9D494A38217F6B63003D66F1 /* LayoutAttribute.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D494A37217F6B63003D66F1 /* LayoutAttribute.swift */; };
+		9D494A3A217F6B70003D66F1 /* LayoutAnchor.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D494A39217F6B70003D66F1 /* LayoutAnchor.swift */; };
+		9D494A3C217F6B7D003D66F1 /* LayoutConstraint.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D494A3B217F6B7D003D66F1 /* LayoutConstraint.swift */; };
 		9D9089B92118914500605DC9 /* Editor.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9D9089B82118914500605DC9 /* Editor.swift */; };
 		9DE25DE02170D7AF000C04DF /* Dialog.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9DE25DDF2170D7AF000C04DF /* Dialog.swift */; };
 		9DE25DE22170D7C0000C04DF /* DialogController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9DE25DE12170D7C0000C04DF /* DialogController.swift */; };
@@ -300,6 +303,9 @@
 		9D054A6320D175AC00D0528D /* Material+UIButton.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "Material+UIButton.swift"; sourceTree = "<group>"; };
 		9D054A6420D175AC00D0528D /* Material+UILabel.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "Material+UILabel.swift"; sourceTree = "<group>"; };
 		9D39A81A20FE8ED100BA8FA1 /* ViewController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ViewController.swift; sourceTree = "<group>"; };
+		9D494A37217F6B63003D66F1 /* LayoutAttribute.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LayoutAttribute.swift; sourceTree = "<group>"; };
+		9D494A39217F6B70003D66F1 /* LayoutAnchor.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LayoutAnchor.swift; sourceTree = "<group>"; };
+		9D494A3B217F6B7D003D66F1 /* LayoutConstraint.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LayoutConstraint.swift; sourceTree = "<group>"; };
 		9D9089B82118914500605DC9 /* Editor.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Editor.swift; sourceTree = "<group>"; };
 		9DE25DDF2170D7AF000C04DF /* Dialog.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Dialog.swift; sourceTree = "<group>"; };
 		9DE25DE12170D7C0000C04DF /* DialogController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = DialogController.swift; sourceTree = "<group>"; };
@@ -652,6 +658,9 @@
 			isa = PBXGroup;
 			children = (
 				96BCB7811CB40DC500C806FE /* Layout.swift */,
+				9D494A39217F6B70003D66F1 /* LayoutAnchor.swift */,
+				9D494A37217F6B63003D66F1 /* LayoutAttribute.swift */,
+				9D494A3B217F6B7D003D66F1 /* LayoutConstraint.swift */,
 			);
 			name = Layout;
 			sourceTree = "<group>";
@@ -993,6 +1002,7 @@
 				9DF352461FED210000B2A11B /* CheckButton.swift in Sources */,
 				965E810A1DD4D5C800D61E4B /* Font.swift in Sources */,
 				965E810B1DD4D5C800D61E4B /* RobotoFont.swift in Sources */,
+				9D494A3C217F6B7D003D66F1 /* LayoutConstraint.swift in Sources */,
 				965E810C1DD4D5C800D61E4B /* DynamicFontType.swift in Sources */,
 				965E81101DD4D5C800D61E4B /* NavigationBar.swift in Sources */,
 				965E81111DD4D5C800D61E4B /* NavigationController.swift in Sources */,
@@ -1015,6 +1025,7 @@
 				965E81211DD4D5C800D61E4B /* TextStorage.swift in Sources */,
 				965E81221DD4D5C800D61E4B /* TextView.swift in Sources */,
 				965E80E71DD4C55200D61E4B /* Material+UIView.swift in Sources */,
+				9D494A38217F6B63003D66F1 /* LayoutAttribute.swift in Sources */,
 				96B8D22D20CF82D5008BD149 /* FABMenu.swift in Sources */,
 				965E80E81DD4C55200D61E4B /* Material+CALayer.swift in Sources */,
 				965E80E91DD4C55200D61E4B /* Material+String.swift in Sources */,
@@ -1054,6 +1065,7 @@
 				965E80D21DD4C50600D61E4B /* Color.swift in Sources */,
 				96BFC1541E5E486F0075DE1F /* SpringAnimation.swift in Sources */,
 				9D054A6620D175AC00D0528D /* Material+UILabel.swift in Sources */,
+				9D494A3A217F6B70003D66F1 /* LayoutAnchor.swift in Sources */,
 				965E80D31DD4C50600D61E4B /* Device.swift in Sources */,
 				965E80FD1DD4D59500D61E4B /* Toolbar.swift in Sources */,
 				965E80D41DD4C50600D61E4B /* Divider.swift in Sources */,

--- a/Sources/iOS/ErrorTextField.swift
+++ b/Sources/iOS/ErrorTextField.swift
@@ -69,9 +69,10 @@ open class ErrorTextField: TextField {
    get {
      return !errorLabel.isHidden
    }
-    set {
+    set(value) {
-      errorLabel.isHidden = !newValue
+      errorLabel.isHidden = !value
-      detailLabel.isHidden = newValue
+      detailLabel.isHidden = value
+      layoutSubviews()
    }
  }

--- a/Sources/iOS/Layout.swift
+++ b/Sources/iOS/Layout.swift
@@ -31,970 +31,797 @@
 import UIKit
 import Motion
-public class Layout {
+/// A protocol that's conformed by UIView and UILayoutGuide.
-  /// Parent UIView context.
+public protocol Constraintable: class { }
-  internal weak var parent: UIView?
-  /// Child UIView context.
+@available(iOS 9.0, *)
-  internal weak var child: UIView?
+extension UILayoutGuide: Constraintable { }
+extension UIView: Constraintable { }
+/// Layout extension for UIView.
+public extension UIView {
  /**
-   An initializer that takes in a parent context.
+   Used to chain layout constraints on a child context.
-   - Parameter parent: An optional parent UIView.
+   - Parameter child: A child UIView to layout.
+   - Returns: A Layout instance.
   */
-  public init(parent: UIView?) {
+  func layout(_ child: UIView) -> Layout {
-    self.parent = parent
+    addSubview(child)
+    child.translatesAutoresizingMaskIntoConstraints = false
+    return child.layout
+  }
+  /// Layout instance for the view.
+  var layout: Layout {
+    return Layout(constraintable: self)
+  }
+  /// Anchor instance for the view.
+  var anchor: LayoutAnchor {
+    return LayoutAnchor(constraintable: self)
  }
  /**
-   An initializer that takes in a parent context and child context.
+   Anchor instance for safeAreaLayoutGuide.
-   - Parameter parent: An optional parent UIView.
+   Below iOS 11, it will be same as view.anchor.
-   - Parameter child: An optional child UIView.
   */
-  public init(parent: UIView?, child: UIView?) {
+  var safeAnchor: LayoutAnchor {
-    self.parent = parent
+    if #available(iOS 11.0, *) {
-    self.child = child
+      return LayoutAnchor(constraintable: safeAreaLayoutGuide)
+    } else {
+      return anchor
+    }
+  }
+}
+public struct Layout {
+  /// A weak reference to the constraintable.
+  weak var constraintable: Constraintable?
+  /// Parent view of the view.
+  var parent: UIView? {
+    return (constraintable as? UIView)?.superview
+  }
+  /// Returns the view that is being laied out.
