Add clip behaviour to Container (#44971)

packages/flutter/lib/src/painting/box_decoration.dart

@@ -213,6 +213,21 @@ class BoxDecoration extends Decoration {
   @override
   EdgeInsetsGeometry get padding => border?.dimensions;
 
+  @override
+  Path getClipPath(Rect rect, TextDirection textDirection) {
+    Path clipPath;
+    switch (shape) {
+      case BoxShape.circle:
+        clipPath = Path()..addOval(rect);
+        break;
+      case BoxShape.rectangle:
+        if (borderRadius != null)
+          clipPath = Path()..addRRect(borderRadius.resolve(textDirection).toRRect(rect));
+        break;
+    }
+    return clipPath;
+  }
+
   /// Returns a new box decoration that is scaled by the given factor.
   BoxDecoration scale(double factor) {
     return BoxDecoration(

packages/flutter/lib/src/painting/decoration.dart

@@ -165,6 +165,9 @@ abstract class Decoration extends Diagnosticable {
   /// omitted if there is no chance that the painter will change (for example,
   /// if it is a [BoxDecoration] with definitely no [DecorationImage]).
   BoxPainter createBoxPainter([ VoidCallback onChanged ]);
+
+  /// Returns a closed [Path] that describes the outer edge of this decoration.
+  Path getClipPath(Rect rect, TextDirection textDirection) => null;
 }
 
 /// A stateful class that can paint a particular [Decoration].

packages/flutter/lib/src/painting/shape_decoration.dart

@@ -122,6 +122,11 @@ class ShapeDecoration extends Decoration {
     );
   }
 
+  @override
+  Path getClipPath(Rect rect, TextDirection textDirection) {
+    return shape.getOuterPath(rect, textDirection: textDirection);
+  }
+
   /// The color to fill in the background of the shape.
   ///
   /// The color is under the [image].

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

@@ -312,10 +312,12 @@ class Container extends StatelessWidget {
     this.margin,
     this.transform,
     this.child,
+    this.clipBehavior = Clip.none,
   }) : assert(margin == null || margin.isNonNegative),
        assert(padding == null || padding.isNonNegative),
        assert(decoration == null || decoration.debugAssertIsValid()),
        assert(constraints == null || constraints.debugAssertIsValid()),
+       assert(clipBehavior != null),
        assert(color == null || decoration == null,
          'Cannot provide both a color and a decoration\n'
          'The color argument is just a shorthand for "decoration: new BoxDecoration(color: color)".'
@@ -388,6 +390,11 @@ class Container extends StatelessWidget {
   /// The transformation matrix to apply before painting the container.
   final Matrix4 transform;
 
+  /// The clip behavior when [Container.decoration] has a clipPath.
+  ///
+  /// Defaults to [Clip.none].
+  final Clip clipBehavior;
+
   EdgeInsetsGeometry get _paddingIncludingDecoration {
     if (decoration == null || decoration.padding == null)
       return padding;
@@ -436,6 +443,17 @@ class Container extends StatelessWidget {
     if (transform != null)
       current = Transform(transform: transform, child: current);
 
+    if (clipBehavior != Clip.none) {
+      current = ClipPath(
+        clipper: _DecorationClipper(
+          textDirection: Directionality.of(context),
+          decoration: decoration
+        ),
+        clipBehavior: clipBehavior,
+        child: current,
+      );
+    }
+
     return current;
   }
 
