TextInput: Don't do an extra round trip to native on focus/blur

Summary:
I wrote up a bunch of context for this in response to #27038 by fat. That comment is reproduced here in this commit message. You can see it in it's original contxt here: https://github.com/facebook/react-native/pull/27038

Okay, here is what I think is happening. For context, here is a diagram I have of how focus and blur propagates through the system. This might be interesting to refer back to as you go through the rest of my explanation.

![graphviz (12)](https://user-images.githubusercontent.com/249164/67992345-982c9d80-fbf9-11e9-96ea-b091210dddbe.png)

ScrollView's scrollResponder is responsible for blurring text inputs when a touch occurs in the ScrollView but outside of the currently focused TextInput. The code for that is here:
6ba2769f0f/Libraries/Components/ScrollResponder.js (L301-L314)

This happens on `scrollResponderHandleResponderRelease` aka, touch up.

It checks for what the currently focused textinput is by calling `TextInputState.currentlyFocusedField()`.

That function is a JS variable that is being updated by calls to `TextInputState.focusTextInput` and `TextInputState.blurTextInput`:

6ba2769f0f/Libraries/Components/TextInput/TextInputState.js (L36-L71)

I added some console logs to those methods to see which ones are being called when running your repro (thanks for the repro!). **This is without your fix**

Click on and off:
```
// Click on input 1
focusTextInput input1
TextInput's _onFocus called

// Click on blank space
scrollResponderHandleResponderRelease blur input1
blurTextInput input1
TextInput's _onBlur called
```

Click on input1, then input 2, then off
```
// Click on input 1
focusTextInput input1
TextInput's _onFocus called for input1

// Click on input 2
focusTextInput input2
TextInput's _onBlur called for input1
TextInput's _onFocus called for input2

// Click on blank space
scrollResponderHandleResponderRelease blur input2
blurTextInput input2
TextInput's _onBlur called for input2
```

And now for the bug. Click on input 1, tab to 2, then off
```
// Click on input 1
focusTextInput input1
TextInput's _onFocus called for input1

// Tab to input 2
TextInput's _onBlur called for input1
TextInput's _onFocus called for input2

// Click on blank space
scrollResponderHandleResponderRelease blur input1
blurTextInput input1
```

Notice how `focusTextInput` was never called with input2 in the last example. Since this is the function that sets the `currentlyFocusedField` when we click on the blank space RN is trying to blur the first input instead of the second.

# The root cause
We are tracking the state of which field is focused in JS which has to stay in sync with what native knows is focused. We [listen to _onPress](6ba2769f0f/Libraries/Components/TextInput/TextInput.js (L1103-L1107)) and call `TextInputState.focusTextInput` in that handler. However, we don't currently have anything listening to other ways for an input to become focused (like tabbing) so it doesn't end up updating the `currentlyFocusedField`.

We have the same problem with blur that we actually fixed the same way you did here in this PR:
6ba2769f0f/Libraries/Components/TextInput/TextInput.js (L1182-L1189)

If you look back at my diagram at the beginning of this post, you'll notice the missing edge from `TextInput._onFocus` to `TextInputState.focusTextInput`. That's the problem. :)

The reason this solution works is because this function **is** the notification from native that an input was focused or blurred. This solution is *fine* because this updates the `currentlyFocusedID` but isn't great because it both sets that value and **calls the native code to focus or blur again**. Luckily the native code doesn't send an event back to JS if you try to blur an already blurred TextInput otherwise we'd have an infinite loop.

# The correct solution
The correct thing would probably be to have all of this tracking in native code and not in JavaScript code. That's a pretty big change though and very out of scope. Something for our team to keep in mind for the future.

A short term term solution would be to refactor `focusTextInput` and `blurTextInput` to pull out the part that sets the `currentlyFocusedID` that we could call from `TextInput` directly from `_onFocus` and `_onBlur`.

# ^This short term term solution is what this commit is doing.

Changelog:
[General][Changed] TextInput no longer does an extra round trip to native on focus/blur

Reviewed By: RSNara

Differential Revision: D18278359

fbshipit-source-id: 417566f25075a847b0f4bac2888f92fbac934096

Libraries/Components/TextInput/TextInput.js

@@ -993,7 +993,7 @@ const TextInput = createReactClass({
   },
 
   _onFocus: function(event: FocusEvent) {
-    this.focus();
+    TextInputState.focusField(ReactNative.findNodeHandle(this._inputRef));
     if (this.props.onFocus) {
       this.props.onFocus(event);
     }
@@ -1079,9 +1079,7 @@ const TextInput = createReactClass({
   },
 
   _onBlur: function(event: BlurEvent) {
-    // This is a hack to fix https://fburl.com/toehyir8
-    // @todo(rsnara) Figure out why this is necessary.
-    this.blur();
+    TextInputState.blurField(ReactNative.findNodeHandle(this._inputRef));
     if (this.props.onBlur) {
       this.props.onBlur(event);
     }

Libraries/Components/TextInput/TextInputState.js

@@ -28,14 +28,26 @@ function currentlyFocusedField(): ?number {
   return currentlyFocusedID;
 }
 
+function focusField(textFieldID: ?number): void {
+  if (currentlyFocusedID !== textFieldID && textFieldID != null) {
+    currentlyFocusedID = textFieldID;
+  }
+}
+
+function blurField(textFieldID: ?number) {
+  if (currentlyFocusedID === textFieldID && textFieldID != null) {
+    currentlyFocusedID = null;
+  }
+}
+
 /**
  * @param {number} TextInputID id of the text field to focus
  * Focuses the specified text field
  * noop if the text field was already focused
  */
 function focusTextInput(textFieldID: ?number) {
-  if (currentlyFocusedID !== textFieldID && textFieldID !== null) {
-    currentlyFocusedID = textFieldID;
+  if (currentlyFocusedID !== textFieldID && textFieldID != null) {
+    focusField(textFieldID);
     if (Platform.OS === 'ios') {
       UIManager.focus(textFieldID);
     } else if (Platform.OS === 'android') {
@@ -55,8 +67,8 @@ function focusTextInput(textFieldID: ?number) {
  * noop if it wasn't focused
  */
 function blurTextInput(textFieldID: ?number) {
-  if (currentlyFocusedID === textFieldID && textFieldID !== null) {
-    currentlyFocusedID = null;
+  if (currentlyFocusedID === textFieldID && textFieldID != null) {
+    blurField(textFieldID);
     if (Platform.OS === 'ios') {
       UIManager.blur(textFieldID);
     } else if (Platform.OS === 'android') {
@@ -84,6 +96,8 @@ function isTextInput(textFieldID: number): boolean {
 
 module.exports = {
   currentlyFocusedField,
+  focusField,
+  blurField,
   focusTextInput,
   blurTextInput,
   registerInput,

Libraries/Components/TextInput/__tests__/TextInput-test.js

@@ -77,7 +77,19 @@ describe('TextInput tests', () => {
     ReactTestRenderer.create(<TextInput ref={textInputRef} value="value1" />);
 
     expect(textInputRef.current.isFocused()).toBe(false);
+    ReactNative.findNodeHandle = jest.fn().mockImplementation(ref => {
+      if (
+        ref === textInputRef.current ||
+        ref === textInputRef.current.getNativeRef()
+      ) {
+        return 1;
+      }
+
+      return 2;
+    });
+
     const inputTag = ReactNative.findNodeHandle(textInputRef.current);
+
     TextInput.State.focusTextInput(inputTag);
     expect(textInputRef.current.isFocused()).toBe(true);
     expect(TextInput.State.currentlyFocusedField()).toBe(inputTag);