Fix the confusing-zero case with NestedScrollView. (#14133)

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

examples/flutter_gallery/lib/demo/material/tabs_demo.dart

@@ -13,6 +13,8 @@ class _Page {
   _Page({ this.label });
   final String label;
   String get id => label[0];
+  @override
+  String toString() => '$runtimeType("$label")';
 }
 
 class _CardData {
@@ -69,6 +71,13 @@ final Map<_Page, List<_CardData>> _allPages = <_Page, List<_CardData>>{
       imageAsset: 'shrine/products/chucks.png',
       imageAssetPackage: _kGalleryAssetsPackage,
     ),
+  ],
+  new _Page(label: 'RIGHT'): <_CardData>[
+    const _CardData(
+      title: 'Beachball',
+      imageAsset: 'shrine/products/beachball.png',
+      imageAssetPackage: _kGalleryAssetsPackage,
+    ),
     const _CardData(
       title: 'Dipped Brush',
       imageAsset: 'shrine/products/brush.png',
@@ -80,13 +89,6 @@ final Map<_Page, List<_CardData>> _allPages = <_Page, List<_CardData>>{
       imageAssetPackage: _kGalleryAssetsPackage,
     ),
   ],
-  new _Page(label: 'RIGHT'): <_CardData>[
-    const _CardData(
-      title: 'Beachball',
-      imageAsset: 'shrine/products/beachball.png',
-      imageAssetPackage: _kGalleryAssetsPackage,
-    ),
-  ],
 };
 
 class _CardDataItem extends StatelessWidget {
@@ -121,7 +123,10 @@ class _CardDataItem extends StatelessWidget {
               ),
             ),
             new Center(
-              child: new Text(data.title, style: Theme.of(context).textTheme.title),
+              child: new Text(
+                data.title,
+                style: Theme.of(context).textTheme.title,
+              ),
             ),
           ],
         ),
@@ -141,13 +146,18 @@ class TabsDemo extends StatelessWidget {
         body: new NestedScrollView(
           headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
             return <Widget>[
-              new SliverAppBar(
-                title: const Text('Tabs and scrolling'),
-                pinned: true,
-                expandedHeight: 150.0,
-                forceElevated: innerBoxIsScrolled,
-                bottom: new TabBar(
-                  tabs: _allPages.keys.map((_Page page) => new Tab(text: page.label)).toList(),
+              new SliverOverlapAbsorber(
+                handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                child: new SliverAppBar(
+                  title: const Text('Tabs and scrolling'),
+                  pinned: true,
+                  expandedHeight: 150.0,
+                  forceElevated: innerBoxIsScrolled,
+                  bottom: new TabBar(
+                    tabs: _allPages.keys.map(
+                      (_Page page) => new Tab(text: page.label),
+                    ).toList(),
+                  ),
                 ),
               ),
             ];
@@ -157,15 +167,41 @@ class TabsDemo extends StatelessWidget {
               return new SafeArea(
                 top: false,
                 bottom: false,
-                child: new ListView(
-                  padding: const EdgeInsets.symmetric(vertical: 8.0, horizontal: 16.0),
-                  itemExtent: _CardDataItem.height,
-                  children: _allPages[page].map((_CardData data) {
-                    return new Padding(
-                      padding: const EdgeInsets.symmetric(vertical: 8.0),
-                      child: new _CardDataItem(page: page, data: data),
+                child: new Builder(
+                  builder: (BuildContext context) {
+                    return new CustomScrollView(
+                      key: new PageStorageKey<_Page>(page),
+                      slivers: <Widget>[
+                        new SliverOverlapInjector(
+                          handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                        ),
+                        new SliverPadding(
+                          padding: const EdgeInsets.symmetric(
+                            vertical: 8.0,
+                            horizontal: 16.0,
+                          ),
+                          sliver: new SliverFixedExtentList(
+                            itemExtent: _CardDataItem.height,
+                            delegate: new SliverChildBuilderDelegate(
+                              (BuildContext context, int index) {
+                                final _CardData data = _allPages[page][index];
+                                return new Padding(
+                                  padding: const EdgeInsets.symmetric(
+                                    vertical: 8.0,
+                                  ),
+                                  child: new _CardDataItem(
+                                    page: page,
+                                    data: data,
+                                  ),
+                                );
+                              },
+                              childCount: _allPages[page].length,
+                            ),
+                          ),
+                        ),
+                      ],
                     );
-                  }).toList(),
+                  },
                 ),
               );
             }).toList(),