+  private var view: UIView? {
+    var  v = constraintable as? UIView
+    if #available(iOS 9.0, *), v == nil {
+      v = (constraintable as? UILayoutGuide)?.owningView
+    }
+    return v
  }
  /**
-   Prints a debug message when the parent context is not available.
+   An initializer taking Constraintable.
-   - Parameter function: A String representation of the function that
+   - Parameter view: A Constraintable.
-   caused the issue.
-   - Returns: The current Layout instance.
   */
-  internal func debugParentNotAvailableMessage(function: String = #function) -> Layout {
+  init(constraintable: Constraintable) {
-    debugPrint("[Material Layout Error: Parent view context is not available for \(function).")
+    self.constraintable = constraintable
-    return self
  }
+}
+public extension Layout {
  /**
-   Prints a debug message when the child context is not available.
+   Sets multiplier of the last created constraint.
-   - Parameter function: A String representation of the function that
+   Not meant for updating the multiplier as it will re-create the constraint.
-   caused the issue.
+   - Parameter _ multiplier: A CGFloat multiplier.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
-  internal func debugChildNotAvailableMessage(function: String = #function) -> Layout {
+  func multiply(_ multiplier: CGFloat) -> Layout {
-    debugPrint("[Material Layout Error: Child view context is not available for \(function).")
+    return resetLastConstraint(multiplier: multiplier)
-    return self
  }
  /**
-   Sets the width of a view.
+   Sets priority of the last created constraint.
-   - Parameter child: A child UIView to layout.
+   Not meant for updating the multiplier as it will re-create the constraint.
-   - Parameter width: A CGFloat value.
+   - Parameter _ value: A Float priority.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
-  @discardableResult
+  func priority(_ value: Float) -> Layout {
-  public func width(_ child: UIView, width: CGFloat) -> Layout {
+    return priority(.init(rawValue: value))
-    guard let v = parent else {
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.width(parent: v, child: child, width: width)
-    return self
  }
  /**
-   Sets the width of a view assuming a child context view.
+   Sets priority of the last created constraint.
-   - Parameter width: A CGFloat value.
+   Not meant for updating the priority as it will re-create the constraint.
-   - Returns: The current Layout instance.
+   - Parameter _ priority: A UILayoutPriority.
+   - Returns: A Layout instance to allow chaining.
   */
-  @discardableResult
+  func priority(_ priority: UILayoutPriority) -> Layout {
-  public func width(_ width: CGFloat) -> Layout {
+    return resetLastConstraint(priority: priority)
-    guard let v = child else {
-      return debugChildNotAvailableMessage()
-    }
-    return self.width(v, width: width)
  }
  /**
-   Sets the height of a view.
+   Removes the last created constraint and creates new one with the new multiplier and/or priority (if provided).
-   - Parameter child: A child UIView to layout.
+   - Parameter multiplier: An optional CGFloat.
-   - Parameter height: A CGFloat value.
+   - Parameter priority: An optional UILayoutPriority.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
-  @discardableResult
+  private func resetLastConstraint(multiplier: CGFloat? = nil, priority: UILayoutPriority? = nil) -> Layout {
-  public func height(_ child: UIView, height: CGFloat) -> Layout {
+    guard let v = view?.lastConstraint, v.isActive else {
-    guard let v = parent else {
+      return self
-      return debugParentNotAvailableMessage()
    }
-    self.child = child
+    v.isActive = false
-    Layout.height(parent: v, child: child, height: height)
+    let newV = NSLayoutConstraint(item: v.firstItem as Any,
+                                  attribute: v.firstAttribute,
+                                  relatedBy: v.relation,
+                                  toItem: v.secondItem,
+                                  attribute: v.secondAttribute,
+                                  multiplier: multiplier ?? v.multiplier,
+                                  constant: v.constant)
+    newV.priority = priority ?? v.priority
+    newV.isActive = true
+    view?.lastConstraint = newV
    return self
  }
+}
+public extension Layout {
  /**
-   Sets the height of a view assuming a child context view.
+   Constraints top of the view to its parent's.
-   - Parameter height: A CGFloat value.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func height(_ height: CGFloat) -> Layout {
+  func top(_ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.top, constant: offset)
-      return debugChildNotAvailableMessage()
-    }
-    return self.height(v, height: height)
  }
  /**
-   Sets the width and height of a view.
+   Constraints left of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter size: A CGSize value.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func size(_ child: UIView, size: CGSize) -> Layout {
+  func left(_ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.left, constant: offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.size(parent: v, child: child, size: size)
-    return self
  }
  /**
-   Sets the width and height of a view assuming a child context view.
+   Constraints right of the view to its parent.
-   - Parameter size: A CGSize value.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func size(_ size: CGSize = CGSize.zero) -> Layout {
+  func right(_ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.right, constant: -offset)
-      return debugChildNotAvailableMessage()
-    }
-    return self.size(v, size: size)
  }
  /**
-   A collection of children views are horizontally stretched with optional left,
+   Constraints leading of the view to its parent's.
-   right padding and interim interimSpace.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter children: An Array UIView to layout.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter left: A CGFloat value for padding the left side.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Parameter interimSpace: A CGFloat value for interim interimSpace.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func horizontally(_ children: [UIView], left: CGFloat = 0, right: CGFloat = 0, interimSpace: InterimSpace = 0) -> Layout {
+  func leading(_ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.leading, constant: offset)
-      return debugParentNotAvailableMessage()
-    }
-    Layout.horizontally(parent: v, children: children, left: left, right: right, interimSpace: interimSpace)
-    return self
  }
  /**
-   A collection of children views are vertically stretched with optional top,
+   Constraints trailing of the view to its parent.
-   bottom padding and interim interimSpace.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter children: An Array UIView to layout.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter top: A CGFloat value for padding the top side.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter interimSpace: A CGFloat value for interim interimSpace.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func vertically(_ children: [UIView], top: CGFloat = 0, bottom: CGFloat = 0, interimSpace: InterimSpace = 0) -> Layout {
+  func trailing(_ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.trailing, constant: -offset)
-      return debugParentNotAvailableMessage()
-    }
-    Layout.vertically(parent: v, children: children, top: top, bottom: bottom, interimSpace: interimSpace)
-    return self
  }
  /**
-   A child view is horizontally stretched with optional left and right padding.
+   Constraints bottom of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func horizontally(_ child: UIView, left: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func bottom(_ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.bottom, constant: -offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.horizontally(parent: v, child: child, left: left, right: right)
-    return self
  }
  /**
-   A child view is horizontally stretched with optional left and right padding.
+   Constraints top-left of the view to its parent's.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Parameter left: A CGFloat offset for left.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func horizontally(left: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func topLeft(top: CGFloat = 0, left: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.topLeft, constants: top, left)
-      return debugChildNotAvailableMessage()
-    }
-    return horizontally(v, left: left, right: right)
  }
  /**
-   A child view is vertically stretched with optional left and right padding.
+   Constraints top-right of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter right: A CGFloat offset for right.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func vertically(_ child: UIView, top: CGFloat = 0, bottom: CGFloat = 0) -> Layout {
+  func topRight(top: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.topRight, constants: top, -right)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.vertically(parent: v, child: child, top: top, bottom: bottom)
-    return self
  }
  /**
-   A child view is vertically stretched with optional left and right padding.
