Description

This PR fixes some edge cases when DragGestureRecognizer interacts with multiple pointers.

It also changes drag_test.dart so that all GestureRecognizer.dispose calls are guarded by addTearDown.

New method resolvePointer

To fix this, a new API OneSequenceRecognizer.resolvePointer is introduced. Here is why this new API is necessary:

Currently OneSequenceRecognizer provides the following ways to control arena entries:

1. startTrackingPointer(pointer) adds one pointer to the arena.
2. resolve(GestureDisposition.rejected) removes all pointers from the arena.

In other words, there is no way to remove one pointer from the arena.

But it is needed for DragGestureRecognizer. Consider the following scenario:

1. Pointer 1 down
2. Pointer 2 down
3. Pointer 1 up
4. Pointer 2 might go up, or might move away.

Expect: If pointer 2 goes up, the drag is canceled. If pointer 2 moves away, the drag is accepted.

This can not be done with the current tools:

• If we don't remove pointer 1 from the arena, then arena 1 will sweep, and this drag gesture might be incorrectly considered as a viable candidate in the sweep and get accepted, violating the expectation when pointer 2 goes up.
• If we do remove pointer 1 from the arena, since we currently have to remove all pointers at once, then pointer 2 will be removed as well, violating the expectation when pointer 2 moves away.

I've also considered embedding this functionality into other methods, mostly stopTrackingPoitner. The problem is gestures that stop tracking pointer do not always want to give up the arena. For example, TapGestureRecognizer stops tracking when the pointer is no longer down, but it does not declare either victory or defeat of the arena, instead waits until itself to be the last member and wins via sweeping.

As a conclusion, we need a new API that can resolve a single pointer. Theoretically we only need to resolve to rejection in this case, but there is no reason not to provide both rejection and acceptance.

Related Issues

• Fixes Another exception was thrown: 'package:flutter/src/gestures/monodrag.dart': Failed assertion: line 387 pos 12: '_initialButtons == kPrimaryButton': is not true. #37466

Tests

I added the following tests:

On multiple pointers:

• DragGestureRecognizer is canceled when all pointers are canceled (FIFO)
• DragGestureRecognizer is canceled when all pointers are canceled (FILO)
• DragGestureRecognizer is accepted when the first pointer is accepted
• canceled pointers (due to up) do not prevent later pointers getting accepted
• canceled pointers (due to buttons) do not prevent later pointers getting accepted

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.