Eliminate rebuilds for Scaffold FAB animation

[nate-thegrate]

This PR contains a tweak to the Scaffold widget's internal logic, reducing the amount of work that needs to be performed each time the FAB animation updates.

resolves #182330

[gemini-code-assist]

Code Review

This pull request optimizes the Scaffold widget to prevent unnecessary rebuilds during the Floating Action Button (FAB) animation. The change replaces an AnimatedBuilder with a Builder and passes the FAB's move animation controller directly to the _ScaffoldLayout delegate's relayout property. This ensures that only the layout is updated when the animation ticks, rather than rebuilding a larger portion of the widget tree. A new test has been added to confirm that the widget building the CustomMultiChildLayout is not marked as dirty during the animation, verifying the performance improvement.

[dkwingsmt]

LGTM. Thank you!

[dkwingsmt]

Do we have a test to verify that the animation is indeed applied (i.e. relayout: is correctly assigned)?

[nate-thegrate]

Yes we do! I was able to find two tests that cover the FAB animation:

flutter/packages/flutter/test/material/scaffold_test.dart

Lines 318 to 366 in d9556bd

	testWidgets('Floating action entrance/exit animation', (WidgetTester tester) async {
	await tester.pumpWidget(
	const MaterialApp(
	home: Scaffold(
	floatingActionButton: FloatingActionButton(
	key: Key('one'),
	onPressed: null,
	child: Text('1'),
	),
	),
	),
	);
	expect(tester.binding.transientCallbackCount, 0);
	await tester.pumpWidget(
	const MaterialApp(
	home: Scaffold(
	floatingActionButton: FloatingActionButton(
	key: Key('two'),
	onPressed: null,
	child: Text('2'),
	),
	),
	),
	);
	expect(tester.binding.transientCallbackCount, greaterThan(0));
	await tester.pumpWidget(Container());
	expect(tester.binding.transientCallbackCount, 0);
	await tester.pumpWidget(const MaterialApp(home: Scaffold()));
	expect(tester.binding.transientCallbackCount, 0);
	await tester.pumpWidget(
	const MaterialApp(
	home: Scaffold(
	floatingActionButton: FloatingActionButton(
	key: Key('one'),
	onPressed: null,
	child: Text('1'),
	),
	),
	),
	);
	expect(tester.binding.transientCallbackCount, greaterThan(0));
	});

flutter/packages/flutter/test/material/scaffold_test.dart

Lines 3553 to 3606 in d9556bd

	testWidgets('FAB default entrance and exit animations', (WidgetTester tester) async {
	var showFab = false;
	await tester.pumpWidget(
	MaterialApp(
	home: StatefulBuilder(
	builder: (BuildContext context, StateSetter setState) {
	return Scaffold(
	body: ElevatedButton(
	onPressed: () {
	setState(() {
	showFab = !showFab;
	});
	},
	child: const Text('Toggle FAB'),
	),
	floatingActionButton: !showFab
	? null
	: FloatingActionButton(onPressed: () {}, child: const Icon(Icons.add)),
	);
	},
	),
	),
	);
	// FAB is not visible.
	expect(find.byType(FloatingActionButton), findsNothing);
	// Tap the button to show the FAB.
	await tester.tap(find.widgetWithText(ElevatedButton, 'Toggle FAB'));
	await tester.pump();
	await tester.pump(const Duration(milliseconds: 100)); // Advance the animation by 100ms.
	// FAB is partially animated in.
	expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, closeTo(743.8, 0.1));
	await tester.pump(const Duration(milliseconds: 100)); // Advance the animation by 100ms.
	// FAB is fully animated in.
	expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, equals(728.0));
	// Tap the button to hide the FAB.
	await tester.tap(find.widgetWithText(ElevatedButton, 'Toggle FAB'));
	await tester.pump();
	await tester.pump(const Duration(milliseconds: 100)); // Advance the animation by 100ms.
	// FAB is partially animated out.
	expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, closeTo(747.1, 0.1));
	await tester.pump(const Duration(milliseconds: 100)); // Advance the animation by 100ms.
	// FAB is fully animated out.
	expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, equals(756.0));
	await tester.pump(const Duration(milliseconds: 50)); // Advance the animation by 50ms.
	// FAB is not visible.
	expect(find.byType(FloatingActionButton), findsNothing);
	});

[justinmc]

LGTM 👍 Good catch!

[auto-submit]

autosubmit label was removed for flutter/flutter/182331, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[nate-thegrate]

Dang it, looks like Google Testing failed. I'll re-trigger the checks in hopes that it was a flake…

[nate-thegrate]

Everything passed! I believe we are ready to land 🫡