Animated: Decouple __attach from new AnimatedProps

Summary:
Decouples `__attach` from the constructor in `AnimatedProps`.

This change will enable the instantiation of `AnimatedProps` (and subsequent invocation of `__getValue()`) without having to trigger side effects until after mount or update. This is important in order for `Animated` to ever become safe for Concurrent Mode.

Changelog:
[Internal]

Reviewed By: lunaleaps

Differential Revision: D28271628

fbshipit-source-id: 6ccfed6de79382cecdfa6939c7dad3134e1ecaaa

Libraries/Animated/__tests__/Animated-test.js

@@ -57,8 +57,6 @@ describe('Animated tests', () => {
         callback,
       );
 
-      expect(anim.__getChildren().length).toBe(3);
-
       expect(node.__getValue()).toEqual({
         style: {
           backgroundColor: 'red',
@@ -71,6 +69,12 @@ describe('Animated tests', () => {
         },
       });
 
+      expect(anim.__getChildren().length).toBe(0);
+
+      node.__attach();
+
+      expect(anim.__getChildren().length).toBe(3);
+
       anim.setValue(0.5);
 
       expect(callback).toBeCalled();
@@ -811,6 +815,10 @@ describe('Animated tests', () => {
         },
       });
 
+      node.__attach();
+
+      expect(callback.mock.calls.length).toBe(0);
+
       vec.setValue({x: 42, y: 1492});
 
       expect(callback.mock.calls.length).toBe(2); // once each for x, y
@@ -904,6 +912,7 @@ describe('Animated tests', () => {
         },
         callback,
       );
+      view.__attach();
       const listener = jest.fn();
       const id = value4.addListener(listener);
       value3.setValue(137);

Libraries/Animated/createAnimatedComponent.js

@@ -182,6 +182,7 @@ function createAnimatedComponent<Props: {+[string]: mixed, ...}, Instance>(
         nextProps,
         this._animatedPropsCallback,
       );
+      this._propsAnimated.__attach();
 
       // When you call detach, it removes the element from the parent list
       // of children. If it goes to 0, then the parent also detaches itself

Libraries/Animated/nodes/AnimatedProps.js

@@ -33,7 +33,6 @@ class AnimatedProps extends AnimatedNode {
     }
     this._props = props;
     this._callback = callback;
-    this.__attach();
   }
 
   __getValue(): Object {