+   Constraints bottom-left of the view to its parent's.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter left: A CGFloat offset for left.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func vertically(top: CGFloat = 0, bottom: CGFloat = 0) -> Layout {
+  func bottomLeft(bottom: CGFloat = 0, left: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.bottomLeft, constants: -bottom, left)
-      return debugChildNotAvailableMessage()
-    }
-    return vertically(v, top: top, bottom: bottom)
  }
  /**
-   A child view is vertically and horizontally stretched with optional top, left, bottom and right padding.
+   Constraints bottom-right of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter right: A CGFloat offset for right.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func edges(_ child: UIView, top: CGFloat = 0, left: CGFloat = 0, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func bottomRight(bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.bottomRight, constants: -bottom, -right)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.edges(parent: v, child: child, top: top, left: left, bottom: bottom, right: right)
-    return self
  }
  /**
-   A child view is vertically and horizontally stretched with optional top, left, bottom and right padding.
+   Constraints left and right of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter left: A CGFloat offset for left.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter right: A CGFloat offset for right.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func edges(top: CGFloat = 0, left: CGFloat = 0, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func leftRight(left: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.leftRight, constants: left, -right)
-      return debugChildNotAvailableMessage()
-    }
-    return edges(v, top: top, left: left, bottom: bottom, right: right)
  }
  /**
-   A child view is aligned from the top with optional top padding.
+   Constraints top-leading of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter leading: A CGFloat offset for leading.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func top(_ child: UIView, top: CGFloat = 0) -> Layout {
+  func topLeading(top: CGFloat = 0, leading: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.topLeading, constants: top, leading)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.top(parent: v, child: child, top: top)
-    return self
  }
  /**
-   A child view is aligned from the top with optional top padding.
+   Constraints top-trailing of the view to its parent's.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter top: A CGFloat offset for top.
-   - Returns: The current Layout instance.
+   - Parameter trailing: A CGFloat offset for trailing.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func top(_ top: CGFloat = 0) -> Layout {
+  func topTrailing(top: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.topTrailing, constants: top, -trailing)
-      return debugChildNotAvailableMessage()
-    }
-    return self.top(v, top: top)
  }
  /**
-   A child view is aligned from the left with optional left padding.
+   Constraints bottom-leading of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Parameter leading: A CGFloat offset for leading.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func left(_ child: UIView, left: CGFloat = 0) -> Layout {
+  func bottomLeading(bottom: CGFloat = 0, leading: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.bottomLeading, constants: -bottom, leading)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.left(parent: v, child: child, left: left)
-    return self
  }
  /**
-   A child view is aligned from the left with optional left padding.
+   Constraints bottom-trailing of the view to its parent's.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Returns: The current Layout instance.
+   - Parameter trailing: A CGFloat offset for trailing.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func left(_ left: CGFloat = 0) -> Layout {
+  func bottomTrailing(bottom: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.bottomTrailing, constants: -bottom, -trailing)
-      return debugChildNotAvailableMessage()
-    }
-    return self.left(v, left: left)
  }
  /**
-   A child view is aligned from the bottom with optional bottom padding.
+   Constraints leading and trailing of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter leading: A CGFloat offset for leading.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter trailing: A CGFloat offset for trailing.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func bottom(_ child: UIView, bottom: CGFloat = 0) -> Layout {
+  func leadingTrailing(leading: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.leadingTrailing, constants: leading, -trailing)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.bottom(parent: v, child: child, bottom: bottom)
-    return self
  }
  /**
-   A child view is aligned from the bottom with optional bottom padding.
+   Constraints top and bottom of the view to its parent's.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter top: A CGFloat offset for top.
-   - Returns: The current Layout instance.
+   - Parameter bottom: A CGFloat offset for bottom.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func bottom(_ bottom: CGFloat = 0) -> Layout {
+  func topBottom(top: CGFloat = 0, bottom: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.topBottom, constants: top, -bottom)
-      return debugChildNotAvailableMessage()
-    }
-    return self.bottom(v, bottom: bottom)
  }
  /**
-   A child view is aligned from the right with optional right padding.
+   Constraints center of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter offsetX: A CGFloat offset for horizontal center.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Parameter offsetY: A CGFloat offset for vertical center.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func right(_ child: UIView, right: CGFloat = 0) -> Layout {
+  func center(offsetX: CGFloat = 0, offsetY: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.center, constants: offsetX, offsetY)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.right(parent: v, child: child, right: right)
-    return self
  }
  /**
-   A child view is aligned from the right with optional right padding.
+   Constraints horizontal center of the view to its parent's.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func right(_ right: CGFloat = 0) -> Layout {
+  func centerX(_ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.centerX, constant: offset)
-      return debugChildNotAvailableMessage()
-    }
-    return self.right(v, right: right)
  }
  /**
-   A child view is aligned from the top left with optional top and left padding.
+   Constraints vertical center of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter left: A CGFloat value for padding the left side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func topLeft(_ child: UIView, top: CGFloat = 0, left: CGFloat = 0) -> Layout {
+  func centerY(_ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.centerY, constant: offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.topLeft(parent: v, child: child, top: top, left: left)
-    return self
  }
  /**
-   A child view is aligned from the top left with optional top and left padding.
+   Constraints width of the view to its parent's.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter offset: A CGFloat offset.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func topLeft(top: CGFloat = 0, left: CGFloat = 0) -> Layout {
+  func width(offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.width, constant: offset)
-      return debugChildNotAvailableMessage()
-    }
-    return topLeft(v, top: top, left: left)
  }
  /**
-   A child view is aligned from the top right with optional top and right padding.
+   Constraints height of the view to its parent's.
-   - Parameter child: A child UIView to layout.
+   - Parameter offset: A CGFloat offset.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func topRight(_ child: UIView, top: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func height(offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.height, constant: offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.topRight(parent: v, child: child, top: top, right: right)
-    return self
  }
  /**
-   A child view is aligned from the top right with optional top and right padding.
+   Constraints edges of the view to its parent's.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Parameter left: A CGFloat offset for left.
-   - Returns: The current Layout instance.
+   - Parameter bottom: A CGFloat offset for bottom.
+   - Parameter right: A CGFloat offset for right.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func topRight(top: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func edges(top: CGFloat = 0, left: CGFloat = 0, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.edges, constants: top, left, -bottom, -right)
-      return debugChildNotAvailableMessage()
-    }
-    return topRight(v, top: top, right: right)
  }
+}
+public extension Layout {
  /**
-   A child view is aligned from the bottom left with optional bottom and left padding.
+   Constraints width of the view to a constant value.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ width: A CGFloat value.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter left: A CGFloat value for padding the left side.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func bottomLeft(_ child: UIView, bottom: CGFloat = 0, left: CGFloat = 0) -> Layout {
+  func width(_ width: CGFloat) -> Layout {
-    guard let v = parent else {
+    return constraint(.constantWidth, constants: width)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.bottomLeft(parent: v, child: child, bottom: bottom, left: left)
-    return self
  }
  /**
-   A child view is aligned from the bottom left with optional bottom and left padding.
