Fix Animate.sequence restart issues (facebook#44031)

Summary:
Currently, if the `Animated.sequence` animation is finished, and then the `start()` method is called without a `reset()` method being invoked beforehand, the failure happens: ```undefined is not an object (evaluating 'animations[current].start')```.
Use cases:
- sequence animation started, finished, and then started again
- sequence animation is used in `Animation.loop` with `resetBeforeIteration` set to `false`, which essentially does the same as the previous case

Related issues:
- facebook#43120
- facebook#37611

## Changelog:

[General] [Fixed] - Fix sequence restart failure

Pull Request resolved: facebook#44031

Test Plan: Test cases are included: 1 for regression and 2 for mentioned use cases in the summary

Reviewed By: cipolleschi

Differential Revision: D56015346

Pulled By: dmytrorykun

fbshipit-source-id: 8b0f46c8a33397fece807634463ce630c89d28af

packages/react-native/Libraries/Animated/AnimatedImplementation.js

@@ -315,6 +315,8 @@ const sequence = function (
         current++;
 
         if (current === animations.length) {
+          // if the start is called, even without a reset, it should start from the beginning
+          current = 0;
           callback && callback(result);
           return;
         }

packages/react-native/Libraries/Animated/__tests__/Animated-test.js

@@ -264,6 +264,50 @@ describe('Animated tests', () => {
 
       expect(cb).toBeCalledWith({finished: false});
     });
+
+    it('supports restarting sequence after it was stopped during execution', () => {
+      const anim1 = {start: jest.fn(), stop: jest.fn()};
+      const anim2 = {start: jest.fn(), stop: jest.fn()};
+      const cb = jest.fn();
+
+      const seq = Animated.sequence([anim1, anim2]);
+
+      seq.start(cb);
+
+      anim1.start.mock.calls[0][0]({finished: true});
+      seq.stop();
+
+      // anim1 should be finished so anim2 should also start
+      expect(anim1.start).toHaveBeenCalledTimes(1);
+      expect(anim2.start).toHaveBeenCalledTimes(1);
+
+      seq.start(cb);
+
+      // after restart the sequence should resume from the anim2
+      expect(anim1.start).toHaveBeenCalledTimes(1);
+      expect(anim2.start).toHaveBeenCalledTimes(2);
+    });
+
+    it('supports restarting sequence after it was finished without a reset', () => {
+      const anim1 = {start: jest.fn(), stop: jest.fn()};
+      const anim2 = {start: jest.fn(), stop: jest.fn()};
+      const cb = jest.fn();
+
+      const seq = Animated.sequence([anim1, anim2]);
+
+      seq.start(cb);
+      anim1.start.mock.calls[0][0]({finished: true});
+      anim2.start.mock.calls[0][0]({finished: true});
+
+      // sequence should be finished
+      expect(cb).toBeCalledWith({finished: true});
+
+      seq.start(cb);
+
+      // sequence should successfully restart from the anim1
+      expect(anim1.start).toHaveBeenCalledTimes(2);
+      expect(anim2.start).toHaveBeenCalledTimes(1);
+    });
   });
 
   describe('Animated Loop', () => {
@@ -506,6 +550,28 @@ describe('Animated tests', () => {
     expect(cb).not.toBeCalled();
   });
 
+  it('restarts sequence normally in a loop if resetBeforeIteration is false', () => {
+    const anim1 = {start: jest.fn(), stop: jest.fn()};
+    const anim2 = {start: jest.fn(), stop: jest.fn()};
+    const seq = Animated.sequence([anim1, anim2]);
+
+    const loop = Animated.loop(seq, {resetBeforeIteration: false});
+
+    loop.start();
+
+    expect(anim1.start).toHaveBeenCalledTimes(1);
+
+    anim1.start.mock.calls[0][0]({finished: true});
+
+    expect(anim2.start).toHaveBeenCalledTimes(1);
+
+    anim2.start.mock.calls[0][0]({finished: true});
+
+    // after anim2 is finished, the sequence is finished,
+    // hence the loop iteration is finished, so the next iteration starts
+    expect(anim1.start).toHaveBeenCalledTimes(2);
+  });
+
   describe('Animated Parallel', () => {
     it('works with an empty parallel', () => {
       const cb = jest.fn();