Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
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
- Loading branch information