[Android] Natively loopable animations

Libraries/Animated/src/AnimatedImplementation.js

@@ -212,13 +212,15 @@ type TimingAnimationConfig =  AnimationConfig & {
   easing?: (value: number) => number,
   duration?: number,
   delay?: number,
+  shouldLoop?: bool,
 };
 
 type TimingAnimationConfigSingle = AnimationConfig & {
   toValue: number | AnimatedValue,
   easing?: (value: number) => number,
   duration?: number,
   delay?: number,
+  shouldLoop?: bool,
 };
 
 let _easeInOut;
@@ -236,6 +238,7 @@ class TimingAnimation extends Animation {
   _toValue: any;
   _duration: number;
   _delay: number;
+  _shouldLoop: bool;
   _easing: (value: number) => number;
   _onUpdate: (value: number) => void;
   _animationFrame: any;
@@ -250,6 +253,7 @@ class TimingAnimation extends Animation {
     this._easing = config.easing !== undefined ? config.easing : easeInOut();
     this._duration = config.duration !== undefined ? config.duration : 500;
     this._delay = config.delay !== undefined ? config.delay : 0;
+    this._shouldLoop = config.shouldLoop !== undefined ? config.shouldLoop : false;
     this.__isInteraction = config.isInteraction !== undefined ? config.isInteraction : true;
     this._useNativeDriver = config.useNativeDriver !== undefined ? config.useNativeDriver : false;
   }
@@ -265,7 +269,8 @@ class TimingAnimation extends Animation {
       type: 'frames',
       frames,
       toValue: this._toValue,
-      delay: this._delay
+      delay: this._delay,
+      shouldLoop: this._shouldLoop
     };
   }
 

ReactAndroid/src/main/java/com/facebook/react/animated/FrameBasedAnimationDriver.java

@@ -26,6 +26,7 @@ class FrameBasedAnimationDriver extends AnimationDriver {
   private final double[] mFrames;
   private final double mToValue;
   private double mFromValue;
+  private boolean mShouldLoop;
 
   FrameBasedAnimationDriver(ReadableMap config) {
     ReadableArray frames = config.getArray("frames");
@@ -35,6 +36,7 @@ class FrameBasedAnimationDriver extends AnimationDriver {
       mFrames[i] = frames.getDouble(i);
     }
     mToValue = config.getDouble("toValue");
+    mShouldLoop = config.getBoolean("shouldLoop");
   }
 
   @Override
@@ -53,9 +55,12 @@ public void runAnimationStep(long frameTimeNanos) {
     }
     double nextValue;
     if (frameIndex >= mFrames.length - 1) {
-      // animation has completed, no more frames left
-      mHasFinished = true;
       nextValue = mToValue;
+      if (mShouldLoop) { // looping animation, return to start
+        mStartFrameTimeNanos = frameTimeNanos;
+      } else { // animation has completed, no more frames left
+        mHasFinished = true;
+      }
     } else {
       nextValue = mFromValue + mFrames[frameIndex] * (mToValue - mFromValue);
     }