+   Constraints height of the view to a constant value.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter _ height: A CGFloat value.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func bottomLeft(bottom: CGFloat = 0, left: CGFloat = 0) -> Layout {
+  func height(_ height: CGFloat) -> Layout {
-    guard let v = child else {
+    return constraint(.constantHeight, constants: height)
-      return debugChildNotAvailableMessage()
-    }
-    return bottomLeft(v, bottom: bottom, left: left)
  }
+}
+public extension Layout {
  /**
-   A child view is aligned from the bottom right with optional bottom and right padding.
+   Constraints top of the view to the given anchor.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func bottomRight(_ child: UIView, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func top(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.top, to: anchor, constant: offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.bottomRight(parent: v, child: child, bottom: bottom, right: right)
-    return self
  }
  /**
-   A child view is aligned from the bottom right with optional bottom and right padding.
+   Constraints left of the view to the given anchor.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func bottomRight(bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
+  func left(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.left, to: anchor, constant: offset)
-      return debugChildNotAvailableMessage()
-    }
-    return bottomRight(v, bottom: bottom, right: right)
  }
  /**
-   A child view is aligned at the center with an optional offsetX and offsetY value.
+   Constraints right of the view to the given anchor.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter offsetX: A CGFloat value for the offset along the x axis.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter offsetX: A CGFloat value for the offset along the y axis.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
  @discardableResult
-  public func center(_ child: UIView, offsetX: CGFloat = 0, offsetY: CGFloat = 0) -> Layout {
+  func right(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.right, to: anchor, constant: -offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.center(parent: v, child: child, offsetX: offsetX, offsetY: offsetY)
-    return self
  }
  /**
-   A child view is aligned at the center with an optional offsetX and offsetY value.
+   Constraints leading of the view to the given anchor.
-   - Parameter offsetX: A CGFloat value for the offset along the x axis.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter offsetX: A CGFloat value for the offset along the y axis.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func center(offsetX: CGFloat = 0, offsetY: CGFloat = 0) -> Layout {
+  func leading(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.leading, to: anchor, constant: offset)
-      return debugChildNotAvailableMessage()
-    }
-    return center(v, offsetX: offsetX, offsetY: offsetY)
  }
  /**
-   A child view is aligned at the center horizontally with an optional offset value.
+   Constraints trailing of the view to the given anchor.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter offset: A CGFloat value for the offset along the x axis.
+   - Parameter _ offset: A CGFloat offset.
-   - Returns: The current Layout instance.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func centerHorizontally(_ child: UIView, offset: CGFloat = 0) -> Layout {
+  func trailing(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.trailing, to: anchor, constant: -offset)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.centerHorizontally(parent: v, child: child, offset: offset)
-    return self
  }
  /**
-   A child view is aligned at the center horizontally with an optional offset value.
+   Constraints bottom of the view to the given anchor.
-   - Parameter offset: A CGFloat value for the offset along the x axis.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Returns: The current Layout instance.
+   - Parameter _ offset: A CGFloat offset.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func centerHorizontally(offset: CGFloat = 0) -> Layout {
+  func bottom(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.bottom, to: anchor, constant: -offset)
-      return debugChildNotAvailableMessage()
-    }
-    return centerHorizontally(v, offset: offset)
  }
  /**
-   A child view is aligned at the center vertically with an optional offset value.
+   Constraints top-leading of the view to the given anchor.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter offset: A CGFloat value for the offset along the y axis.
+   - Parameter top: A CGFloat offset for top.
-   - Returns: The current Layout instance.
+   - Parameter leading: A CGFloat offset for leading.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func centerVertically(_ child: UIView, offset: CGFloat = 0) -> Layout {
+  func topLeading(_ anchor: LayoutAnchorable, top: CGFloat = 0, leading: CGFloat = 0) -> Layout {
-    guard let v = parent else {
+    return constraint(.topLeading, to: anchor, constants: top, leading)
-      return debugParentNotAvailableMessage()
-    }
-    self.child = child
-    Layout.centerVertically(parent: v, child: child, offset: offset)
-    return self
  }
  /**
-   A child view is aligned at the center vertically with an optional offset value.
+   Constraints top-trailing of the view to the given anchor.
-   - Parameter offset: A CGFloat value for the offset along the y axis.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Returns: The current Layout instance.
+   - Parameter top: A CGFloat offset for top.
+   - Parameter trailing: A CGFloat offset for trailing.
+   - Returns: A Layout instance to allow chaining.
   */
  @discardableResult
-  public func centerVertically(offset: CGFloat = 0) -> Layout {
+  func topTrailing(_ anchor: LayoutAnchorable, top: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-    guard let v = child else {
+    return constraint(.topTrailing, to: anchor, constants: top, -trailing)
-      return debugChildNotAvailableMessage()
-    }
-    return centerVertically(v, offset: offset)
  }
-}
-fileprivate extension Layout {
  /**
-   Updates the consraints for a given view.
+   Constraints bottom-leading of the view to the given anchor.
-   - Parameter for view: A UIView.
+   - Parameter _ anchor: A LayoutAnchorable.
+   - Parameter bottom: A CGFloat offset for bottom.
+   - Parameter leading: A CGFloat offset for leading.
+   - Returns: A Layout instance to allow chaining.
   */
-  class func updateConstraints(for view: UIView) {
+  @discardableResult
-    view.setNeedsUpdateConstraints()
+  func bottomLeading(_ anchor: LayoutAnchorable, bottom: CGFloat = 0, leading: CGFloat = 0) -> Layout {
-    view.updateConstraintsIfNeeded()
+    return constraint(.bottomLeading, to: anchor, constants: -bottom, leading)
-    view.setNeedsLayout()
-    view.layoutIfNeeded()
  }
  /**
-   Updates the constraints for a given Array of views.
+   Constraints bottom-trailing of the view to the given anchor.
-   - Parameter for [view]: An Array of UIViews.
+   - Parameter _ anchor: A LayoutAnchorable.
+   - Parameter bottom: A CGFloat offset for bottom.
+   - Parameter trailing: A CGFloat offset for trailing.
+   - Returns: A Layout instance to allow chaining.
   */
-  class func updateConstraints(for views: [UIView]) {
+  @discardableResult
-    for v in views {
+  func bottomTrailing(_ anchor: LayoutAnchorable, bottom: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-      updateConstraints(for: v)
+    return constraint(.bottomTrailing, to: anchor, constants: -bottom, -trailing)
-    }
  }
-}
-/// Layout
-extension Layout {
  /**
-   Sets the width of a view.
+   Constraints leading and trailing of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter leading: A CGFloat offset for leading.
-   - Parameter width: A CGFloat value.
+   - Parameter trailing: A CGFloat offset for trailing.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func width(parent: UIView, child: UIView, width: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func leadingTrailing(_ anchor: LayoutAnchorable, leading: CGFloat = 0, trailing: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .width, multiplier: 1, constant: width))
+    return constraint(.leadingTrailing, to: anchor, constants: leading, -trailing)
-    updateConstraints(for: child)
  }
  /**
-   Sets the height of a view.
+   Constraints top-left of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter height: A CGFloat value.
+   - Parameter left: A CGFloat offset for left.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func height(parent: UIView, child: UIView, height: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func topLeft(_ anchor: LayoutAnchorable, top: CGFloat = 0, left: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .height, multiplier: 1, constant: height))
+    return constraint(.topLeft, to: anchor, constants: top, left)
-    updateConstraints(for: child)
  }
  /**
-   Sets the width and height of a view.
+   Constraints top-right of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter size: A CGSize value.
