fixes CupertinoFullscreenDialogTransition leaks (#147168)

packages/flutter/lib/src/cupertino/route.dart

@@ -506,44 +506,96 @@ class _CupertinoPageTransitionState extends State<CupertinoPageTransition> {
 ///
 /// For example, used when creating a new calendar event by bringing in the next
 /// screen from the bottom.
-class CupertinoFullscreenDialogTransition extends StatelessWidget {
+class CupertinoFullscreenDialogTransition extends StatefulWidget {
   /// Creates an iOS-style transition used for summoning fullscreen dialogs.
   ///
+  const CupertinoFullscreenDialogTransition({
+    super.key,
+    required this.primaryRouteAnimation,
+    required this.secondaryRouteAnimation,
+    required this.child,
+    required this.linearTransition,
+  });
+
   ///  * `primaryRouteAnimation` is a linear route animation from 0.0 to 1.0
   ///    when this screen is being pushed.
+  final Animation<double> primaryRouteAnimation;
+
   ///  * `secondaryRouteAnimation` is a linear route animation from 0.0 to 1.0
   ///    when another screen is being pushed on top of this one.
-  ///  * `linearTransition` is whether to perform the secondary transition linearly.
+  final Animation<double> secondaryRouteAnimation;
+
+  ///  * `linearTransition` is whether to perform the transitions linearly.
   ///    Used to precisely track back gesture drags.
-  CupertinoFullscreenDialogTransition({
-    super.key,
-    required Animation<double> primaryRouteAnimation,
-    required Animation<double> secondaryRouteAnimation,
-    required this.child,
-    required bool linearTransition,
-  }) : _positionAnimation = CurvedAnimation(
-         parent: primaryRouteAnimation,
+  final bool linearTransition;
+
+  /// The widget below this widget in the tree.
+  final Widget child;
+
+  @override
+  State<CupertinoFullscreenDialogTransition> createState() => _CupertinoFullscreenDialogTransitionState();
+}
+
+class _CupertinoFullscreenDialogTransitionState extends State<CupertinoFullscreenDialogTransition> {
+  /// When this page is coming in to cover another page.
+  late Animation<Offset> _primaryPositionAnimation;
+  /// When this page is becoming covered by another page.
+  late Animation<Offset> _secondaryPositionAnimation;
+  /// Curve of primary page which is coming in to cover another page.
+  CurvedAnimation? _primaryPositionCurve;
+  /// Curve of secondary page which is becoming covered by another page.
+  CurvedAnimation? _secondaryPositionCurve;
+
+  @override
+  void initState() {
+    super.initState();
+    _setupAnimation();
+  }
+
+  @override
+  void didUpdateWidget(covariant CupertinoFullscreenDialogTransition oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (oldWidget.primaryRouteAnimation != widget.primaryRouteAnimation ||
+        oldWidget.secondaryRouteAnimation != widget.secondaryRouteAnimation ||
+        oldWidget.child != widget.child ||
+        oldWidget.linearTransition != widget.linearTransition) {
+        _disposeCurve();
+        _setupAnimation();
+    }
+  }
+
+   @override
+  void dispose() {
+    _disposeCurve();
+    super.dispose();
+  }
+
+  void _disposeCurve() {
+    _primaryPositionCurve?.dispose();
+    _secondaryPositionCurve?.dispose();
+    _primaryPositionCurve = null;
+    _secondaryPositionCurve = null;
+  }
+
+  void _setupAnimation() {
+    _primaryPositionAnimation =  (_primaryPositionCurve = CurvedAnimation(
+         parent: widget.primaryRouteAnimation,
          curve: Curves.linearToEaseOut,
          // The curve must be flipped so that the reverse animation doesn't play
          // an ease-in curve, which iOS does not use.
          reverseCurve: Curves.linearToEaseOut.flipped,
-       ).drive(_kBottomUpTween),
+       )).drive(_kBottomUpTween);
        _secondaryPositionAnimation =
-           (linearTransition
-             ? secondaryRouteAnimation
-             : CurvedAnimation(
-                 parent: secondaryRouteAnimation,
+           (widget.linearTransition
+             ? widget.secondaryRouteAnimation
+             : _secondaryPositionCurve = CurvedAnimation(
+                 parent: widget.secondaryRouteAnimation,
                  curve: Curves.linearToEaseOut,
                  reverseCurve: Curves.easeInToLinear,
                )
            ).drive(_kMiddleLeftTween);
+  }
 
-  final Animation<Offset> _positionAnimation;
-  // When this page is becoming covered by another page.
-  final Animation<Offset> _secondaryPositionAnimation;
-
-  /// The widget below this widget in the tree.
-  final Widget child;
 
   @override
   Widget build(BuildContext context) {
@@ -554,8 +606,8 @@ class CupertinoFullscreenDialogTransition extends StatelessWidget {
       textDirection: textDirection,
       transformHitTests: false,
       child: SlideTransition(
-        position: _positionAnimation,
-        child: child,
+        position: _primaryPositionAnimation,
+        child: widget.child,
       ),
     );
   }

packages/flutter/test/cupertino/route_test.dart

@@ -12,6 +12,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 import '../widgets/semantics_tester.dart';
 
@@ -2431,6 +2432,53 @@ void main() {
       expect(tester.getBottomRight(find.byType(Placeholder)).dx, 390.0);
     });
   });
+
+  testWidgets(
+  // TODO(polina-c): remove when fixed https://github.com/flutter/flutter/issues/145600 [leak-tracking-opt-in]
+  experimentalLeakTesting: LeakTesting.settings.withTracked(classes: <String>['CurvedAnimation']),
+  'Fullscreen route does not leak CurveAnimation', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Builder(
+          builder: (BuildContext context) {
+            return CupertinoButton(
+              child: const Text('Button'),
+              onPressed: () {
+                Navigator.push<void>(context, CupertinoPageRoute<void>(
+                  fullscreenDialog: true,
+                  builder: (BuildContext context) {
+                    return Column(
+                      children: <Widget>[
+                        const Placeholder(),
+                        CupertinoButton(
+                          child: const Text('Close'),
+                          onPressed: () {
+                            Navigator.pop<void>(context);
+                          },
+                        ),
+                      ],
+                    );
+                  },
+                ));
+              },
+            );
+          },
+        ),
+      ),
+    );
+
+    // Enter animation.
+    await tester.tap(find.text('Button'));
+    await tester.pump();
+
+    await tester.pump(const Duration(milliseconds: 400));
+
+    // Exit animation
+    await tester.tap(find.text('Close'));
+    await tester.pump();
+
+    await tester.pump(const Duration(milliseconds: 400));
+  });
 }
 
 class MockNavigatorObserver extends NavigatorObserver {