[material_ui] Allow custom MaterialInkController implementations

packages/material_ui/lib/src/material.dart

@@ -95,6 +95,13 @@ abstract class MaterialInkController {
   /// The ink feature will paint as part of this controller.
   void addInkFeature(InkFeature feature);
 
+  /// Stop painting the given ink feature and release any references to it.
+  ///
+  /// Called from [InkFeature.dispose]. Controllers that implement this
+  /// interface (including custom ones) must remove the feature from any
+  /// internal list used for painting.
+  void removeInkFeature(InkFeature feature);
+
   /// Notifies the controller that one of its ink features needs to repaint.
   void markNeedsPaint();
 }
@@ -590,14 +597,18 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
   @override
   void addInkFeature(InkFeature feature) {
     assert(!feature._debugDisposed);
-    assert(feature._controller == this);
+    // Custom controllers that are not RenderObjects may delegate here so ink
+    // can still paint on the host Material. Only require identity when the
+    // feature's controller is itself a RenderObject.
+    assert(feature.controller is! RenderObject || identical(feature.controller, this));
     _inkFeatures ??= <InkFeature>[];
     assert(!_inkFeatures!.contains(feature));
     _inkFeatures!.add(feature);
     markNeedsPaint();
   }
 
-  void _removeFeature(InkFeature feature) {
+  @override
+  void removeInkFeature(InkFeature feature) {
     assert(_inkFeatures != null);
     _inkFeatures!.remove(feature);
     markNeedsPaint();
@@ -670,19 +681,23 @@ class _InkFeatures extends SingleChildRenderObjectWidget {
 abstract class InkFeature {
   /// Initializes fields for subclasses.
   InkFeature({
-    required MaterialInkController controller,
+    required this.controller,
     required this.referenceBox,
     this.onRemoved,
-  }) : _controller = controller as _RenderInkFeatures {
+  }) {
     assert(debugMaybeDispatchCreated('material', 'InkFeature', this));
   }
 
   /// The [MaterialInkController] associated with this [InkFeature].
   ///
   /// Typically used by subclasses to call
   /// [MaterialInkController.markNeedsPaint] when they need to repaint.
-  MaterialInkController get controller => _controller;
-  final _RenderInkFeatures _controller;
+  ///
+  /// Controllers returned by [Material.of] are [RenderObject]s and can paint
+  /// ink. Custom [MaterialInkController] implementations may delegate
+  /// [MaterialInkController.addInkFeature] to the host controller; painting
+  /// then uses the enclosing Material render object.
+  final MaterialInkController controller;
 
   /// The render box whose visual position defines the frame of reference for this ink feature.
   final RenderBox referenceBox;
@@ -701,7 +716,7 @@ abstract class InkFeature {
       return true;
     }());
     assert(debugMaybeDispatchDisposed(this));
-    _controller._removeFeature(this);
+    controller.removeInkFeature(this);
     onRemoved?.call();
   }
 
@@ -762,8 +777,24 @@ abstract class InkFeature {
   void _paint(Canvas canvas) {
     assert(referenceBox.attached);
     assert(!_debugDisposed);
+    // Prefer the controller when it is a RenderObject (Material.of). Custom
+    // delegating controllers are not RenderObjects, so fall back to the
+    // enclosing _RenderInkFeatures ancestor of the reference box.
+    RenderObject? paintContext;
+    if (controller is RenderObject) {
+      paintContext = controller as RenderObject;
+    } else {
+      RenderObject? ancestor = referenceBox.parent;
+      while (ancestor != null && ancestor is! _RenderInkFeatures) {
+        ancestor = ancestor.parent;
+      }
+      paintContext = ancestor;
+    }
+    if (paintContext == null) {
+      return;
+    }
     // determine the transform that gets our coordinate system to be like theirs
-    final Matrix4? transform = _getPaintTransform(_controller, referenceBox);
+    final Matrix4? transform = _getPaintTransform(paintContext, referenceBox);
     if (transform != null) {
       paintFeature(canvas, transform);
     }

packages/material_ui/pending_changelogs/change_2026_09_11_1789108378458.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Allows custom `MaterialInkController` implementations by removing a private type cast in `InkFeature` and adding `MaterialInkController.removeInkFeature`.
+version: patch

packages/material_ui/test/material_test.dart

@@ -1128,6 +1128,40 @@ void main() {
     );
   });
 
+  testWidgets('InkFeature accepts a custom MaterialInkController', (WidgetTester tester) async {
+    await tester.pumpWidget(const Material(child: SizedBox(width: 40, height: 40)));
+
+    final Element element = tester.element(find.byType(SizedBox));
+    final MaterialInkController host = Material.of(element);
+    final referenceBox = element.findRenderObject()! as RenderBox;
+    final customController = _DelegatingMaterialInkController(host);
+
+    expect(
+      () => _InkFeature(controller: customController, referenceBox: referenceBox),
+      returnsNormally,
+    );
+    expect(customController.added, hasLength(1));
+
+    customController.added.single.dispose();
+    expect(customController.removed, hasLength(1));
+    expect(identical(customController.added.single, customController.removed.single), isTrue);
+  });
+
+  testWidgets('InkFeature with delegating controller still paints', (WidgetTester tester) async {
+    await tester.pumpWidget(const Material(child: SizedBox(width: 40, height: 40)));
+
+    final Element element = tester.element(find.byType(SizedBox));
+    final MaterialInkController host = Material.of(element);
+    final referenceBox = element.findRenderObject()! as RenderBox;
+    final customController = _DelegatingMaterialInkController(host);
+    final tracker = TrackPaintInkFeature(controller: customController, referenceBox: referenceBox);
+    customController.addInkFeature(tracker);
+
+    await tester.pump();
+    expect(tracker.paintCount, greaterThan(0));
+    tracker.dispose();
+  });
+
   group('LookupBoundary', () {
     testWidgets('hides Material from Material.maybeOf', (WidgetTester tester) async {
       MaterialInkController? material;
@@ -1289,3 +1323,34 @@ class _InkFeature extends InkFeature {
   @override
   void paintFeature(Canvas canvas, Matrix4 transform) {}
 }
+
+class _DelegatingMaterialInkController implements MaterialInkController {
+  _DelegatingMaterialInkController(this._host);
+
+  final MaterialInkController _host;
+  final List<InkFeature> added = <InkFeature>[];
+  final List<InkFeature> removed = <InkFeature>[];
+
+  @override
+  Color? get color => _host.color;
+
+  @override
+  TickerProvider get vsync => _host.vsync;
+
+  @override
+  void addInkFeature(InkFeature feature) {
+    added.add(feature);
+    _host.addInkFeature(feature);
+  }
+
+  @override
+  void removeInkFeature(InkFeature feature) {
+    removed.add(feature);
+    _host.removeInkFeature(feature);
+  }
+
+  @override
+  void markNeedsPaint() {
+    _host.markNeedsPaint();
+  }
+}