Introduce MotionObservableConvertible.

Summary:
This type makes it possible to create meta-types that have streams. By extending MotionObservableConvertible with operators, we can use operators on meta-types. We can also accept meta-types anywhere that we'd normally accept a MotionObservable.

This change sets the foundation for dramatically simplifying the overall usage of streams. Some examples of possible changes:

```
// Before
runtime.write(someProperty.stream, to: someProperty)
// After
runtime.write(someProperty, to: someProperty)

// Before
someProperty.stream.x()
// After
someProperty.x()
```

Depends on D2562.
Depends on D2561.

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

Reviewed By: chuga, O4 Material Apple platform reviewers

Subscribers: appsforartists, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2563

src/MotionObservable.swift

@@ -38,7 +38,7 @@ public typealias CoreAnimationChannel = (CoreAnimationChannelEvent) -> Void
 
  Throughout this documentation we will treat the words "observable" and "stream" as synonyms.
  */
-public final class MotionObservable<T>: IndefiniteObservable<MotionObserver<T>>, ExtendableMotionObservable {
+public final class MotionObservable<T>: IndefiniteObservable<MotionObserver<T>> {
   /** Sugar for subscribing a MotionObserver. */
   public func subscribe(next: @escaping NextChannel<T>,
                         state: @escaping StateChannel,
@@ -89,21 +89,16 @@ public final class MotionObserver<T>: Observer {
   public let coreAnimation: CoreAnimationChannel
 }
 
-/**
- This type is used for extending MotionObservable using generics.
-
- This is required to be able to do extensions where T == some value, such as CGPoint. See
- https://twitter.com/dgregor79/status/646167048645554176 for discussion of what appears to be a
- bug in swift.
- */
-public protocol ExtendableMotionObservable {
+/** A MotionObservableConvertible has a canonical MotionObservable that it can return. */
+public protocol MotionObservableConvertible {
   associatedtype T
 
-  /**
-   We define this only so that T can be inferred by the compiler so that we don't have to
-   introduce a new generic type such as Value in the associatedtype here.
-   */
-  func subscribe(next: @escaping NextChannel<T>,
-                 state: @escaping StateChannel,
-                 coreAnimation: @escaping CoreAnimationChannel) -> Subscription
+  /** Returns the canonical MotionObservable for this object. */
+  func asStream() -> MotionObservable<T>
+}
+
+extension MotionObservable: MotionObservableConvertible {
+  public func asStream() -> MotionObservable<T> {
+    return self
+  }
 }

src/MotionRuntime.swift

@@ -42,9 +42,9 @@ public class MotionRuntime {
   }
 
   /** Subscribes to the stream, writes its output to the given property, and observes its state. */
-  public func write<O: ExtendableMotionObservable, T>(_ stream: O, to property: ReactiveProperty<T>) where O.T == T {
+  public func write<O: MotionObservableConvertible, T>(_ stream: O, to property: ReactiveProperty<T>) where O.T == T {
     let token = NSUUID().uuidString
-    subscriptions.append(stream.subscribe(next: property.setValue, state: { [weak self] state in
+    subscriptions.append(stream.asStream().subscribe(next: property.setValue, state: { [weak self] state in
       property.state(state)
 
       guard let strongSelf = self else { return }

src/operators/CGPoint.swift

@@ -14,11 +14,11 @@
  limitations under the License.
  */
 
-extension ExtendableMotionObservable where T == CGPoint {
+extension MotionObservableConvertible where T == CGPoint {
 
   /** Extract the x value from a CGPoint. */
-  public func x() -> MotionObservable<CGFloat> { return _map { $0.x } }
+  public func x() -> MotionObservable<CGFloat> { return asStream()._map { $0.x } }
 
   /** Extract the y value from a CGPoint. */
-  public func y() -> MotionObservable<CGFloat> { return _map { $0.y } }
+  public func y() -> MotionObservable<CGFloat> { return asStream()._map { $0.y } }
 }

src/operators/arithmetic.swift

@@ -16,26 +16,26 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T == CGFloat {
+extension MotionObservableConvertible where T == CGFloat {
 
