Material should unconditionally repaint on LayoutChangedNotification (#8107)

When a ListView scrolls, it generates a LayoutChangedNotification, which was
causing Material to repaint unconditionally. That's not necessary if there are
no ink effects that need to be moved. This patch skips the repaint in that
case.

Fixes #7937

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

@@ -216,7 +216,8 @@ class _MaterialState extends State<Material> with TickerProviderStateMixin {
     }
     contents = new NotificationListener<LayoutChangedNotification>(
       onNotification: (LayoutChangedNotification notification) {
-        _inkFeatureRenderer.currentContext.findRenderObject().markNeedsPaint();
+        _RenderInkFeatures renderer = _inkFeatureRenderer.currentContext.findRenderObject();
+        renderer._didChangeLayout();
         return true;
       },
       child: new _InkFeatures(
@@ -292,6 +293,11 @@ class _RenderInkFeatures extends RenderProxyBox implements MaterialInkController
     markNeedsPaint();
   }
 
+  void _didChangeLayout() {
+    if (_inkFeatures.isNotEmpty)
+      markNeedsPaint();
+  }
+
   @override
   bool hitTestSelf(Point position) => true;
 

packages/flutter/test/material/material_test.dart

@@ -13,12 +13,86 @@ class NotifyMaterial extends StatelessWidget {
   }
 }
 
+class PaintRecorder extends CustomPainter {
+  PaintRecorder(this.log);
+
+  List<Size> log;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    log.add(size);
+    final Paint paint = new Paint()..color = const Color(0xFF0000FF);
+    canvas.drawRect(Point.origin & size, paint);
+  }
+
+  @override
+  bool shouldRepaint(PaintRecorder oldDelegate) => false;
+}
+
 void main() {
   testWidgets('LayoutChangedNotificaion test', (WidgetTester tester) async {
     await tester.pumpWidget(
       new Material(
-        child: new NotifyMaterial()
-      )
+        child: new NotifyMaterial(),
+      ),
     );
   });
+
+  testWidgets('ListView scroll does not repaint', (WidgetTester tester) async {
+    List<Size> log = <Size>[];
+
+    await tester.pumpWidget(
+      new Column(
+        children: <Widget>[
+          new SizedBox(
+            width: 150.0,
+            height: 150.0,
+            child: new CustomPaint(
+              painter: new PaintRecorder(log),
+            ),
+          ),
+          new Expanded(
+            child: new Material(
+              child: new Column(
+                children: <Widget>[
+                  new Expanded(
+                    child: new ListView(
+                      children: <Widget>[
+                        new Container(
+                          height: 2000.0,
+                          decoration: const BoxDecoration(
+                            backgroundColor: const Color(0xFF00FF00),
+                          ),
+                        ),
+                      ],
+                    ),
+                  ),
+                  new SizedBox(
+                    width: 100.0,
+                    height: 100.0,
+                    child: new CustomPaint(
+                      painter: new PaintRecorder(log),
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+
+    // We paint twice because we have two CustomPaint widgets in the tree above
+    // to test repainting both inside and outside the Material widget.
+    expect(log, equals(<Size>[
+      const Size(150.0, 150.0),
+      const Size(100.0, 100.0),
+    ]));
+    log.clear();
+
+    await tester.scroll(find.byType(ListView), const Offset(0.0, -300.0));
+    await tester.pump();
+
+    expect(log, isEmpty);
+  });
 }