Add undoStackModifier to UndoHistory

[Renzo-Olivares]

This change adds a feature to UndoHistory that allows the user to modify the value being pushed onto the undo stack.

This is used by the framework to ignore the composing region when pushing history entries to the Undo stack on Android. This is so an undo does not trigger an input connection restart by the Android TextInputPlugin, which occurs when the framework changes the composing region. This is also the native platform behavior observed in Google Keep app on Android, where doing an undo during composing reverts to the previous state but with composing inactive and a subsequent redo does not bring back the composing region.

Fixes #130881
Partial fix for #134398

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[Renzo-Olivares]

I think another way to solve the issue of discarding the composing region so the Android text input plugin does not trigger a restart is to have EditableText use a private implementation of UndoHistory. This would also involve probably making _push on UndoHistoryState public. Curious to hear any thoughts on any of these approaches.

[LongCatIsLooong]

Does this dedup like the ValueNotifier class does? If you map all inputs to the same value, does it push one entry to the stack, or multiple entries that are all identical?

[Renzo-Olivares]

In that case it would just push one entry.

flutter/packages/flutter/lib/src/widgets/undo_history.dart

Lines 169 to 171 in db7ef5b

	if (widget.value.value == _lastValue) {
	return;
	}

[Renzo-Olivares]

Oh I was mistaken, this happens before the modifier gets a chance to do anything. I can make sure the modified value is taken into account for this logic.

There is also this logic in _UndoStack.push that prevents this.

flutter/packages/flutter/lib/src/widgets/undo_history.dart

Lines 390 to 392 in db7ef5b

	if (value == currentValue) {
	return;
	}

[hellohuanlin]

this is likely something that wants detailed documentation (for future readers and maybe future you)

[LongCatIsLooong]

why invoke this twice?

[Renzo-Olivares]

Good catch, i'll use the variable already defined.

[LongCatIsLooong]

The PR description seems to indicate clearing the composing region was to prevent the android input connection from restarting but the comment is a bit different? Is it the TextInputPlugin that's resetting the composing region or it's the undo mechanism?
(also IIRC android doesn't have a built-in undo/redo functionality, the native behavior refers to what Gboard does right?)

[Renzo-Olivares]

It is the TextInputPlugin restarting the input here due to the composing region being changed when we do an undo/redo on the framework side.

The Google Keep app has in-app Undo/Redo controls and this is the behavior I observed.

[LongCatIsLooong]

The Google Keep app has in-app Undo/Redo

I'm being pedantic, but that's not the "native" behavior right? It's just one app that chose to implement it that way.

It is the TextInputPlugin restarting the input here due to the composing region being changed when we do an undo/redo on the framework side.

but the comment is saying this is to prevent the text input plugin from resetting the composing region?

[Renzo-Olivares]

Yeah that is true, I have updated the comments to better reflect this and match the pr description.

[LongCatIsLooong]

LGTM modulo comments.