   /** Emits the incoming value + amount. */
-  public func offset(by amount: CGFloat) -> MotionObservable<CGFloat> { return _map { $0 + amount } }
+  public func offset(by amount: CGFloat) -> MotionObservable<CGFloat> { return asStream()._map { $0 + amount } }
 
   /** Emits the incoming value * amount. */
-  public func scaled(by amount: CGFloat) -> MotionObservable<CGFloat> { return _map { $0 * amount } }
+  public func scaled(by amount: CGFloat) -> MotionObservable<CGFloat> { return asStream()._map { $0 * amount } }
 
   /** Emits the incoming value / amount. */
-  public func normalized(by amount: CGFloat) -> MotionObservable<CGFloat> { return _map { $0 / amount } }
+  public func normalized(by amount: CGFloat) -> MotionObservable<CGFloat> { return asStream()._map { $0 / amount } }
 
   /** Subtract the incoming value from the provided value. */
-  public func subtracted(from value: CGFloat) -> MotionObservable<CGFloat> { return _map { value - $0 } }
+  public func subtracted(from value: CGFloat) -> MotionObservable<CGFloat> { return asStream()._map { value - $0 } }
 }
 
-extension ExtendableMotionObservable where T == CGPoint {
+extension MotionObservableConvertible where T == CGPoint {
 
   /** Emits the incoming value / amount. */
   public func normalized(by amount: CGSize) -> MotionObservable<CGPoint> {
-    return _map {
+    return asStream()._map {
       return CGPoint(x: $0.x / amount.width,
                      y: $0.y / amount.height)
     }

src/operators/debugging.swift

@@ -16,11 +16,11 @@
 
 import Foundation
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /** Writes any incoming value to the console and then passes the value on. */
   public func log(_ context: String? = nil) -> MotionObservable<T> {
-    return _nextOperator({ value, next in
+    return asStream()._nextOperator({ value, next in
       if let context = context {
         print(context, value)
       } else {

src/operators/distance.swift

@@ -16,21 +16,21 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T == CGFloat {
+extension MotionObservableConvertible where T == CGFloat {
 
   /** Emits the distance between the incoming value and the location. */
   public func distance(from location: CGFloat) -> MotionObservable<CGFloat> {
-    return _map {
+    return asStream()._map {
       fabs($0 - location)
     }
   }
 }
 
-extension ExtendableMotionObservable where T == CGPoint {
+extension MotionObservableConvertible where T == CGPoint {
 
   /** Emits the distance between the incoming value and the location. */
   public func distance(from location: CGPoint) -> MotionObservable<CGFloat> {
-    return _map {
+    return asStream()._map {
       let xDelta = $0.x - location.x
       let yDelta = $0.y - location.y
       return sqrt(xDelta * xDelta + yDelta * yDelta)

src/operators/foundation/_filter.swift

@@ -16,11 +16,11 @@
 
 import Foundation
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /** Only emit those items from an Observable that pass a test. */
   public func _filter(_ predicate: @escaping (T) -> Bool) -> MotionObservable<T> {
-    return _nextOperator { value, next in
+    return asStream()._nextOperator { value, next in
       if predicate(value) {
         next(value)
       }

src/operators/foundation/_map.swift

@@ -16,11 +16,11 @@
 
 import Foundation
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /** Transform the items emitted by an Observable by applying a function to each item. */
   func _map<U>(_ transform: @escaping (T) -> U) -> MotionObservable<U> {
-    return _nextOperator({ value, next in
+    return asStream()._nextOperator({ value, next in
       next(transform(value))
 