@@ -444,6 +462,7 @@ class Container extends StatelessWidget {
     super.debugFillProperties(properties);
     properties.add(DiagnosticsProperty<AlignmentGeometry>('alignment', alignment, showName: false, defaultValue: null));
     properties.add(DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding, defaultValue: null));
+    properties.add(DiagnosticsProperty<Clip>('clipBehavior', clipBehavior, defaultValue: Clip.none));
     properties.add(DiagnosticsProperty<Decoration>('bg', decoration, defaultValue: null));
     properties.add(DiagnosticsProperty<Decoration>('fg', foregroundDecoration, defaultValue: null));
     properties.add(DiagnosticsProperty<BoxConstraints>('constraints', constraints, defaultValue: null));
@@ -451,3 +470,26 @@ class Container extends StatelessWidget {
     properties.add(ObjectFlagProperty<Matrix4>.has('transform', transform));
   }
 }
+
+/// A clipper that uses [Decoration.getClipPath] to clip.
+class _DecorationClipper extends CustomClipper<Path> {
+  _DecorationClipper({
+    TextDirection textDirection,
+    @required this.decoration
+  }) : assert (decoration != null),
+       textDirection = textDirection ?? TextDirection.ltr;
+
+  final TextDirection textDirection;
+  final Decoration decoration;
+
+  @override
+  Path getClip(Size size) {
+    return decoration.getClipPath(Offset.zero & size, textDirection);
+  }
+
+  @override
+  bool shouldReclip(_DecorationClipper oldClipper) {
+    return oldClipper.decoration != decoration
+        || oldClipper.textDirection != textDirection;
+  }
+}

packages/flutter/test/painting/box_decoration_test.dart

@@ -68,4 +68,18 @@ void main() {
       paints..rect(rect: Offset.zero & size),
     );
   });
+
+  test('BoxDecoration.getClipPath', () {
+    const double radius = 10;
+    final BoxDecoration decoration = BoxDecoration(
+      borderRadius: BorderRadius.circular(radius),
+    );
+    const Rect rect = Rect.fromLTWH(0.0, 0.0, 100.0, 20.0);
+    final Path clipPath = decoration.getClipPath(rect, TextDirection.ltr);
+    final Matcher isLookLikeExpectedPath = isPathThat(
+      includes: const <Offset>[ Offset(30.0, 10.0), Offset(50.0, 10.0), ],
+      excludes: const <Offset>[ Offset(1.0, 1.0), Offset(99.0, 19.0), ],
+    );
+    expect(clipPath, isLookLikeExpectedPath);
+  });
 }

packages/flutter/test/painting/shape_decoration_test.dart

@@ -98,6 +98,17 @@ void main() {
     );
     expect(log, isEmpty);
   });
+
+  test('ShapeDecoration.getClipPath', () {
+    const ShapeDecoration decoration = ShapeDecoration(shape: CircleBorder(side: BorderSide.none));
+    const Rect rect = Rect.fromLTWH(0.0, 0.0, 100.0, 20.0);
+    final Path clipPath = decoration.getClipPath(rect, TextDirection.ltr);
+    final Matcher isLookLikeExpectedPath = isPathThat(
+      includes: const <Offset>[ Offset(50.0, 10.0), ],
+      excludes: const <Offset>[ Offset(1.0, 1.0), Offset(30.0, 10.0), Offset(99.0, 19.0), ],
+    );
+    expect(clipPath, isLookLikeExpectedPath);
+  });
 }
 
 class TestImageProvider extends ImageProvider<TestImageProvider> {

packages/flutter/test/widgets/container_test.dart

@@ -498,6 +498,38 @@ void main() {
       ),
     );
   });
+
+  testWidgets('giving clipBehaviour Clip.None, will not add a ClipPath to the tree', (WidgetTester tester) async {
+    await tester.pumpWidget(Container(
+      clipBehavior: Clip.none,
+      decoration: BoxDecoration(
+        borderRadius: BorderRadius.circular(1),
+      ),
+      child: const SizedBox(),
+    ));
+
+    expect(
+      find.byType(ClipPath),
+      findsNothing,
+    );
+  });
+
+  testWidgets('giving clipBehaviour not a Clip.None, will add a ClipPath to the tree', (WidgetTester tester) async {
+    final Container container = Container(
+      clipBehavior: Clip.hardEdge,
+      decoration: BoxDecoration(
+        borderRadius: BorderRadius.circular(1),
+      ),
+      child: const SizedBox(),
+    );
+
+    await tester.pumpWidget(container);
+
+    expect(
+      find.byType(ClipPath),
+      findsOneWidget,
+    );
+  });
 }
 
 class _MockPaintingContext extends Mock implements PaintingContext {}