+   - Parameter right: A CGFloat offset for right.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func size(parent: UIView, child: UIView, size: CGSize = CGSize.zero) {
+  @discardableResult
-    Layout.width(parent: parent, child: child, width: size.width)
+  func topRight(_ anchor: LayoutAnchorable, top: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    Layout.height(parent: parent, child: child, height: size.height)
+    return constraint(.topRight, to: anchor, constants: top, -right)
  }
  /**
-   A collection of children views are horizontally stretched with optional left,
+   Constraints bottom-left of the view to the given anchor.
-   right padding and interim interimSpace.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter parent: A parent UIView context.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter children: An Array UIView to layout.
+   - Parameter left: A CGFloat offset for left.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter right: A CGFloat value for padding the right side.
-   - Parameter interimSpace: A CGFloat value for interim interimSpace.
   */
-  public class func horizontally(parent: UIView, children: [UIView], left: CGFloat = 0, right: CGFloat = 0, interimSpace: InterimSpace = 0) {
+  @discardableResult
-    prepareForConstraint(parent, children: children)
+  func bottomLeft(_ anchor: LayoutAnchorable, bottom: CGFloat = 0, left: CGFloat = 0) -> Layout {
+    return constraint(.bottomLeft, to: anchor, constants: -bottom, left)
-    if 0 < children.count {
-      parent.addConstraint(NSLayoutConstraint(item: children[0], attribute: .left, relatedBy: .equal, toItem: parent, attribute: .left, multiplier: 1, constant: left))
-      for i in 1..<children.count {
-        parent.addConstraint(NSLayoutConstraint(item: children[i], attribute: .left, relatedBy: .equal, toItem: children[i - 1], attribute: .right, multiplier: 1, constant: interimSpace))
-        parent.addConstraint(NSLayoutConstraint(item: children[i], attribute: .width, relatedBy: .equal, toItem: children[0], attribute: .width, multiplier: 1, constant: 0))
-      }
-      parent.addConstraint(NSLayoutConstraint(item: children[children.count - 1], attribute: .right, relatedBy: .equal, toItem: parent, attribute: .right, multiplier: 1, constant: -right))
-    }
-    updateConstraints(for: children)
  }
  /**
-   A collection of children views are vertically stretched with optional top,
+   Constraints bottom-right of the view to the given anchor.
-   bottom padding and interim interimSpace.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter parent: A parent UIView context.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter children: An Array UIView to layout.
+   - Parameter right: A CGFloat offset for right.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter interimSpace: A CGFloat value for interim interimSpace.
   */
-  public class func vertically(parent: UIView, children: [UIView], top: CGFloat = 0, bottom: CGFloat = 0, interimSpace: InterimSpace = 0) {
+  @discardableResult
-    prepareForConstraint(parent, children: children)
+  func bottomRight(_ anchor: LayoutAnchorable, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
+    return constraint(.bottomRight, to: anchor, constants: -bottom, -right)
-    if 0 < children.count {
-      parent.addConstraint(NSLayoutConstraint(item: children[0], attribute: .top, relatedBy: .equal, toItem: parent, attribute: .top, multiplier: 1, constant: top))
-      for i in 1..<children.count {
-        parent.addConstraint(NSLayoutConstraint(item: children[i], attribute: .top, relatedBy: .equal, toItem: children[i - 1], attribute: .bottom, multiplier: 1, constant: interimSpace))
-        parent.addConstraint(NSLayoutConstraint(item: children[i], attribute: .height, relatedBy: .equal, toItem: children[0], attribute: .height, multiplier: 1, constant: 0))
-      }
-      parent.addConstraint(NSLayoutConstraint(item: children[children.count - 1], attribute: .bottom, relatedBy: .equal, toItem: parent, attribute: .bottom, multiplier: 1, constant: -bottom))
-    }
-    updateConstraints(for: children)
  }
  /**
-   A child view is horizontally stretched with optional left and right padding.
+   Constraints left and right of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter left: A CGFloat offset for left.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Parameter right: A CGFloat offset for right.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func horizontally(parent: UIView, child: UIView, left: CGFloat = 0, right: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func leftRight(_ anchor: LayoutAnchorable, left: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .left, relatedBy: .equal, toItem: parent, attribute: .left, multiplier: 1, constant: left))
+    return constraint(.leftRight, to: anchor, constants: left, -right)
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .right, relatedBy: .equal, toItem: parent, attribute: .right, multiplier: 1, constant: -right))
-    updateConstraints(for: child)
  }
  /**
-   A child view is vertically stretched with optional left and right padding.
+   Constraints top and bottom of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func vertically(parent: UIView, child: UIView, top: CGFloat = 0, bottom: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func topBottom(_ anchor: LayoutAnchorable, top: CGFloat = 0, bottom: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .top, relatedBy: .equal, toItem: parent, attribute: .top, multiplier: 1, constant: top))
+    return constraint(.topBottom, to: anchor, constants: top, -bottom)
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .bottom, relatedBy: .equal, toItem: parent, attribute: .bottom, multiplier: 1, constant: -bottom))
-    updateConstraints(for: child)
  }
  /**
-   A child view is vertically and horizontally stretched with optional top, left, bottom and right padding.
+   Constraints center of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter offsetX: A CGFloat offset for horizontal center.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter offsetY: A CGFloat offset for vertical center.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter right: A CGFloat value for padding the right side.
   */
-  public class func edges(parent: UIView, child: UIView, top: CGFloat = 0, left: CGFloat = 0, bottom: CGFloat = 0, right: CGFloat = 0) {
+  @discardableResult
-    horizontally(parent: parent, child: child, left: left, right: right)
+  func center(_ anchor: LayoutAnchorable, offsetX: CGFloat = 0, offsetY: CGFloat = 0) -> Layout {
-    vertically(parent: parent, child: child, top: top, bottom: bottom)
+    return constraint(.center, to: anchor, constants: offsetX, offsetY)
  }
  /**
-   A child view is aligned from the top with optional top padding.
+   Constraints horizontal center of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func top(parent: UIView, child: UIView, top: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func centerX(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .top, relatedBy: .equal, toItem: parent, attribute: .top, multiplier: 1, constant: top))
+    return constraint(.centerX, to: anchor, constant: offset)
-    updateConstraints(for: child)
  }
  /**
-   A child view is aligned from the left with optional left padding.
+   Constraints vertical center of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ offset: A CGFloat offset.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func left(parent: UIView, child: UIView, left: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func centerY(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .left, relatedBy: .equal, toItem: parent, attribute: .left, multiplier: 1, constant: left))
+    return constraint(.centerY, to: anchor, constant: offset)
-    updateConstraints(for: child)
  }
  /**
-   A child view is aligned from the bottom with optional bottom padding.
+   Constraints height of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter offset: A CGFloat offset.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func bottom(parent: UIView, child: UIView, bottom: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func width(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .bottom, relatedBy: .equal, toItem: parent, attribute: .bottom, multiplier: 1, constant: -bottom))
+    return constraint(.width, to: anchor, constant: offset)
-    updateConstraints(for: child)
  }
  /**
-   A child view is aligned from the right with optional right padding.
