Deduct offset from getValue result when detaching

Summary:
The NativeAnimated `getValue` API returns `value + offset`:
Android:
iOS: https://github.com/facebook/react-native/blob/main/Libraries/NativeAnimation/Nodes/RCTValueAnimatedNode.m#L44
Android: https://github.com/facebook/react-native/blob/main/ReactAndroid/src/main/java/com/facebook/react/animated/ValueAnimatedNode.java#L36

When we store the value after detaching the NativeAnimated node, it stores the result of the NativeAnimated `getValue` call to the `_value` property, so if we call `__getValue` at some later point on the `AnimatedValue`, we will count the offset twice.

This change deducts the offset value from the result returned from `getValue` when storing the latest value.

Changelog:
[General][Fixed] - AnimatedValue.__detach should store getValue result with offset deducted

Reviewed By: yungsters

Differential Revision: D32987003

fbshipit-source-id: 488d1fe512f886c7a9de1e5a4de8f19441ebd81e

Libraries/Animated/__tests__/AnimatedNative-test.js

@@ -134,6 +134,28 @@ describe('Native Animated', () => {
       expect(opacity.__getValue()).toBe(1);
     });
 
+    it('should deduct offset when saving value on unmount', () => {
+      NativeAnimatedModule.getValue = jest.fn((tag, saveCallback) => {
+        // Assume current raw value of value node is 0.5, the NativeAnimated
+        // getValue API returns the sum of raw value and offset, so return 1.
+        saveCallback(1);
+      });
+      const opacity = new Animated.Value(0);
+      opacity.setOffset(0.5);
+      opacity.__makeNative();
+
+      const root = TestRenderer.create(<Animated.View style={{opacity}} />);
+      const tag = opacity.__getNativeTag();
+
+      root.unmount();
+
+      expect(NativeAnimatedModule.getValue).toBeCalledWith(
+        tag,
+        expect.any(Function),
+      );
+      expect(opacity.__getValue()).toBe(1);
+    });
+
     it('should extract offset', () => {
       const opacity = new Animated.Value(0);
       opacity.__makeNative();

Libraries/Animated/nodes/AnimatedValue.js

@@ -100,7 +100,7 @@ class AnimatedValue extends AnimatedWithChildren {
   __detach() {
     if (this.__isNative) {
       NativeAnimatedAPI.getValue(this.__getNativeTag(), value => {
-        this._value = value;
+        this._value = value - this._offset;
       });
     }
     this.stopAnimation();