Summary:
Have ScrollView use forwardRef so that the host component methods like `measure` and `measureLayout` are available without having to call `getNativeScrollRef`. Instead, you can use `<ScrollView ref={myRef} />` and directly call all methods of ScrollView and host components on `myRef`.
Previous usage:
```
const myRef = React.createRef<React.ElementRef<typeof ScrollView>>();
<ScrollView ref={myRef} />
const innerViewRef = myRef.current.getNativeScrollRef();
innerViewRef.measure();
```
New usage:
```
const myRef = React.createRef<React.ElementRef<typeof View>>();
<ScrollView ref={myRef} />
// now, myRef.current can be used directly as the ref
myRef.current.measure();
myRef.current.measureLayout();
// Additionally, myRef still has access to ScrollView methods
myRef.current.scrollTo(...);
```
Changes:
* Added deprecation warnings to ScrollView methods `getNativeScrollRef`, `getScrollableNode`, and `getScrollResponder`
* Added the forwardRef call to create `ForwardedScrollView` - this takes in `ref` and passes it into the class ScrollView as `scrollViewRef`.
* Forwarded the ref to the native scroll view using `setAndForwardRef`.
* Added statics onto `ForwardedScrollView` so that `ScrollView.Context` can still be accessed.
* Added type `ScrollViewImperativeMethods`, which lists the public methods of ScrollView.
* Converted all public methods of ScrollView to arrow functions. This is because they need to be bound to the forwarded ref.
* Bound all public methods of ScrollView to the forwarded ref in the `setAndForwardRef` call.
* Flow typed the final output (ForwardedScrollView) as an abstract component that takes in the props of the `ScrollView` class, and has all methods of both the inner host component (`measure`, `measureLayout`, etc) and the public methods (`scrollTo`, etc).
Changes to mockScrollView:
* Changed mockScrollView to be able to mock the function component instead of a class component
* Updated necessary tests
Changelog:
[General] [Changed] - Make ScrollView use forwardRef
Reviewed By: TheSavior
Differential Revision: D19304480
fbshipit-source-id: 6c359897526d9d5ac6bc6ab6d5f9d82bfc0d8af4
Vladislav Shabanov
Kacie Bawiec