     }, coreAnimation: { event, coreAnimation in

src/operators/foundation/_nextOperator.swift

@@ -16,7 +16,7 @@
 
 import Foundation
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /**
    A light-weight operator builder.
@@ -26,7 +26,7 @@ extension ExtendableMotionObservable {
    */
   func _nextOperator<U>(_ operation: @escaping (T, (U) -> Void) -> Void) -> MotionObservable<U> {
     return MotionObservable<U> { observer in
-      return self.subscribe(next: {
+      return self.asStream().subscribe(next: {
         return operation($0, observer.next)
       }, state: observer.state, coreAnimation: { _ in
         assertionFailure("Core animation is not supported by this operator.")
@@ -43,7 +43,7 @@ extension ExtendableMotionObservable {
    */
   func _nextOperator<U>(_ operation: @escaping (T, (U) -> Void) -> Void, coreAnimation: @escaping (CoreAnimationChannelEvent, CoreAnimationChannel) -> Void) -> MotionObservable<U> {
     return MotionObservable<U> { observer in
-      return self.subscribe(next: {
+      return self.asStream().subscribe(next: {
         return operation($0, observer.next)
       }, state: observer.state, coreAnimation: {
         return coreAnimation($0, observer.coreAnimation)

src/operators/gestures/centroid.swift

@@ -16,11 +16,11 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIGestureRecognizer {
+extension MotionObservableConvertible where T: UIGestureRecognizer {
 
   /** Extract centroid from the incoming gesture recognizer. */
   public func centroid(in view: UIView) -> MotionObservable<CGPoint> {
-    return _map { value in
+    return asStream()._map { value in
       value.location(in: view)
     }
   }

src/operators/gestures/onRecognitionState.swift

@@ -16,18 +16,18 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIGestureRecognizer {
+extension MotionObservableConvertible where T: UIGestureRecognizer {
 
   /** Only forwards the gesture recognizer if its state matches the provided value. */
   public func onRecognitionState(_ state: UIGestureRecognizerState) -> MotionObservable<T> {
-    return _filter { value in
+    return asStream()._filter { value in
       return value.state == state
     }
   }
 
   /** Only forwards the gesture recognizer if its state matches any of the provided values. */
   public func onRecognitionStates(_ states: [UIGestureRecognizerState]) -> MotionObservable<T> {
-    return _filter { value in
+    return asStream()._filter { value in
       return states.contains(value.state)
     }
   }

src/operators/gestures/rotated.swift

@@ -16,17 +16,18 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIRotationGestureRecognizer {
+extension MotionObservableConvertible where T: UIRotationGestureRecognizer {
 
   /**
    Adds the current translation to the initial position and emits the result while the gesture
    recognizer is active.
    */
-  func rotated(from initialRotation: MotionObservable<CGFloat>) -> MotionObservable<CGFloat> {
+  func rotated<O: MotionObservableConvertible>(from initialRotation: O) -> MotionObservable<CGFloat> where O.T == CGFloat {
+    let initialRotationStream = initialRotation.asStream()
     var cachedInitialRotation: CGFloat?
-    return _nextOperator { value, next in
+    return asStream()._nextOperator { value, next in
       if value.state == .began || (value.state == .changed && cachedInitialRotation == nil)  {
-        cachedInitialRotation = initialRotation.read()
+        cachedInitialRotation = initialRotationStream.read()
       } else if value.state != .began && value.state != .changed {
         cachedInitialRotation = nil
       }

src/operators/gestures/scaled.swift

@@ -16,21 +16,23 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIPinchGestureRecognizer {
+extension MotionObservableConvertible where T: UIPinchGestureRecognizer {
 
   /**
    Multiplies the current scale by the initial scale and emits the result while the gesture
    recognizer is active.
    */
-  func scaled(from initialScale: MotionObservable<CGFloat>) -> MotionObservable<CGFloat> {
-    var cachedInitialScale: CGFloat?
-    return _nextOperator { value, next in
-      if value.state == .began || (value.state == .changed && cachedInitialScale == nil)  {
-        cachedInitialScale = initialScale.read()
+  func scaled<O: MotionObservableConvertible>(from initialScale: O) -> MotionObservable<CGFloat> where O.T == CGFloat {
+    let initialScaleStream = initialScale.asStream()
+    var initialScale: CGFloat?
+    return asStream()._nextOperator { value, next in
+      if value.state == .began || (value.state == .changed && initialScale == nil)  {
+        initialScale = initialScaleStream.read()
+
       } else if value.state != .began && value.state != .changed {
-        cachedInitialScale = nil
+        initialScale = nil
       }
-      if let cachedInitialScale = cachedInitialScale {
+      if let cachedInitialScale = initialScale {
         let scale = value.scale
         next(cachedInitialScale * scale)
       }

src/operators/gestures/translated.swift

@@ -16,17 +16,19 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIPanGestureRecognizer {
+extension MotionObservableConvertible where T: UIPanGestureRecognizer {
 
