IOS-80: cache user defaults last known valus to avoid unarchiving it every time.

1Weather/Storage/UserDefaultsWrapper.swift

@@ -11,6 +11,10 @@ import Foundation
 struct UserDefaultsUnitValue<T> {
     var wrappedValue: T {
         get {
+            // TODO: If we will ever have 2 instances of UserDefaultsUnitValue with the same key at the same time, this cache will become desynchronized.
+            if let knownValue = lastKnownValue {
+                return knownValue
+            }
             guard
                 let data = UserDefaults.standard.data(forKey: key),
                 let decoded  = NSKeyedUnarchiver.unarchiveObject(with: data) as? T
@@ -21,7 +25,7 @@ struct UserDefaultsUnitValue<T> {
             return decoded
         }
         set {
-            
+            lastKnownValue = newValue
             let data = NSKeyedArchiver.archivedData(withRootObject: newValue)
             UserDefaults.standard.set(data, forKey: key)
             UserDefaults.standard.synchronize()
@@ -34,6 +38,7 @@ struct UserDefaultsUnitValue<T> {
     
     private let key: String
     private let defaultValue: T
+    private var lastKnownValue: T?
 
     init(wrappedValue defaultValue:T, key:String) {
         self.defaultValue = defaultValue