Provide the model value in the core animation channel.

Summary:
Prior to this diff we were piggybacking off the next channel in order to set the desired model value on the layer. After this change, core animation animations will no longer send any values down the next channel. Instead, the desired model value is sent along with the core animation .add event.

This change keeps the next channel focused on providing real-time values, e.g. from POP or a gesture recognizer.

Reviewers: O2 Material Motion, #material_motion, O4 Material Apple platform reviewers, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2448

src/MotionObservable.swift

@@ -24,7 +24,7 @@ public typealias StateChannel = (MotionState) -> Void
 /** A Core Animation channel event. */
 public enum CoreAnimationChannelEvent {
   /** The provided animation is expected to be added to a layer. */
-  case add(CAPropertyAnimation, String, initialVelocity: Any?)
+  case add(CAPropertyAnimation, String, modelValue: Any, initialVelocity: Any?)
 
   /** Any animation with the given key is expected to be removed from a layer. */
   case remove(String)

src/operators/foundation/_map.swift

@@ -26,12 +26,13 @@ extension ExtendableMotionObservable {
     }, coreAnimation: { event, coreAnimation in
       let transformedInitialVelocity: Any?
       switch event {
-      case .add(let animation, let key, let initialVelocity):
+      case .add(let animation, let key, let modelValue, let initialVelocity):
         if let initialVelocity = initialVelocity {
           transformedInitialVelocity = transform(initialVelocity as! T)
         } else {
           transformedInitialVelocity = nil
         }
+        let transformedModelValue = transform(modelValue as! T)
 
         let copy = animation.copy() as! CAPropertyAnimation
         switch copy {
@@ -45,11 +46,15 @@ extension ExtendableMotionObservable {
           if let byValue = basicAnimation.byValue {
             basicAnimation.byValue = transform(byValue as! T)
           }
-          coreAnimation(.add(basicAnimation, key, initialVelocity: transformedInitialVelocity))
+          coreAnimation(.add(basicAnimation, key,
+                             modelValue: transformedModelValue,
+                             initialVelocity: transformedInitialVelocity))
 
         case let keyframeAnimation as CAKeyframeAnimation:
           keyframeAnimation.values = keyframeAnimation.values?.map { transform($0 as! T) }
-          coreAnimation(.add(keyframeAnimation, key, initialVelocity: transformedInitialVelocity))
+          coreAnimation(.add(keyframeAnimation, key,
+                             modelValue: transformedModelValue,
+                             initialVelocity: transformedInitialVelocity))
 
         default:
           assertionFailure("Unsupported animation type: \(type(of: animation))")

src/properties/CALayer+Properties.swift

@@ -53,12 +53,13 @@ public class CALayerReactivePropertyBuilder {
     var lastAnimationKey: String?
     return ReactiveProperty(read: read, write: write, coreAnimation: { event in
       switch event {
-      case .add(let animation, let key, let initialVelocity):
+      case .add(let animation, let key, let modelValue, let initialVelocity):
         if let initialVelocity = initialVelocity {
           applyInitialVelocity(initialVelocity, to: animation)
         }
 
         animation.keyPath = keyPath
+        layer.setValue(modelValue, forKeyPath: keyPath)
         layer.add(animation, forKey: key)
 
       case .remove(let key):

src/sources/coreAnimationSpringSource.swift

@@ -48,11 +48,12 @@ public func coreAnimationSpringSource<T where T: Subtractable, T: Zeroable>(_ sp
       CATransaction.setCompletionBlock {
         observer.state(.atRest)
       }
-      observer.next($0)
 
       let key = NSUUID().uuidString
       animationKeys.append(key)
-      observer.coreAnimation(.add(animation, key, initialVelocity: spring.initialVelocity.read()))
+      observer.coreAnimation(.add(animation, key,
+                                  modelValue: $0,
+                                  initialVelocity: spring.initialVelocity.read()))
 
       CATransaction.commit()
     }

src/sources/coreAnimationTweenSource.swift

@@ -45,9 +45,10 @@ public func coreAnimationTweenSource<T>(_ tween: Tween<T>) -> MotionObservable<T
       observer.state(.atRest)
     }
 
-    observer.next(values.last!)
     let key = NSUUID().uuidString
-    observer.coreAnimation(.add(animation, key, initialVelocity: nil))
+    observer.coreAnimation(.add(animation, key,
+                                modelValue: values.last!,
+                                initialVelocity: nil))
 
     CATransaction.commit()