   /**
    Adds the current translation to the initial position and emits the result while the gesture
    recognizer is active.
    */
-  func translated(from initialPosition: MotionObservable<CGPoint>, in view: UIView) -> MotionObservable<CGPoint> {
+  func translated<O: MotionObservableConvertible>(from initialPosition: O, in view: UIView) -> MotionObservable<CGPoint> where O.T == CGPoint {
+    let initialPositionStream = initialPosition.asStream()
     var cachedInitialPosition: CGPoint?
-    return _nextOperator { value, next in
+    return asStream()._nextOperator { value, next in
       if value.state == .began || (value.state == .changed && cachedInitialPosition == nil)  {
-        cachedInitialPosition = initialPosition.read()
+        cachedInitialPosition = initialPositionStream.read()
+
       } else if value.state != .began && value.state != .changed {
         cachedInitialPosition = nil
       }

src/operators/gestures/velocity.swift

@@ -16,28 +16,28 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: UIPanGestureRecognizer {
+extension MotionObservableConvertible where T: UIPanGestureRecognizer {
 
   /** Extract translational velocity from the incoming pan gesture recognizer. */
   public func velocity(in view: UIView) -> MotionObservable<CGPoint> {
-    return _map { value in
+    return asStream()._map { value in
       value.velocity(in: view)
     }
   }
 }
 
-extension ExtendableMotionObservable where T: UIRotationGestureRecognizer {
+extension MotionObservableConvertible where T: UIRotationGestureRecognizer {
 
   /** Extract rotational velocity from the incoming rotation gesture recognizer. */
   public func velocity() -> MotionObservable<CGFloat> {
-    return _map { value in value.velocity }
+    return asStream()._map { value in value.velocity }
   }
 }
 
-extension ExtendableMotionObservable where T: UIPinchGestureRecognizer {
+extension MotionObservableConvertible where T: UIPinchGestureRecognizer {
 
   /** Extract scale velocity from the incoming pinch gesture recognizer. */
   public func velocity() -> MotionObservable<CGFloat> {
-    return _map { value in value.velocity }
+    return asStream()._map { value in value.velocity }
   }
 }

src/operators/mapRange.swift

@@ -16,7 +16,7 @@
 
 import Foundation
 
-extension ExtendableMotionObservable where T: Subtractable, T: Lerpable {
+extension MotionObservableConvertible where T: Subtractable, T: Lerpable {
 
   /** Linearly interpolate the incoming value along the given range to the destination range. */
   public func mapRange<U>(
@@ -25,7 +25,7 @@ extension ExtendableMotionObservable where T: Subtractable, T: Lerpable {
     destinationStart: U,
     destinationEnd: U) -> MotionObservable<U>
   where U: Lerpable, U: Subtractable, U: Addable {
-    return _map {
+    return asStream()._map {
       let position = $0 - rangeStart
 
       let vector = rangeEnd - rangeStart

src/operators/mapTo.swift

@@ -16,10 +16,10 @@
 
 import Foundation
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /** Emit a constant value each time this operator receives a value. */
   public func mapTo<U>(_ value: U) -> MotionObservable<U> {
-    return _map { _ in value }
+    return asStream()._map { _ in value }
   }
 }

src/operators/multicast.swift

@@ -16,7 +16,7 @@
 
 import IndefiniteObservable
 
-extension ExtendableMotionObservable {
+extension MotionObservableConvertible {
 
   /**
    Turns a stream into a multicast stream.
@@ -33,7 +33,7 @@ extension ExtendableMotionObservable {
     var lastCoreAnimationEvent: CoreAnimationChannelEvent?
 
     let subscribe = {
-      subscription = self.subscribe(next: { value in
+      subscription = self.asStream().subscribe(next: { value in
         lastValue = value
         for observer in observers {
           observer.next(value)