packages/flutter/lib/painting.dart

@@ -41,6 +41,7 @@ export 'src/painting/image_provider.dart';
 export 'src/painting/image_resolution.dart';
 export 'src/painting/image_stream.dart';
 export 'src/painting/matrix_utils.dart';
+export 'src/painting/paint_utilities.dart';
 export 'src/painting/rounded_rectangle_border.dart';
 export 'src/painting/shape_decoration.dart';
 export 'src/painting/stadium_border.dart';

packages/flutter/lib/src/animation/animation.dart

@@ -38,7 +38,7 @@ typedef void AnimationStatusListener(AnimationStatus status);
 ///
 /// To create a new animation that you can run forward and backward, consider
 /// using [AnimationController].
-abstract class Animation<T> extends Listenable {
+abstract class Animation<T> extends Listenable implements ValueListenable<T> {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
   const Animation();
@@ -71,6 +71,7 @@ abstract class Animation<T> extends Listenable {
   AnimationStatus get status;
 
   /// The current value of the animation.
+  @override
   T get value;
 
   /// Whether this animation is stopped at the beginning.

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -32,6 +32,16 @@ abstract class Listenable {
   void removeListener(VoidCallback listener);
 }
 
+/// An interface for subclasses of [Listenable] that expose a [value].
+///
+/// This interface is implemented by [ValueNotifier<T>] and [Animation<T>], and
+/// allows other APIs to accept either of those implementations interchangeably.
+abstract class ValueListenable<T> extends Listenable {
+  /// The current value of the object. When the value changes, the callbacks
+  /// registered with [addListener] will be invoked.
+  T get value;
+}
+
 /// A class that can be extended or mixed in that provides a change notification
 /// API using [VoidCallback] for notifications.
 ///
@@ -169,13 +179,14 @@ class _MergingListenable extends ChangeNotifier {
 /// A [ChangeNotifier] that holds a single value.
 ///
 /// When [value] is replaced, this class notifies its listeners.
-class ValueNotifier<T> extends ChangeNotifier {
+class ValueNotifier<T> extends ChangeNotifier implements ValueListenable<T> {
   /// Creates a [ChangeNotifier] that wraps this value.
   ValueNotifier(this._value);
 
   /// The current value stored in this notifier.
   ///
   /// When the value is replaced, this class notifies its listeners.
+  @override
   T get value => _value;
   T _value;
   set value(T newValue) {

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

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:math' as math;
+
+import 'basic_types.dart';
+
+/// Draw a line between two points, which cuts diagonally back and forth across
+/// the line that connects the two points.
+///
+/// The line will cross the line `zigs - 1` times.
+///
+/// If `zigs` is 1, then this will draw two sides of a triangle from `start` to
+/// `end`, with the third point being `width` away from the line, as measured
+/// perpendicular to that line.
+///
+/// If `width` is positive, the first `zig` will be to the left of the `start`
+/// point when facing the `end` point. To reverse the zigging polarity, provide
+/// a negative `width`.
+///
+/// The line is drawn using the provided `paint` on the provided `canvas`.
+void paintZigZag(Canvas canvas, Paint paint, Offset start, Offset end, int zigs, double width) {
+  assert(zigs.isFinite);
+  assert(zigs > 0);
+  canvas.save();
+  canvas.translate(start.dx, start.dy);
+  end = end - start;
+  canvas.rotate(math.atan2(end.dy, end.dx));
+  final double length = end.distance;
+  final double spacing = length / (zigs * 2.0);
+  final Path path = new Path()
+    ..moveTo(0.0, 0.0);
+  for (int index = 0; index < zigs; index += 1) {
+    final double x = (index * 2.0 + 1.0) * spacing;
+    final double y = width * ((index % 2.0) * 2.0 - 1.0);
+    path.lineTo(x, y);
+  }
+  path.lineTo(length, 0.0);
+  canvas.drawPath(path, paint);
+  canvas.restore();
+}
\ No newline at end of file

packages/flutter/lib/src/rendering/sliver_padding.dart

@@ -242,7 +242,7 @@ class RenderSliverPadding extends RenderSliver with RenderObjectWithChildMixin<R
 