+   Constraints height of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter offset: A CGFloat offset.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func right(parent: UIView, child: UIView, right: CGFloat = 0) {
+  @discardableResult
-    prepareForConstraint(parent, child: child)
+  func height(_ anchor: LayoutAnchorable, _ offset: CGFloat = 0) -> Layout {
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .right, relatedBy: .equal, toItem: parent, attribute: .right, multiplier: 1, constant: -right))
+    return constraint(.height, to: anchor, constant: offset)
-    updateConstraints(for: child)
  }
  /**
-   A child view is aligned from the top left with optional top and left padding.
+   Constraints edges of the view to the given anchor.
-   - Parameter parent: A parent UIView context.
+   - Parameter _ anchor: A LayoutAnchorable.
-   - Parameter child: A child UIView to layout.
+   - Parameter top: A CGFloat offset for top.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter left: A CGFloat offset for left.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Parameter bottom: A CGFloat offset for bottom.
-   - Returns: The current Layout instance.
+   - Parameter right: A CGFloat offset for right.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func topLeft(parent: UIView, child: UIView, top t: CGFloat = 0, left l: CGFloat = 0) {
+  @discardableResult
-    top(parent: parent, child: child, top: t)
+  func edges(_ anchor: LayoutAnchorable, top: CGFloat = 0, left: CGFloat = 0, bottom: CGFloat = 0, right: CGFloat = 0) -> Layout {
-    left(parent: parent, child: child, left: l)
+    return constraint(.edges, to: anchor, constants: top, left, -bottom, -right)
  }
+}
+private extension Layout {
  /**
-   A child view is aligned from the top right with optional top and right padding.
+   Constraints the view to its parent according to the provided attribute.
-   - Parameter parent: A parent UIView context.
+   If the constraint already exists, will update its constant.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ attribute: A LayoutAttribute.
-   - Parameter top: A CGFloat value for padding the top side.
+   - Parameter constant: A CGFloat.
-   - Parameter right: A CGFloat value for padding the right side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func topRight(parent: UIView, child: UIView, top t: CGFloat = 0, right r: CGFloat = 0) {
+  func constraint(_ attribute: LayoutAttribute, constant: CGFloat) -> Layout {
-    top(parent: parent, child: child, top: t)
+    return constraint([attribute], constants: constant)
-    right(parent: parent, child: child, right: r)
  }
  /**
-   A child view is aligned from the bottom left with optional bottom and left padding.
+   Constraints the view to its parent according to the provided attributes.
-   - Parameter parent: A parent UIView context.
+   If any of the constraints already exists, will update its constant.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ attributes: An array of LayoutAttribute.
-   - Parameter bottom: A CGFloat value for padding the bottom side.
+   - Parameter constants: A list of CGFloat.
-   - Parameter left: A CGFloat value for padding the left side.
+   - Returns: A Layout instance to allow chaining.
-   - Returns: The current Layout instance.
   */
-  public class func bottomLeft(parent: UIView, child: UIView, bottom b: CGFloat = 0, left l: CGFloat = 0) {
+  func constraint(_ attributes: [LayoutAttribute], constants: CGFloat...) -> Layout {
-    bottom(parent: parent, child: child, bottom: b)
+    var attributes = attributes
-    left(parent: parent, child: child, left: l)
+    var anchor: LayoutAnchor!
-  }
-  /**
+    if attributes == .constantHeight || attributes == .constantWidth {
-   A child view is aligned from the bottom right with optional bottom and right padding.
+      attributes.removeLast()
-   - Parameter parent: A parent UIView context.
+      anchor = LayoutAnchor(constraintable: nil, attributes: [.notAnAttribute])
-   - Parameter child: A child UIView to layout.
+    } else {
-   - Parameter bottom: A CGFloat value for padding the bottom side.
-   - Parameter right: A CGFloat value for padding the right side.
+      guard parent != nil else {
-   - Returns: The current Layout instance.
+        fatalError("[Material Error: Constraint requires view to have parent.")
-   */
-  public class func bottomRight(parent: UIView, child: UIView, bottom b: CGFloat = 0, right r: CGFloat = 0) {
-    bottom(parent: parent, child: child, bottom: b)
-    right(parent: parent, child: child, right: r)
      }
-  /**
+      anchor = LayoutAnchor(constraintable: parent, attributes: attributes)
-   A child view is aligned at the center with an optional offsetX and offsetY value.
+    }
-   - Parameter parent: A parent UIView context.
+    return constraint(attributes, to: anchor, constants: constants)
-   - Parameter child: A child UIView to layout.
-   - Parameter offsetX: A CGFloat value for the offset along the x axis.
-   - Parameter offsetX: A CGFloat value for the offset along the y axis.
-   - Returns: The current Layout instance.
-   */
-  public class func center(parent: UIView, child: UIView, offsetX: CGFloat = 0, offsetY: CGFloat = 0) {
-    centerHorizontally(parent: parent, child: child, offset: offsetX)
-    centerVertically(parent: parent, child: child, offset: offsetY)
  }
  /**
-   A child view is aligned at the center horizontally with an optional offset value.
+   Constraints the view to the given anchor according to the provided attribute.
-   - Parameter parent: A parent UIView context.
+   If the constraint already exists, will update its constant.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ attribute: A LayoutAttribute.
-   - Parameter offset: A CGFloat value for the offset along the y axis.
+   - Parameter to anchor: A LayoutAnchorable.
-   - Returns: The current Layout instance.
+   - Parameter constant: A CGFloat.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func centerHorizontally(parent: UIView, child: UIView, offset: CGFloat = 0) {
+  func constraint(_ attribute: LayoutAttribute, to anchor: LayoutAnchorable, constant: CGFloat) -> Layout {
-    prepareForConstraint(parent, child: child)
+    return constraint([attribute], to: anchor, constants: constant)
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .centerX, relatedBy: .equal, toItem: parent, attribute: .centerX, multiplier: 1, constant: offset))
-    updateConstraints(for: child)
  }
  /**
-   A child view is aligned at the center vertically with an optional offset value.
+   Constraints the view to the given anchor according to the provided attributes.
-   - Parameter parent: A parent UIView context.
+   If any of the constraints already exists, will update its constant.
-   - Parameter child: A child UIView to layout.
+   - Parameter _ attributes: An array of LayoutAttribute.
-   - Parameter offset: A CGFloat value for the offset along the y axis.
+   - Parameter to anchor: A LayoutAnchorable.
-   - Returns: The current Layout instance.
+   - Parameter constants: A list of CGFloat.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func centerVertically(parent: UIView, child: UIView, offset: CGFloat = 0) {
+  func constraint(_ attributes: [LayoutAttribute], to anchor: LayoutAnchorable, constants: CGFloat...) -> Layout {
-    prepareForConstraint(parent, child: child)
+    return constraint(attributes, to: anchor, constants: constants)
-    parent.addConstraint(NSLayoutConstraint(item: child, attribute: .centerY, relatedBy: .equal, toItem: parent, attribute: .centerY, multiplier: 1, constant: offset))
-    updateConstraints(for: child)
  }
  /**
-   Creats an Array with a NSLayoutConstraint value.
+   Constraints the view to the given anchor according to the provided attributes.
-   - Parameter format: The VFL format string.
+   If any of the constraints already exists, will update its constant.
-   - Parameter options: Additional NSLayoutFormatOptions.
+   - Parameter _ attributes: An array of LayoutAttribute.
