Fix memory leak in ReactiveProperty.

Summary: ReactiveProperty must hold on to its stream but its stream shouldn't hold a reference to the property.

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

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Subscribers: markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D2703

examples/ModalDialogExample.swift

@@ -130,7 +130,7 @@ class ModalDialogTransitionDirector: SelfDismissingTransitionDirector {
     runtime.add(spring, to: reactiveForeLayer.positionY)
 
     if mainThreadReactive {
-      let rotation = reactiveForeLayer.positionY.stream
+      let rotation = reactiveForeLayer.positionY
         .mapRange(rangeStart: spring.backwardDestination,
                   rangeEnd: spring.forwardDestination,
                   destinationStart: CGFloat(M_PI / 8),

examples/SwipeExample.swift

@@ -64,7 +64,7 @@ class TossableStackedCard: ViewInteraction {
           ])
     runtime.add(destinationStream, to: destination)
 
-    let gestureEnabledStream = tossDirection.stream.rewrite([
+    let gestureEnabledStream = tossDirection.rewrite([
       .none: true,
       .left: false,
       .right: false

src/ReactiveProperty.swift

@@ -51,21 +51,6 @@ public final class ReactiveProperty<T> {
     }
   }
 
-  public lazy var stream: MotionObservable<T> = {
-    let stream = MotionObservable<T> { observer in
-      self.observers.append(observer)
-
-      observer.next(self.value)
-
-      return {
-        if let index = self.observers.index(where: { $0 === observer }) {
-          self.observers.remove(at: index)
-        }
-      }
-    }
-    return stream
-  }()
-
   /** Initializes a new instance with the given initial value and write function. */
   public init(initialValue: T, write: @escaping ScopedWrite<T>) {
     self._value = initialValue
@@ -108,7 +93,7 @@ public final class ReactiveProperty<T> {
   private var state = MotionState.atRest
   private var coreAnimationEvent: CoreAnimationChannelEvent?
 
-  private var observers: [MotionObserver<T>] = []
+  fileprivate var observers: [MotionObserver<T>] = []
 }
 
 public func == <T: Equatable> (left: ReactiveProperty<T>, right: T) -> Bool {
@@ -117,7 +102,17 @@ public func == <T: Equatable> (left: ReactiveProperty<T>, right: T) -> Bool {
 
 extension ReactiveProperty: MotionObservableConvertible {
   public func asStream() -> MotionObservable<T> {
-    return stream
+    return MotionObservable<T> { observer in
+      self.observers.append(observer)
+
+      observer.next(self.value)
+
+      return {
+        if let index = self.observers.index(where: { $0 === observer }) {
+          self.observers.remove(at: index)
+        }
+      }
+    }
   }
 }
 

src/interactions/Tossable.swift

@@ -28,7 +28,7 @@ public class Destination: MotionObservableConvertible {
   public let property: ReactiveProperty<CGPoint>
 
   public func asStream() -> MotionObservable<CGPoint> {
-    return property.stream
+    return property.asStream()
   }
 }
 

src/interactions/TransitionSpring.swift

@@ -41,8 +41,8 @@ public class TransitionSpring<T: Zeroable>: Spring<T>, TransitionInteraction {
     self.forwardDestination = forwardDestination
     self._initialValue = direction == .forward ? backwardDestination : forwardDestination
 
-    self.toggledDestination = direction.stream.destinations(back: backwardDestination,
-                                                            fore: forwardDestination)
+    self.toggledDestination = direction.destinations(back: backwardDestination,
+                                                     fore: forwardDestination)
     super.init(threshold: threshold, system: system)
   }
 

tests/unit/properties/PropertyObservation.swift

@@ -24,7 +24,7 @@ class PropertyObservation: XCTestCase {
     let property = createProperty(withInitialValue: point)
 
     var observedValue: CGPoint? = nil
-    let _ = property.stream.subscribe(next: { value in
+    let _ = property.asStream().subscribe(next: { value in
       observedValue = value
     }, coreAnimation: { _ in })
     XCTAssertEqual(observedValue!, point)