   @override
   bool hitTestChildren(HitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) {
-    if (child.geometry.hitTestExtent > 0.0)
+    if (child != null && child.geometry.hitTestExtent > 0.0)
       return child.hitTest(result, mainAxisPosition: mainAxisPosition - childMainAxisPosition(child), crossAxisPosition: crossAxisPosition - childCrossAxisPosition(child));
     return false;
   }

packages/flutter/lib/src/scheduler/binding.dart

@@ -580,12 +580,27 @@ abstract class SchedulerBinding extends BindingBase with ServicesBinding {
   /// Schedules a new frame using [scheduleFrame] if this object is not
   /// currently producing a frame.
   ///
-  /// After this is called, the framework ensures that the end of the
-  /// [handleBeginFrame] function will (eventually) be reached.
+  /// Calling this method ensures that [handleDrawFrame] will eventually be
+  /// called, unless it's already in progress.
+  ///
+  /// This has no effect if [schedulerPhase] is
+  /// [SchedulerPhase.transientCallbacks] or [SchedulerPhase.midFrameMicrotasks]
+  /// (because a frame is already being prepared in that case), or
+  /// [SchedulerPhase.persistentCallbacks] (because a frame is actively being
+  /// rendered in that case). It will schedule a frame if the [schedulerPhase]
+  /// is [SchedulerPhase.idle] (in between frames) or
+  /// [SchedulerPhase.postFrameCallbacks] (after a frame).
   void ensureVisualUpdate() {
-    if (schedulerPhase != SchedulerPhase.idle)
-      return;
-    scheduleFrame();
+    switch (schedulerPhase) {
+      case SchedulerPhase.idle:
+      case SchedulerPhase.postFrameCallbacks:
+        scheduleFrame();
+        return;
+      case SchedulerPhase.transientCallbacks:
+      case SchedulerPhase.midFrameMicrotasks:
+      case SchedulerPhase.persistentCallbacks:
+        return;
+    }
   }
 
   /// If necessary, schedules a new frame by calling

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

@@ -508,7 +508,7 @@ abstract class WidgetsBinding extends BindingBase with SchedulerBinding, Gesture
     _deferFirstFrameReportCount -= 1;
   }
 
-    void _handleBuildScheduled() {
+  void _handleBuildScheduled() {
     // If we're in the process of building dirty elements, then changes
     // should not trigger a new frame.
     assert(() {

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

@@ -26,7 +26,8 @@ import 'scroll_position_with_single_context.dart';
 ///
 /// A [ScrollController] is a [Listenable]. It notifies its listeners whenever
 /// any of the attached [ScrollPosition]s notify _their_ listeners (i.e.
-/// whenever any of them scroll).
+/// whenever any of them scroll). It does not notify its listeners when the list
+/// of attached [ScrollPosition]s changes.
 ///
 /// Typically used with [ListView], [GridView], [CustomScrollView].
 ///

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

@@ -479,7 +479,7 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// This notifier's value is true if a scroll is underway and false if the scroll
   /// position is idle.
   ///
-  /// Listeners added by stateful widgets should be in the widget's
+  /// Listeners added by stateful widgets should be removed in the widget's
   /// [State.dispose] method.
   final ValueNotifier<bool> isScrollingNotifier = new ValueNotifier<bool>(false);
 

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

@@ -182,11 +182,39 @@ abstract class ScrollView extends StatelessWidget {
     return getAxisDirectionFromAxisReverseAndDirectionality(context, scrollDirection, reverse);
   }
 
+  /// Build the list of widgets to place inside the viewport.
+  ///
   /// Subclasses should override this method to build the slivers for the inside
   /// of the viewport.
   @protected
   List<Widget> buildSlivers(BuildContext context);
 
+  /// Build the viewport.
+  ///
+  /// Subclasses may override this method to change how the viewport is built.
+  /// The default implementation uses a [ShrinkWrappingViewport] if [shrinkWrap]
+  /// is true, and a regular [Viewport] otherwise.
+  @protected
+  Widget buildViewport(
+    BuildContext context,
+    ViewportOffset offset,
+    AxisDirection axisDirection,
+    List<Widget> slivers,
+  ) {
+    if (shrinkWrap) {
+      return new ShrinkWrappingViewport(
+        axisDirection: axisDirection,
+        offset: offset,
+        slivers: slivers,
+      );
+    }
+    return new Viewport(
+      axisDirection: axisDirection,
+      offset: offset,
+      slivers: slivers,
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     final List<Widget> slivers = buildSlivers(context);
@@ -200,20 +228,8 @@ abstract class ScrollView extends StatelessWidget {
       controller: scrollController,
       physics: physics,
       viewportBuilder: (BuildContext context, ViewportOffset offset) {
-        if (shrinkWrap) {
-          return new ShrinkWrappingViewport(
-            axisDirection: axisDirection,
-            offset: offset,
-            slivers: slivers,
-          );
-        } else {
-          return new Viewport(
-            axisDirection: axisDirection,
-            offset: offset,
-            slivers: slivers,
-          );
-        }
-      }
+        return buildViewport(context, offset, axisDirection, slivers);
+      },
     );
     return primary && scrollController != null
       ? new PrimaryScrollController.none(child: scrollable)

