add new config resetBeforeIteration for Animated.loop (#20561)

Summary:
Fixes #20560
Pull Request resolved: https://github.com/facebook/react-native/pull/20561

Differential Revision: D14103437

Pulled By: cpojer

fbshipit-source-id: 35f2b3426304248e1a28f6a88812f07414618ea2

Libraries/Animated/src/AnimatedImplementation.js

@@ -417,11 +417,14 @@ const stagger = function(
   );
 };
 
-type LoopAnimationConfig = {iterations: number};
+type LoopAnimationConfig = {
+  iterations: number,
+  resetBeforeIteration?: boolean,
+};
 
 const loop = function(
   animation: CompositeAnimation,
-  {iterations = -1}: LoopAnimationConfig = {},
+  {iterations = -1, resetBeforeIteration = true}: LoopAnimationConfig = {},
 ): CompositeAnimation {
   let isFinished = false;
   let iterationsSoFar = 0;
@@ -436,7 +439,7 @@ const loop = function(
           callback && callback(result);
         } else {
           iterationsSoFar++;
-          animation.reset();
+          resetBeforeIteration && animation.reset();
           animation.start(restart);
         }
       };

Libraries/Animated/src/__tests__/Animated-test.js

@@ -445,6 +445,37 @@ describe('Animated tests', () => {
     });
   });
 
+  it('does not reset animation in a loop if resetBeforeIteration is false', () => {
+    const animation = {
+      start: jest.fn(),
+      reset: jest.fn(),
+      _isUsingNativeDriver: () => false,
+    };
+    const cb = jest.fn();
+
+    const loop = Animated.loop(animation, {resetBeforeIteration: false});
+
+    expect(animation.start).not.toBeCalled();
+
+    loop.start(cb);
+
+    expect(animation.start).toBeCalled();
+    expect(animation.reset).not.toBeCalled();
+    expect(cb).not.toBeCalled();
+
+    animation.start.mock.calls[0][0]({finished: true}); // End of loop 1
+    expect(animation.reset).not.toBeCalled();
+    expect(cb).not.toBeCalled();
+
+    animation.start.mock.calls[0][0]({finished: true}); // End of loop 2
+    expect(animation.reset).not.toBeCalled();
+    expect(cb).not.toBeCalled();
+
+    animation.start.mock.calls[0][0]({finished: true}); // End of loop 3
+    expect(animation.reset).not.toBeCalled();
+    expect(cb).not.toBeCalled();
+  });
+
   describe('Animated Parallel', () => {
     it('works with an empty parallel', () => {
       const cb = jest.fn();