-   - Parameter metrics: An optional Dictionary<String, Any> of metric key / value pairs.
+   - Parameter to anchor: A LayoutAnchorable.
-   - Parameter views: A Dictionary<String, Any> of view key / value pairs.
+   - Parameter constants: An array of CGFloat.
-   - Returns: The Array<NSLayoutConstraint> instance.
+   - Returns: A Layout instance to allow chaining.
   */
-  public class func constraint(format: String, options: NSLayoutConstraint.FormatOptions, metrics: [String: Any]?, views: [String: Any]) -> [NSLayoutConstraint] {
+  func constraint(_ attributes: [LayoutAttribute], to anchor: LayoutAnchorable, constants: [CGFloat]) -> Layout {
-    for (_, a) in views {
+    let from = LayoutAnchor(constraintable: constraintable, attributes: attributes)
-      if let v = a as? UIView {
+    var to = anchor as? LayoutAnchor
-        v.translatesAutoresizingMaskIntoConstraints = false
+    if to?.attributes.isEmpty ?? true {
-      }
+      let v = (anchor as? UIView) ?? (anchor as? LayoutAnchor)?.constraintable
+      to = LayoutAnchor(constraintable: v, attributes: attributes)
    }
-    return NSLayoutConstraint.constraints(
+    let constraint = LayoutConstraint(fromAnchor: from, toAnchor: to!, constants: constants)
-      withVisualFormat: format,
-      options: options,
-      metrics: metrics,
+    let constraints = (view?.constraints ?? []) + (view?.superview?.constraints ?? [])
-      views: views
+    let newConstraints = constraint.constraints
-    )
+    for newConstraint in newConstraints {
+      guard let activeConstraint = constraints.first(where: { $0.equalTo(newConstraint) }) else {
+        newConstraint.isActive = true
+        view?.lastConstraint = newConstraint
+        continue
      }
-  /**
+      activeConstraint.constant = newConstraint.constant
-   Prepares the relationship between the parent view context and child view
-   to layout. If the child is not already added to the view hierarchy as the
-   parent's child, then it is added.
-   - Parameter parent: A parent UIView context.
-   - Parameter child: A child UIView to layout.
-   */
-  private class func prepareForConstraint(_ parent: UIView, child: UIView) {
-    if parent != child.superview {
-      child.removeFromSuperview()
-      parent.addSubview(child)
    }
-    child.translatesAutoresizingMaskIntoConstraints = false
+    return self
  }
+}
+private extension NSLayoutConstraint {
  /**
-   Prepares the relationship between the parent view context and an Array of
+   Checks if the constraint is equal to given constraint.
-   child UIViews.
+   - Parameter _ other: An NSLayoutConstraint.
-   - Parameter parent: A parent UIView context.
+   - Returns: A Bool indicating whether constraints are equal.
-   - Parameter children: An Array of UIViews.
   */
-  private class func prepareForConstraint(_ parent: UIView, children: [UIView]) {
+  func equalTo(_ other: NSLayoutConstraint) -> Bool {
-    for v in children {
+    return firstItem === other.firstItem
-      prepareForConstraint(parent, child: v)
+      && secondItem === other.secondItem
-    }
+      && firstAttribute == other.firstAttribute
+      && secondAttribute == other.secondAttribute
  }
 }
-/// A memory reference to the LayoutKey instance for UIView extensions.
+/// A memory reference to the lastConstraint of UIView.
-fileprivate var LayoutKey: UInt8 = 0
+private var LastConstraintKey: UInt8 = 0
-/// Layout extension for UIView.
+private extension UIView {
-extension UIView {
+  /**
-  /// Layout reference.
+   The last consntraint that's created by Layout system.
-  public private(set) var layout: Layout {
+   Used to set multiplier/priority on the last constraint.
+   */
+  var lastConstraint: NSLayoutConstraint? {
    get {
-      return AssociatedObject.get(base: self, key: &LayoutKey) {
+      return AssociatedObject.get(base: self, key: &LastConstraintKey) {
-        return Layout(parent: self)
+        nil
      }
    }
    set(value) {
-      AssociatedObject.set(base: self, key: &LayoutKey, value: value)
+      AssociatedObject.set(base: self, key: &LastConstraintKey, value: value)
-    }
    }
-  /**
-   Used to chain layout constraints on a child context.
-   - Parameter child: A child UIView to layout.
-   - Returns: The current Layout instance.
-   */
-  public func layout(_ child: UIView) -> Layout {
-    return Layout(parent: self, child: child)
  }
 }
--- a/Sources/iOS/LayoutAnchor.swift
+++ b/Sources/iOS/LayoutAnchor.swift
+/*
+ * Copyright (C) 2015 - 2018, Daniel Dahan and CosmicMind, Inc. <http://cosmicmind.com>.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *  * Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ *  * Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ *  * Neither the name of CosmicMind nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+import UIKit
+/// A protocol that's conformed by UIView, LayoutAnchor, and Layout.
+public protocol LayoutAnchorable { }
+extension UIView: LayoutAnchorable { }
+extension Layout: LayoutAnchorable { }
+extension LayoutAnchor: LayoutAnchorable { }
+public struct LayoutAnchor {
+  /// A weak reference to the constraintable.
+  weak var constraintable: Constraintable?
+  /// An array of LayoutAttribute for the view.
+  let attributes: [LayoutAttribute]
+  /**
+   An initializer taking constraintable and anchor attributes.
+   - Parameter view: A Constraintable.
+   - Parameter attributes: An array of LayoutAtrribute.
+  */
+  init(constraintable: Constraintable?, attributes: [LayoutAttribute] = []) {
+    self.constraintable = constraintable
+    self.attributes = attributes
+  }
+}
+public extension LayoutAnchor {
+  /// A layout anchor representing top of the view.
+  var top: LayoutAnchor {
+    return anchor(.top)
+  }
+  /// A layout anchor representing bottom of the view.
+  var bottom: LayoutAnchor {
+    return anchor(.bottom)
+  }
+  /// A layout anchor representing left of the view.
+  var left: LayoutAnchor {
+    return anchor(.left)
+  }
+  /// A layout anchor representing right of the view.
+  var right: LayoutAnchor {
+    return anchor(.right)
+  }
+  /// A layout anchor representing leading of the view.
+  var leading: LayoutAnchor {
+    return anchor(.leading)
+  }
+  /// A layout anchor representing trailing of the view.
+  var trailing: LayoutAnchor {
+    return anchor(.trailing)
+  }
+  /// A layout anchor representing top-left of the view.
+  var topLeft: LayoutAnchor {
+    return acnhor(.topLeft)
+  }
+  /// A layout anchor representing top-right of the view.
+  var topRight: LayoutAnchor {
+    return acnhor(.topRight)
+  }
+  /// A layout anchor representing bottom-left of the view.
+  var bottomLeft: LayoutAnchor {
+    return acnhor(.bottomLeft)
+  }
+  /// A layout anchor representing bottom-right of the view.
+  var bottomRight: LayoutAnchor {
+    return acnhor(.bottomRight)
+  }
+  /// A layout anchor representing top-leading of the view.
+  var topLeading: LayoutAnchor {
+    return acnhor(.topLeading)
+  }
+  /// A layout anchor representing top-trailing of the view.
+  var topTrailing: LayoutAnchor {
+    return acnhor(.topTrailing)
+  }
+  /// A layout anchor representing bottom-leading of the view.