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

@@ -12,21 +12,6 @@ export 'package:flutter/rendering.dart' show
   AxisDirection,
   GrowthDirection;
 
-AxisDirection _getDefaultCrossAxisDirection(BuildContext context, AxisDirection axisDirection) {
-  assert(axisDirection != null);
-  switch (axisDirection) {
-    case AxisDirection.up:
-      return textDirectionToAxisDirection(Directionality.of(context));
-    case AxisDirection.right:
-      return AxisDirection.down;
-    case AxisDirection.down:
-      return textDirectionToAxisDirection(Directionality.of(context));
-    case AxisDirection.left:
-      return AxisDirection.down;
-  }
-  return null;
-}
-
 /// A widget that is bigger on the inside.
 ///
 /// [Viewport] is the visual workhorse of the scrolling machinery. It displays a
@@ -124,11 +109,31 @@ class Viewport extends MultiChildRenderObjectWidget {
   /// The [center] must be the key of a child of the viewport.
   final Key center;
 
+  /// Given a [BuildContext] and an [AxisDirection], determine the correct cross
+  /// axis direction.
+  ///
+  /// This depends on the [Directionality] if the `axisDirection` is vertical;
+  /// otherwise, the default cross axis direction is downwards.
+  static AxisDirection getDefaultCrossAxisDirection(BuildContext context, AxisDirection axisDirection) {
+    assert(axisDirection != null);
+    switch (axisDirection) {
+      case AxisDirection.up:
+        return textDirectionToAxisDirection(Directionality.of(context));
+      case AxisDirection.right:
+        return AxisDirection.down;
+      case AxisDirection.down:
+        return textDirectionToAxisDirection(Directionality.of(context));
+      case AxisDirection.left:
+        return AxisDirection.down;
+    }
+    return null;
+  }
+
   @override
   RenderViewport createRenderObject(BuildContext context) {
     return new RenderViewport(
       axisDirection: axisDirection,
-      crossAxisDirection: crossAxisDirection ?? _getDefaultCrossAxisDirection(context, axisDirection),
+      crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
       anchor: anchor,
       offset: offset,
     );
@@ -138,7 +143,7 @@ class Viewport extends MultiChildRenderObjectWidget {
   void updateRenderObject(BuildContext context, RenderViewport renderObject) {
     renderObject
       ..axisDirection = axisDirection
-      ..crossAxisDirection = crossAxisDirection ?? _getDefaultCrossAxisDirection(context, axisDirection)
+      ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
       ..anchor = anchor
       ..offset = offset;
   }
@@ -271,7 +276,7 @@ class ShrinkWrappingViewport extends MultiChildRenderObjectWidget {
   RenderShrinkWrappingViewport createRenderObject(BuildContext context) {
     return new RenderShrinkWrappingViewport(
       axisDirection: axisDirection,
-      crossAxisDirection: crossAxisDirection ?? _getDefaultCrossAxisDirection(context, axisDirection),
+      crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
       offset: offset,
     );
   }
@@ -280,7 +285,7 @@ class ShrinkWrappingViewport extends MultiChildRenderObjectWidget {
   void updateRenderObject(BuildContext context, RenderShrinkWrappingViewport renderObject) {
     renderObject
       ..axisDirection = axisDirection
-      ..crossAxisDirection = crossAxisDirection ?? _getDefaultCrossAxisDirection(context, axisDirection)
+      ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
       ..offset = offset;
   }
 

packages/flutter/test/rendering/mock_canvas.dart

@@ -292,6 +292,23 @@ abstract class PaintPattern {
   /// If no call to [Canvas.drawParagraph] was made, then this results in failure.
   void paragraph({ ui.Paragraph paragraph, dynamic offset });
 
