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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: #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:
https://github.com/facebook/react-native/blob/6ba2769f0f92ca75fb0eb60ccb8337920a9c31eb/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`:

https://github.com/facebook/react-native/blob/6ba2769f0f92ca75fb0eb60ccb8337920a9c31eb/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](https://github.com/facebook/react-native/blob/6ba2769f0f92ca75fb0eb60ccb8337920a9c31eb/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:
https://github.com/facebook/react-native/blob/6ba2769f0f92ca75fb0eb60ccb8337920a9c31eb/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
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TheSavior authored and facebook-github-bot committed Nov 4, 2019
1 parent dfba312 commit e9b4928311513d3cbbd9d875827694eab6cfa932
Showing 3 changed files with 32 additions and 8 deletions.
@@ -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);
}
@@ -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,
@@ -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);

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