Make `DraggableScrollableController` a `ChangeNotifier` (#96089)

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

@@ -44,7 +44,14 @@ typedef ScrollableWidgetBuilder = Widget Function(
 ///
 /// The controller's methods cannot be used until after the controller has been
 /// passed into a [DraggableScrollableSheet] and the sheet has run initState.
-class DraggableScrollableController {
+///
+/// A [DraggableScrollableController] is a [Listenable]. It notifies its
+/// listeners whenever an attached sheet changes sizes. It does not notify its
+/// listeners when a sheet is first attached or when an attached sheet's
+/// parameters change without affecting the sheet's current size. It does not
+/// fire when [pixels] changes without [size] changing. For example, if the
+/// constraints provided to an attached sheet change.
+class DraggableScrollableController extends ChangeNotifier {
   _DraggableScrollableSheetScrollController? _attachedController;
 
   /// Get the current size (as a fraction of the parent height) of the attached sheet.
@@ -160,9 +167,23 @@ class DraggableScrollableController {
   void _attach(_DraggableScrollableSheetScrollController scrollController) {
     assert(_attachedController == null, 'Draggable scrollable controller is already attached to a sheet.');
     _attachedController = scrollController;
+    _attachedController!.extent._currentSize.addListener(notifyListeners);
+  }
+
+  void _onExtentReplaced(_DraggableSheetExtent previousExtent) {
+    // When the extent has been replaced, the old extent is already disposed and
+    // the controller will point to a new extent. We have to add our listener to
+    // the new extent.
+    _attachedController!.extent._currentSize.addListener(notifyListeners);
+    if (previousExtent.currentSize != _attachedController!.extent.currentSize) {
+      // The listener won't fire for a change in size between two extent
+      // objects so we have to fire it manually here.
+      notifyListeners();
+    }
   }
 
   void _detach() {
+    _attachedController?.extent._currentSize.removeListener(notifyListeners);
     _attachedController = null;
   }
 }
@@ -635,6 +656,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   }
 
   void _replaceExtent() {
+    final _DraggableSheetExtent previousExtent = _extent;
     _extent.dispose();
     _extent = _extent.copyWith(
       minSize: widget.minChildSize,
@@ -647,6 +669,9 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
     // Modify the existing scroll controller instead of replacing it so that
     // developers listening to the controller do not have to rebuild their listeners.
     _scrollController.extent = _extent;
+    // If an external facing controller was provided, let it know that the
+    // extent has been replaced.
+    widget.controller?._onExtentReplaced(previousExtent);
     if (widget.snap) {
       // Trigger a snap in case snap or snapSizes has changed. We put this in a
       // post frame callback so that `build` can update `_extent.availablePixels`

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -976,6 +976,147 @@ void main() {
     expect(tester.takeException(), isAssertionError);
   });
 
+  testWidgets('Can listen for changes in sheet size', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final List<double> loggedSizes = <double>[];
+    final DraggableScrollableController controller = DraggableScrollableController();
+    controller.addListener(() {
+      loggedSizes.add(controller.size);
+    });
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester
+        .getSize(find.byKey(stackKey))
+        .height;
+
+    // The initial size shouldn't be logged because no change has occurred yet.
+    expect(loggedSizes.isEmpty, true);
+
+    await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight), touchSlopY: 0);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.4].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    await tester.timedDrag(find.text('Item 1'), Offset(0, -.1 * screenHeight), const Duration(seconds: 1), frequency: 2);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.45, .5].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    controller.jumpTo(.6);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.6].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    controller.animateTo(1, duration: const Duration(milliseconds: 400), curve: Curves.linear);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.7, .8, .9, 1].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    DraggableScrollableActuator.reset(tester.element(find.byKey(containerKey)));
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.5].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+  });
+
+  testWidgets('Listener does not fire on parameter change and persists after change', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final List<double> loggedSizes = <double>[];
+    final DraggableScrollableController controller = DraggableScrollableController();
+    controller.addListener(() {
+      loggedSizes.add(controller.size);
+    });
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester
+        .getSize(find.byKey(stackKey))
+        .height;
+
+    expect(loggedSizes.isEmpty, true);
+
+    await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight), touchSlopY: 0);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.4].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    // Update a parameter without forcing a change in the current size.
+    await tester.pumpWidget(_boilerplate(
+      null,
+      minChildSize: .1,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    expect(loggedSizes.isEmpty, true);
+
+    await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight), touchSlopY: 0);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.3].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+  });
+
+  testWidgets('Listener fires if a parameter change forces a change in size', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final List<double> loggedSizes = <double>[];
+    final DraggableScrollableController controller = DraggableScrollableController();
+    controller.addListener(() {
+      loggedSizes.add(controller.size);
+    });
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester
+        .getSize(find.byKey(stackKey))
+        .height;
+
+    expect(loggedSizes.isEmpty, true);
+
+    // Set a new `initialChildSize` which will trigger a size change because we
+    // haven't moved away initial size yet.
+    await tester.pumpWidget(_boilerplate(
+      null,
+      initialChildSize: .6,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    expect(loggedSizes, <double>[.6].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    // Move away from initial child size.
+    await tester.drag(find.text('Item 1'), Offset(0, .3 * screenHeight), touchSlopY: 0);
+    await tester.pumpAndSettle();
+    expect(loggedSizes, <double>[.3].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+
+    // Set a `minChildSize` greater than the current size.
+    await tester.pumpWidget(_boilerplate(
+      null,
+      minChildSize: .4,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    expect(loggedSizes, <double>[.4].map((double v) => closeTo(v, precisionErrorTolerance)));
+    loggedSizes.clear();
+  });
+
   testWidgets('Invalid controller interactions throw assertion errors', (WidgetTester tester) async {
     final DraggableScrollableController controller = DraggableScrollableController();
     // Can't use a controller before attaching it.