+  /// Indicates that a shadow is expected next.
+  ///
+  /// The next shadow is examined. Any arguments that are passed to this method
+  /// are compared to the actual [Canvas.drawShadow] call's `paint` argument,
+  /// and any mismatches result in failure.
+  ///
+  /// To introspect the Path object (as it stands after the painting has
+  /// completed), the `includes` and `excludes` arguments can be provided to
+  /// specify points that should be considered inside or outside the path
+  /// (respectively).
+  ///
+  /// If no call to [Canvas.drawShadow] was made, then this results in failure.
+  ///
+  /// Any calls made between the last matched call (if any) and the
+  /// [Canvas.drawShadow] call are ignored.
+  void shadow({ Iterable<Offset> includes, Iterable<Offset> excludes, Color color, double elevation, bool transparentOccluder });
+
   /// Indicates that an image is expected next.
   ///
   /// The next call to [Canvas.drawImage] is examined, and its arguments
@@ -637,6 +654,11 @@ class _TestRecordingCanvasPatternMatcher extends _TestRecordingCanvasMatcher imp
     _predicates.add(new _FunctionPaintPredicate(#drawParagraph, <dynamic>[paragraph, offset]));
   }
 
+  @override
+  void shadow({ Iterable<Offset> includes, Iterable<Offset> excludes, Color color, double elevation, bool transparentOccluder }) {
+    _predicates.add(new _ShadowPredicate(includes: includes, excludes: excludes, color: color, elevation: elevation, transparentOccluder: transparentOccluder));
+  }
+
   @override
   void image({ ui.Image image, double x, double y, Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) {
     _predicates.add(new _DrawImagePaintPredicate(image: image, x: x, y: y, color: color, strokeWidth: strokeWidth, hasMaskFilter: hasMaskFilter, style: style));
@@ -709,6 +731,24 @@ class _TestRecordingCanvasPatternMatcher extends _TestRecordingCanvasMatcher imp
 abstract class _PaintPredicate {
   void match(Iterator<RecordedInvocation> call);
 
+  @protected
+  void checkMethod(Iterator<RecordedInvocation> call, Symbol symbol) {
+    int others = 0;
+    final RecordedInvocation firstCall = call.current;
+    while (!call.current.invocation.isMethod || call.current.invocation.memberName != symbol) {
+      others += 1;
+      if (!call.moveNext())
+        throw new _MismatchedCall(
+          'It called $others other method${ others == 1 ? "" : "s" } on the canvas, '
+          'the first of which was $firstCall, but did not '
+          'call ${_symbolName(symbol)}() at the time where $this was expected.',
+          'The first method that was called when the call to ${_symbolName(symbol)}() '
+          'was expected, $firstCall, was called with the following stack:',
+          firstCall,
+        );
+    }
+  }
+
   @override
   String toString() {
     throw new FlutterError('$runtimeType does not implement toString.');
@@ -734,19 +774,7 @@ abstract class _DrawCommandPaintPredicate extends _PaintPredicate {
 
   @override
   void match(Iterator<RecordedInvocation> call) {
-    int others = 0;
-    final RecordedInvocation firstCall = call.current;
-    while (!call.current.invocation.isMethod || call.current.invocation.memberName != symbol) {
-      others += 1;
-      if (!call.moveNext())
-        throw new _MismatchedCall(
-          'It called $others other method${ others == 1 ? "" : "s" } on the canvas, '
-          'the first of which was $firstCall, but did not '
-          'call $methodName at the time where $this was expected.',
-          'The stack for the call to $firstCall was:',
-          firstCall,
-        );
-    }
+    checkMethod(call, symbol);
     final int actualArgumentCount = call.current.invocation.positionalArguments.length;
     if (actualArgumentCount != argumentCount)
       throw 'It called $methodName with $actualArgumentCount argument${actualArgumentCount == 1 ? "" : "s"}; expected $argumentCount.';
@@ -1008,6 +1036,81 @@ class _ArcPaintPredicate extends _DrawCommandPaintPredicate {
   );
 }
 
