Fixed issue-29, unbalanced calls to begin/end appearance transitions

Calling {begin,end}AppearanceTransition methods for modal transitions
was causing the issue since UIKit was already calling them that internally

Sources/Transition/MotionTransition+Complete.swift

@@ -65,6 +65,7 @@ extension MotionTransition {
       progress = 0
       progress = 0
       totalDuration = 0
       totalDuration = 0
       state = .possible
       state = .possible
+      isModalTransition = false
     }
     }
     
     
     state = .completing
     state = .completing

Sources/Transition/MotionTransition+UIViewControllerTransitioningDelegate.swift

@@ -41,6 +41,7 @@ extension MotionTransition: UIViewControllerTransitioningDelegate {
     
     
     state = .notified
     state = .notified
     isPresenting = true
     isPresenting = true
+    isModalTransition = true
     fromViewController = fromViewController ?? presenting
     fromViewController = fromViewController ?? presenting
     toViewController = toViewController ?? presented
     toViewController = toViewController ?? presented
     
     
@@ -54,6 +55,7 @@ extension MotionTransition: UIViewControllerTransitioningDelegate {
     
     
     state = .notified
     state = .notified
     isPresenting = false
     isPresenting = false
+    isModalTransition = true
     fromViewController = fromViewController ?? dismissed
     fromViewController = fromViewController ?? dismissed
     return self
     return self
   }
   }

Sources/Transition/MotionTransition.swift

@@ -271,6 +271,13 @@ open class MotionTransition: NSObject {
   
   
   /// Whether or not we are presenting the destination view controller.
   /// Whether or not we are presenting the destination view controller.
   public internal(set) var isPresenting = true
   public internal(set) var isPresenting = true
+    
+  /**
+   A boolean indicating if the transition is of modal type.
+   True if `viewController.present(_:animated:completion:)` or
+   `viewController.dismiss(animated:completion:)` is called, false otherwise.
+   */
+  public internal(set) var isModalTransition = false
   
   
   /// A boolean indicating whether the transition interactive or not.
   /// A boolean indicating whether the transition interactive or not.
   public var isInteractive: Bool {
   public var isInteractive: Bool {
@@ -588,8 +595,10 @@ internal extension MotionTransition {
       return
       return
     }
     }
     
     
-    fvc.beginAppearanceTransition(false, animated: true)
-    tvc.beginAppearanceTransition(true, animated: true)
+    if !isModalTransition {
+        fvc.beginAppearanceTransition(false, animated: true)
+        tvc.beginAppearanceTransition(true, animated: true)
+    }
     
     
     processForMotionDelegate(viewController: fvc) { [weak self] in
     processForMotionDelegate(viewController: fvc) { [weak self] in
       guard let `self` = self else {
       guard let `self` = self else {
@@ -625,8 +634,10 @@ internal extension MotionTransition {
       return
       return
     }
     }
     
     
-    tvc.endAppearanceTransition()
-    fvc.endAppearanceTransition()
+    if !isModalTransition {
+        tvc.endAppearanceTransition()
+        fvc.endAppearanceTransition()
+    }
     
     
     processForMotionDelegate(viewController: fvc) { [weak self] in
     processForMotionDelegate(viewController: fvc) { [weak self] in
       guard let `self` = self else {
       guard let `self` = self else {