Fix `floatingActionButtonAnimator` not being updated (#110067)

packages/flutter/lib/src/material/scaffold.dart

@@ -1305,15 +1305,15 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
   @override
   void didUpdateWidget(_FloatingActionButtonTransition oldWidget) {
     super.didUpdateWidget(oldWidget);
+    if (oldWidget.fabMotionAnimator != widget.fabMotionAnimator || oldWidget.fabMoveAnimation != widget.fabMoveAnimation) {
+      // Get the right scale and rotation animations to use for this widget.
+      _updateAnimations();
+    }
     final bool oldChildIsNull = oldWidget.child == null;
     final bool newChildIsNull = widget.child == null;
     if (oldChildIsNull == newChildIsNull && oldWidget.child?.key == widget.child?.key) {
       return;
     }
-    if (oldWidget.fabMotionAnimator != widget.fabMotionAnimator || oldWidget.fabMoveAnimation != widget.fabMoveAnimation) {
-      // Get the right scale and rotation animations to use for this widget.
-      _updateAnimations();
-    }
     if (_previousController.status == AnimationStatus.dismissed) {
       final double currentValue = widget.currentController.value;
       if (currentValue == 0.0 || oldWidget.child == null) {

packages/flutter/test/material/floating_action_button_location_test.dart

@@ -607,6 +607,58 @@ void main() {
 
       expect(tester.binding.transientCallbackCount, 0);
     });
+
+    testWidgets('Animator can be updated', (WidgetTester tester) async {
+      FloatingActionButtonAnimator fabAnimator = FloatingActionButtonAnimator.scaling;
+      FloatingActionButtonLocation fabLocation = FloatingActionButtonLocation.startFloat;
+
+      final Duration animationDuration = kFloatingActionButtonSegue * 2;
+
+      await tester.pumpWidget(_singleFabScaffold(
+        fabLocation,
+        animator: fabAnimator,
+      ));
+
+      expect(find.byType(FloatingActionButton), findsOneWidget);
+      expect(tester.binding.transientCallbackCount, 0);
+      expect(tester.getCenter(find.byType(FloatingActionButton)).dx, 44.0);
+
+      fabLocation = FloatingActionButtonLocation.endFloat;
+      await tester.pumpWidget(_singleFabScaffold(
+        fabLocation,
+        animator: fabAnimator,
+      ));
+
+      expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, lessThan(16.0));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, greaterThan(16));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getCenter(find.byType(FloatingActionButton)).dx, 756.0);
+      expect(tester.getTopRight(find.byType(FloatingActionButton)).dx, lessThan(800 - 16));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getTopRight(find.byType(FloatingActionButton)).dx, lessThan(800 - 16));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getTopRight(find.byType(FloatingActionButton)).dx, equals(800 - 16));
+
+      fabLocation = FloatingActionButtonLocation.startFloat;
+      fabAnimator = _NoScalingFabAnimator();
+      await tester.pumpWidget(_singleFabScaffold(
+        fabLocation,
+        animator: fabAnimator,
+      ));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getCenter(find.byType(FloatingActionButton)).dx, 756.0);
+      expect(tester.getTopRight(find.byType(FloatingActionButton)).dx, equals(800 - 16));
+
+      await tester.pump(animationDuration * 0.25);
+      expect(tester.getCenter(find.byType(FloatingActionButton)).dx, 44.0);
+      expect(tester.getTopLeft(find.byType(FloatingActionButton)).dx, lessThan(16.0));
+    });
   });
 
   group('Locations account for safe interactive areas', () {
@@ -1717,3 +1769,20 @@ class _LinearMovementFabAnimator extends FloatingActionButtonAnimator {
     return const AlwaysStoppedAnimation<double>(1.0);
   }
 }
+
+class _NoScalingFabAnimator extends FloatingActionButtonAnimator {
+  @override
+  Offset getOffset({required Offset begin, required Offset end, required double progress}) {
+    return progress < 0.5 ? begin : end;
+  }
+
+  @override
+  Animation<double> getScaleAnimation({required Animation<double> parent}) {
+    return const AlwaysStoppedAnimation<double>(1.0);
+  }
+
+  @override
+  Animation<double> getRotationAnimation({required Animation<double> parent}) {
+    return const AlwaysStoppedAnimation<double>(1.0);
+  }
+}