+class _ShadowPredicate extends _PaintPredicate {
+  _ShadowPredicate({ this.includes, this.excludes, this.color, this.elevation, this.transparentOccluder });
+
+  final Iterable<Offset> includes;
+  final Iterable<Offset> excludes;
+  final Color color;
+  final double elevation;
+  final bool transparentOccluder;
+
+  static const Symbol symbol = #drawShadow;
+  String get methodName => _symbolName(symbol);
+
+  @protected
+  void verifyArguments(List<dynamic> arguments) {
+    if (arguments.length != 4)
+      throw 'It called $methodName with ${arguments.length} arguments; expected 4.';
+    final Path pathArgument = arguments[0];
+    if (includes != null) {
+      for (Offset offset in includes) {
+        if (!pathArgument.contains(offset))
+          throw 'It called $methodName with a path that unexpectedly did not contain $offset.';
+      }
+    }
+    if (excludes != null) {
+      for (Offset offset in excludes) {
+        if (pathArgument.contains(offset))
+          throw 'It called $methodName with a path that unexpectedly contained $offset.';
+      }
+    }
+    final Color actualColor = arguments[1];
+    if (color != null && actualColor != color)
+      throw 'It called $methodName with a color, $actualColor, which was not exactly the expected color ($color).';
+    final double actualElevation = arguments[2];
+    if (elevation != null && actualElevation != elevation)
+      throw 'It called $methodName with an elevation, $actualElevation, which was not exactly the expected value ($elevation).';
+    final bool actualTransparentOccluder = arguments[3];
+    if (transparentOccluder != null && actualTransparentOccluder != transparentOccluder)
+      throw 'It called $methodName with a transparentOccluder value, $actualTransparentOccluder, which was not exactly the expected value ($transparentOccluder).';
+  }
+
+  @override
+  void match(Iterator<RecordedInvocation> call) {
+    checkMethod(call, symbol);
+    verifyArguments(call.current.invocation.positionalArguments);
+    call.moveNext();
+  }
+
+  @protected
+  void debugFillDescription(List<String> description) {
+    if (includes != null && excludes != null) {
+      description.add('that contains $includes and does not contain $excludes');
+    } else if (includes != null) {
+      description.add('that contains $includes');
+    } else if (excludes != null) {
+      description.add('that does not contain $excludes');
+    }
+    if (color != null)
+      description.add('$color');
+    if (elevation != null)
+      description.add('elevation: $elevation');
+    if (transparentOccluder != null)
+      description.add('transparentOccluder: $transparentOccluder');
+  }
+
+  @override
+  String toString() {
+    final List<String> description = <String>[];
+    debugFillDescription(description);
+    String result = methodName;
+    if (description.isNotEmpty)
+      result += ' with ${description.join(", ")}';
+    return result;
+  }
+}
+
 class _DrawImagePaintPredicate extends _DrawCommandPaintPredicate {
   _DrawImagePaintPredicate({ this.image, this.x, this.y, Color color, double strokeWidth, bool hasMaskFilter, PaintingStyle style }) : super(
     #drawImage, 'an image', 3, 2, color: color, strokeWidth: strokeWidth, hasMaskFilter: hasMaskFilter, style: style
@@ -1128,20 +1231,7 @@ class _FunctionPaintPredicate extends _PaintPredicate {
 
   @override
   void match(Iterator<RecordedInvocation> call) {
-    int others = 0;
-    final RecordedInvocation firstCall = call.current;
-    while (!call.current.invocation.isMethod || call.current.invocation.memberName != symbol) {
-      others += 1;
-      if (!call.moveNext())
-        throw new _MismatchedCall(
-          'It called $others other method${ others == 1 ? "" : "s" } on the canvas, '
-          'the first of which was $firstCall, but did not '
-          'call ${_symbolName(symbol)}() at the time where $this was expected.',
-          'The first method that was called when the call to ${_symbolName(symbol)}() '
-          'was expected, $firstCall, was called with the following stack:',
-          firstCall,
-        );
-    }
+    checkMethod(call, symbol);
     if (call.current.invocation.positionalArguments.length != arguments.length)
       throw 'It called ${_symbolName(symbol)} with ${call.current.invocation.positionalArguments.length} arguments; expected ${arguments.length}.';
     for (int index = 0; index < arguments.length; index += 1) {
@@ -1169,20 +1259,7 @@ class _FunctionPaintPredicate extends _PaintPredicate {
 class _SaveRestorePairPaintPredicate extends _PaintPredicate {
   @override
   void match(Iterator<RecordedInvocation> call) {
-    int others = 0;
-    final RecordedInvocation firstCall = call.current;
-    while (!call.current.invocation.isMethod || call.current.invocation.memberName != #save) {
-      others += 1;
-      if (!call.moveNext())
-        throw new _MismatchedCall(
-          'It called $others other method${ others == 1 ? "" : "s" } on the canvas, '
-          'the first of which was $firstCall, but did not '
-          'call save() at the time where $this was expected.',
-          'The first method that was called when the call to save() '
-          'was expected, $firstCall, was called with the following stack:',
-          firstCall,
-        );
-    }
+    checkMethod(call, #save);
     int depth = 1;
     while (depth > 0) {
       if (!call.moveNext())