Transform entire MotionEvent instead of single pointer only

[j-piasecki]

Description

Gesture Handler on Android intercepts events via the GestureHandlerRootView, which means that we need to transform the touch coordinated by ourselves to check if it's inside the view. Similarly, we need to transform the event when dispatching it to one of the gesture handlers in order to correctly calculate the changes and send touch events.

The current method works well until there is more than one pointer at once on the screen. When there is more than one pointer, only the first one will be correctly transformed, while others will simply be moved by the offset calculated when transforming the first pointer.

This PR changes the logic when delivering events to gesture handlers - the whole event is transformed, and handlers receive both events - transformed and untransformed one. This way things like touch events and NativeViewGestureHandler which rely on the location in the coordinate space of the view keep working, and other gestures get to work in the coordinate space of the root view.

Fixes #2138.

Test plan

Tested on the Example app.

[wood1986]

I tested in my app. it works. But I do not know other area

[wood1986]

Hey @j-piasecki as this PR has been a month, May I know when you merge into master and publish a new version? Thanks

[kmagiera]

Most of the changes look good. Accepting to unblock. Please take a look at my comments to GestureHandlerOrchestrator.kt as some of the changes there look a bit suspicious

[kmagiera]

are you sure it is safe to recycle event here? It seem like transormEventToViewCoords may sometimes create a new event that should be recycled and other times it may reuse the one provided to it. The latter case matches the previous behavior of this method and we never called recycle here. Not saying this is bad but just look suspicious.

[j-piasecki]

transormEventToViewCoords never creates a new event, it always transforms the one it receives but I can see how the code of the method might've suggested otherwise. To be hones I don't remember why I've done it like that (it might've been a result of some logic that used to be there), but I've simplified it in 78c37cd so it should be much clearer that it doesn't create a new event.

That said, It should be safe to recycle there since transformEventToViewCoords receives (and returns transformed) a copy of the source event:

react-native-gesture-handler/android/lib/src/main/java/com/swmansion/gesturehandler/GestureHandlerOrchestrator.kt

Line 255 in 78c37cd

val event = transformEventToViewCoords(handler.view, MotionEvent.obtain(sourceEvent))

[kmagiera]

not sure about the naming of this method. It suggests that it mutates the provided object while sometimes it doesn't. Also, it looks like it mutates the event in a very few scenarios. Unless I don't understand this well it appears like it only mutates if the view is the wrapperView (which happens only at the very top of the view hierarchy) and when view is null which I don't think should ever happen?

[j-piasecki]

It traverses the view hierarchy upwards until it finds wrapperView (which is an instance of GestureHandlerRootView) or null (in case there is no root view we don't want to crash the app) and then comes back recursively transforming the event using the same matrices that are used to transform views.

It will not mutate the provided event in the following cases:

• null is passed as an argument for view
• all the views on the path from the wrapperView to the provided view aren't transformed (their matrix is identity) and are positioned at the top-left corner of their parents

As for the name, I wanted to make It obvious that it doesn't return a new object but may transform the one provided as an argument so it's not safe to use if the untransformed event is also needed. Semantically I believe it's fine since the only case where the event is not transformed is when the coordinate space of the provided view is the same as the wrapper view.

[kmagiera]

well, forgot to check 'approve' with my last comment

[wood1986]

Thanks. May I know when it will be released? Is it a patch or minor or major release?

[j-piasecki]

@wood1986 Most probably in Gesture Handler 2.7.0, but at the moment I cannot give you a timeframe.

[j-piasecki]

@wood1986 Just a heads up, GH 2.7.0 has been released yesterday.