Animated: Change onEnd to Private Property

Summary:
Currently, the `Animation` class (from Animated) defines an unofficially protected `__onEnd` property that subclasses are expected to populate. However, subclasses are expected to *not* invoke `__onEnd`, but instead call `__debouncedOnEnd`.

In order to make this code less fragile, this commit refactors the `Animation` class and its subclasses so that `onEnd` is stored in a private property and initialized by the `start` method in the `Animation` parent class. Now, the only way to invoke the `onEnd` callback is by calling `__debouncedOnEnd`.

The subclasses have been adjusted to call the `start` superclass method to initialize this.

Changelog:
[General][Changed] - The `Animation` superclass no longer exposes `__onEnd` as a property. Subclasses must instead invoke `super.start(…)` in their `start()` implementation.

Differential Revision: D63572063

packages/react-native/Libraries/Animated/animations/Animation.js

@@ -36,9 +36,10 @@ let startNativeAnimationNextId = 1;
 // Once an animation has been stopped or finished its course, it will
 // not be reused.
 export default class Animation {
+  #onEnd: ?EndCallback;
+
   __active: boolean;
   __isInteraction: boolean;
-  __onEnd: ?EndCallback;
   __iterations: number;
   __isLooping: ?boolean;
 
@@ -50,7 +51,9 @@ export default class Animation {
     onEnd: ?EndCallback,
     previousAnimation: ?Animation,
     animatedValue: AnimatedValue,
-  ): void {}
+  ): void {
+    this.#onEnd = onEnd;
+  }
 
   stop(): void {
     if (this._nativeId) {
@@ -66,8 +69,8 @@ export default class Animation {
 
   // Helper function for subclasses to make sure onEnd is only called once.
   __debouncedOnEnd(result: EndResult): void {
-    const onEnd = this.__onEnd;
-    this.__onEnd = null;
+    const onEnd = this.#onEnd;
+    this.#onEnd = null;
     onEnd && onEnd(result);
   }
 

packages/react-native/Libraries/Animated/animations/DecayAnimation.js

@@ -79,11 +79,12 @@ export default class DecayAnimation extends Animation {
     previousAnimation: ?Animation,
     animatedValue: AnimatedValue,
   ): void {
+    super.start(fromValue, onUpdate, onEnd, previousAnimation, animatedValue);
+
     this.__active = true;
     this._lastValue = fromValue;
     this._fromValue = fromValue;
     this._onUpdate = onUpdate;
-    this.__onEnd = onEnd;
     this._startTime = Date.now();
 
     if (!this._useNativeDriver && animatedValue.__isNative === true) {

packages/react-native/Libraries/Animated/animations/SpringAnimation.js

@@ -202,12 +202,13 @@ export default class SpringAnimation extends Animation {
     previousAnimation: ?Animation,
     animatedValue: AnimatedValue,
   ): void {
+    super.start(fromValue, onUpdate, onEnd, previousAnimation, animatedValue);
+
     this.__active = true;
     this._startPosition = fromValue;
     this._lastPosition = this._startPosition;
 
     this._onUpdate = onUpdate;
-    this.__onEnd = onEnd;
     this._lastTime = Date.now();
     this._frameTime = 0.0;
 

packages/react-native/Libraries/Animated/animations/TimingAnimation.js

@@ -107,10 +107,11 @@ export default class TimingAnimation extends Animation {
     previousAnimation: ?Animation,
     animatedValue: AnimatedValue,
   ): void {
+    super.start(fromValue, onUpdate, onEnd, previousAnimation, animatedValue);
+
     this.__active = true;
     this._fromValue = fromValue;
     this._onUpdate = onUpdate;
-    this.__onEnd = onEnd;
 
     const start = () => {
       if (!this._useNativeDriver && animatedValue.__isNative === true) {

packages/react-native/Libraries/__tests__/__snapshots__/public-api-test.js.snap

@@ -427,7 +427,6 @@ export type AnimationConfig = {
 declare export default class Animation {
   __active: boolean;
   __isInteraction: boolean;
-  __onEnd: ?EndCallback;
   __iterations: number;
   __isLooping: ?boolean;
   _nativeId: number;