Description

While I'm writing #32770, which involves a custom gesture recognizer, I discovered that it is impossible for custom gestures to provide semantics information, because RawGestureDetector hard-coded how it generates them and only supports the "official" gesture recognizers.

This PR adds a new parameter to RawGestureDetector, SemanticsGestureDelegate semantics configures how detectors handle semantics gestures. It has a default delegate that keep the current behavior.

SemanticsGestureDelegate has a method that allows custom GestureDetectors to define the semantics annotations to add.

Regarding the changed test

A test "cache unchanged callbacks" has been changed so that it is no longer required to cache callbacks.

• Why it is safe to remove:
  • It was brought up by Caching callbacks in RenderSemanticsGestureHandler doesn't work as expected #11552 as a concern with performance, but was later retracted.
  • The callbacks are already cached by RenderSemanticsGestureHandler. It will invoke markNeedsSemanticsUpdate only when it is changed from null to non-null and vice-versa, but not if callback changes in other cases, according to code.
• Why it is preferable to remove:
  • It brings unnecessary overhead in terms of time and memory.
  • It makes writing a custom SemanticsGestureDelegate more complicated. To cache it, the delegate must be stored in a State, which requires the custom gesture detector to be stateful.

This change has been discussed with and approved by @goderbauer.

Related Issues

• Move declaration of semantic handlers from detectors to recognizers #33475: the previous attempt to solve the mentioned problem. It was reverted because we decided that gesture recognizers should not know about semantics.
• Blocks Dismiss modal with any button press #32770

Tests

I added the following tests:

• RawGestureDetector
  • A semantic gesture triggers all handlers of that kind
  • Replacing gesture recognizers should update semantic handlers
  • RawGestureDetector caches handlers returned by GestureSemanticsMapping
• DefaultGestureSemanticsMapping, for each of the methods
  • It should return null when there is no recognizer
  • It should return non-null when there is a recognizer (with empty callback)
  • It should correctly call callbacks

I changed the following tests:

• "cache unchanged callbacks" no longer requires caching callbacks

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