+  var bottomLeading: LayoutAnchor {
+    return acnhor(.bottomLeading)
+  }
+  /// A layout anchor representing bottom-trailing of the view.
+  var bottomTrailing: LayoutAnchor {
+    return acnhor(.bottomTrailing)
+  }
+  /// A layout anchor representing top and bottom of the view.
+  var topBottom: LayoutAnchor {
+    return acnhor(.topBottom)
+  }
+  /// A layout anchor representing left and right of the view.
+  var leftRight: LayoutAnchor {
+    return acnhor(.leftRight)
+  }
+  /// A layout anchor representing leading and trailing of the view.
+  var leadingTrailing: LayoutAnchor {
+    return acnhor(.leadingTrailing)
+  }
+  /// A layout anchor representing center of the view.
+  var center: LayoutAnchor {
+    return acnhor(.center)
+  }
+  /// A layout anchor representing horizontal center of the view.
+  var centerX: LayoutAnchor {
+    return anchor(.centerX)
+  }
+  /// A layout anchor representing vertical center of the view.
+  var centerY: LayoutAnchor {
+    return anchor(.centerY)
+  }
+  /// A layout anchor representing top, left, bottom and right of the view.
+  var edges: LayoutAnchor {
+    return acnhor(.edges)
+  }
+  /// A layout anchor representing width of the view.
+  var width: LayoutAnchor {
+    return anchor(.width)
+  }
+  /// A layout anchor representing height of the view.
+  var height: LayoutAnchor {
+    return anchor(.height)
+  }
+}
+private extension LayoutAnchor {
+  /**
+   Creates LayoutAnchor with the given attribute.
+   - Parameter attribute: A LayoutAttribute.
+   - Returns: A LayoutAnchor.
+   */
+  func anchor(_ attribute: LayoutAttribute) -> LayoutAnchor {
+    return LayoutAnchor(constraintable: constraintable, attributes: [attribute])
+  }
+  /**
+   Creates LayoutAnchor with the given attributes.
+   - Parameter attributes: An array of LayoutAttribute.
+   - Returns: A LayoutAnchor.
+   */
+  func acnhor(_ attributes: [LayoutAttribute]) -> LayoutAnchor {
+    return LayoutAnchor(constraintable: constraintable, attributes: attributes)
+  }
+}
--- a/Sources/iOS/LayoutAttribute.swift
+++ b/Sources/iOS/LayoutAttribute.swift
+/*
+ * Copyright (C) 2015 - 2018, Daniel Dahan and CosmicMind, Inc. <http://cosmicmind.com>.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *  * Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ *  * Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ *  * Neither the name of CosmicMind nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+import UIKit
+/// A typealias for NSLayoutConstraint.Attribute
+internal typealias LayoutAttribute = NSLayoutConstraint.Attribute
+internal extension Array where Element == LayoutAttribute {
+  /// A LayoutAttribute array containing top and left.
+  static var topLeft: [LayoutAttribute] {
+    return [.top, .left]
+  }
+  /// A LayoutAttribute array containing top and right.
+  static var topRight: [LayoutAttribute] {
+    return [.top, .right]
+  }
+  /// A LayoutAttribute array containing bottom and left.
+  static var bottomLeft: [LayoutAttribute] {
+    return [.bottom, .left]
+  }
+  /// A LayoutAttribute array containing bottom and right.
+  static var bottomRight: [LayoutAttribute] {
+    return [.bottom, .right]
+  }
+  /// A LayoutAttribute array containing left and right.
+  static var leftRight: [LayoutAttribute] {
+    return [.left, .right]
+  }
+  /// A LayoutAttribute array containing top and leading.
+  static var topLeading: [LayoutAttribute] {
+    return [.top, .leading]
+  }
+  /// A LayoutAttribute array containing top and trailing.
+  static var topTrailing: [LayoutAttribute] {
+    return [.top, .trailing]
+  }
+  /// A LayoutAttribute array containing bottom and leading.
+  static var bottomLeading: [LayoutAttribute] {
+    return [.bottom, .leading]
+  }
+  /// A LayoutAttribute array containing bottom and trailing.
+  static var bottomTrailing: [LayoutAttribute] {
+    return [.bottom, .trailing]
+  }
+  /// A LayoutAttribute array containing left and trailing.
+  static var leadingTrailing: [LayoutAttribute] {
+    return [.leading, .trailing]
+  }
+  /// A LayoutAttribute array containing top and bottom.
+  static var topBottom: [LayoutAttribute] {
+    return [.top, .bottom]
+  }
+  /// A LayoutAttribute array containing centerX and centerY.
+  static var center: [LayoutAttribute] {
+    return [.centerX, .centerY]
+  }
+  /// A LayoutAttribute array containing top, left, bottom and right.
+  static var edges: [LayoutAttribute] {
+    return [.top, .left, .bottom, .right]
+  }
+  /// A LayoutAttribute array for constant height.
+  static var constantHeight: [LayoutAttribute] {
+    return [.height, .notAnAttribute]
+  }
+  /// A LayoutAttribute array for constant width.
+  static var constantWidth: [LayoutAttribute] {
+    return [.width, .notAnAttribute]
+  }
+}
--- a/Sources/iOS/LayoutConstraint.swift
+++ b/Sources/iOS/LayoutConstraint.swift
+/*
+ * Copyright (C) 2015 - 2018, Daniel Dahan and CosmicMind, Inc. <http://cosmicmind.com>.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *  * Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ *  * Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ *  * Neither the name of CosmicMind nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+import UIKit
+internal struct LayoutConstraint {
+  /// `From` anchor for the constraint.
+  private let fromAnchor: LayoutAnchor
+  /// `To` anchor for the constraint.
+  private let toAnchor: LayoutAnchor
+  /// An array of constants for the constraint.
+  private var constants: [CGFloat]
+  /**
+   An initializer taking `from` and `to` anchors and constants for the constraint.
+   - Parameter fromAnchor: A LayoutAnchor.
+   - Parameter toAnchor: A LayoutAnchor.
+   - Parameter constants: An array of CGFloat.
+   */
+  init(fromAnchor: LayoutAnchor, toAnchor: LayoutAnchor, constants: [CGFloat]) {
+    self.fromAnchor = fromAnchor
+    self.toAnchor = toAnchor
+    self.constants = constants
+  }
+}
+internal extension LayoutConstraint {
+  /// Creates an array of NSLayoutConstraint from a LayoutConstraint.
+  var constraints: [NSLayoutConstraint] {
+    guard fromAnchor.attributes.count == toAnchor.attributes.count else {
+      fatalError("[Material Error: The number of attributes of anchors does not match.]")
+    }
+    guard fromAnchor.attributes.count == constants.count else {
+      fatalError("[Material Error: The number of constants does not match the number of constraints.]")
+    }
+    var v: [NSLayoutConstraint] = []
+    zip(zip(fromAnchor.attributes, toAnchor.attributes), constants).forEach {
+      v.append(NSLayoutConstraint(item: fromAnchor.constraintable as Any,
+                                  attribute: $0.0,
+                                  relatedBy: .equal,
+                                  toItem: toAnchor.constraintable,
+                                  attribute: $0.1,
+                                  multiplier: 1,
+                                  constant: $1))
+    }
+    return v
+  }
+}