Add Cupertino scrollbar (#13290)

* Create CupertinoScrollbar

* handle main axis margin

* Adaptive material scrollbar and tests

* Small tweaks

* reapply changes on head

* Docs

* start

* Refactored ScrollbarPainter to be more immutable

* fix tests

* fix bug: one animationcontroller pointed to multiple painters

* some docs tweak

* remove unused import

* review

* review

* add dispose

packages/flutter/lib/cupertino.dart

@@ -16,6 +16,7 @@ export 'src/cupertino/icons.dart';
 export 'src/cupertino/nav_bar.dart';
 export 'src/cupertino/page_scaffold.dart';
 export 'src/cupertino/route.dart';
+export 'src/cupertino/scrollbar.dart';
 export 'src/cupertino/slider.dart';
 export 'src/cupertino/switch.dart';
 export 'src/cupertino/tab_scaffold.dart';

packages/flutter/lib/src/cupertino/scrollbar.dart

@@ -0,0 +1,140 @@
+// Copyright 2017 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:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+// All values eyeballed.
+const Color _kScrollbarColor = const Color(0x99777777);
+const double _kScrollbarThickness = 2.5;
+const double _kScrollbarMainAxisMargin = 4.0;
+const double _kScrollbarCrossAxisMargin = 2.5;
+const double _kScrollbarMinLength = 4.0;
+const Radius _kScrollbarRadius = const Radius.circular(1.25);
+const Duration _kScrollbarTimeToFade = const Duration(milliseconds: 50);
+const Duration _kScrollbarFadeDuration = const Duration(milliseconds: 250);
+
+/// A iOS style scrollbar.
+///
+/// A scrollbar indicates which portion of a [Scrollable] widget is actually
+/// visible.
+///
+/// To add a scrollbar to a [ScrollView], simply wrap the scroll view widget in
+/// a [CupertinoScrollbar] widget.
+///
+/// See also:
+///
+///  * [ListView], which display a linear, scrollable list of children.
+///  * [GridView], which display a 2 dimensional, scrollable array of children.
+///  * [Scrollbar], a Material Design scrollbar that dynamically adapts to the
+///    platform showing either an Android style or iOS style scrollbar.
+class CupertinoScrollbar extends StatefulWidget {
+  /// Creates an iOS style scrollbar that wraps the given [child].
+  ///
+  /// The [child] should be a source of [ScrollNotification] notifications,
+  /// typically a [Scrollable] widget.
+  const CupertinoScrollbar({
+    Key key,
+    @required this.child,
+  }) : super(key: key);
+
+  /// The subtree to place inside the [CupertinoScrollbar].
+  ///
+  /// This should include a source of [ScrollNotification] notifications,
+  /// typically a [Scrollable] widget.
+  final Widget child;
+
+  @override
+  _CupertinoScrollbarState createState() => new _CupertinoScrollbarState();
+}
+
+class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProviderStateMixin {
+  ScrollbarPainter _painter;
+  TextDirection _textDirection;
+
+  AnimationController _fadeoutAnimationController;
+  Animation<double> _fadeoutOpacityAnimation;
+  Timer _fadeoutTimer;
+
+  @override
+  void initState() {
+    super.initState();
+    _fadeoutAnimationController = new AnimationController(
+      vsync: this,
+      duration: _kScrollbarFadeDuration,
+    );
+    _fadeoutOpacityAnimation = new CurvedAnimation(
+      parent: _fadeoutAnimationController,
+      curve: Curves.fastOutSlowIn
+    );
+  }
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    _textDirection = Directionality.of(context);
+    _painter = _buildCupertinoScrollbarPainter();
+  }
+
+  /// Returns a [ScrollbarPainter] visually styled like the iOS scrollbar.
+  ScrollbarPainter _buildCupertinoScrollbarPainter() {
+    return new ScrollbarPainter(
+      color: _kScrollbarColor,
+      textDirection: _textDirection,
+      thickness: _kScrollbarThickness,
+      fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
+      mainAxisMargin: _kScrollbarMainAxisMargin,
+      crossAxisMargin: _kScrollbarCrossAxisMargin,
+      radius: _kScrollbarRadius,
+      minLength: _kScrollbarMinLength,
+    );
+  }
+
+  bool _handleScrollNotification(ScrollNotification notification) {
+    if (notification is ScrollUpdateNotification ||
+        notification is OverscrollNotification) {
+      // Any movements always makes the scrollbar start showing up.
+      if (_fadeoutAnimationController.status != AnimationStatus.forward) {
+        _fadeoutAnimationController.forward();
+      }
+
+      _fadeoutTimer?.cancel();
+      _painter.update(notification.metrics, notification.metrics.axisDirection);
+    } else if (notification is ScrollEndNotification) {
+      // On iOS, the scrollbar can only go away once the user lifted the finger.
+
+      _fadeoutTimer?.cancel();
+      _fadeoutTimer = new Timer(_kScrollbarTimeToFade, () {
+        _fadeoutAnimationController.reverse();
+        _fadeoutTimer = null;
+      });
+    }
+    return false;
+  }
+
+  @override
+  void dispose() {
+    _fadeoutAnimationController.dispose();
+    _fadeoutTimer?.cancel();
+    _painter.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new NotificationListener<ScrollNotification>(
+      onNotification: _handleScrollNotification,
+      child: new RepaintBoundary(
+        child: new CustomPaint(
+          foregroundPainter: _painter,
+          child: new RepaintBoundary(
+            child: widget.child,
+          ),
+        ),
+      ),
+    );
+  }
+}

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

@@ -3,19 +3,25 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:math' as math;
 
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
-import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 
+const double _kScrollbarThickness = 6.0;
+const Duration _kScrollbarFadeDuration = const Duration(milliseconds: 300);
+const Duration _kScrollbarTimeToFade = const Duration(milliseconds: 600);
+
 /// A material design scrollbar.
 ///
 /// A scrollbar indicates which portion of a [Scrollable] widget is actually
 /// visible.
 ///
+/// Dynamically changes to a iOS style scrollbar that looks like
+/// [CupertinoScrollbar] on iOS platform.
+///
 /// To add a scrollbar to a [ScrollView], simply wrap the scroll view widget in
 /// a [Scrollbar] widget.
 ///
@@ -45,182 +51,112 @@ class Scrollbar extends StatefulWidget {
   _ScrollbarState createState() => new _ScrollbarState();
 }
 
+
 class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
-  _ScrollbarPainter _painter;
+  ScrollbarPainter _materialPainter;
+  TargetPlatform _currentPlatform;
+  TextDirection _textDirection;
+  Color _themeColor;
+
+  AnimationController _fadeoutAnimationController;
+  Animation<double> _fadeoutOpacityAnimation;
+  Timer _fadeoutTimer;
+
+  @override
+  void initState() {
+    super.initState();
+    _fadeoutAnimationController = new AnimationController(
+      vsync: this,
+      duration: _kScrollbarFadeDuration,
+    );
+    _fadeoutOpacityAnimation = new CurvedAnimation(
+      parent: _fadeoutAnimationController,
+      curve: Curves.fastOutSlowIn
+    );
+  }
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _painter ??= new _ScrollbarPainter(this);
-    _painter
-      ..color = Theme.of(context).highlightColor
-      ..textDirection = Directionality.of(context);
+
+    final ThemeData theme = Theme.of(context);
+    _currentPlatform = theme.platform;
+
+    switch (_currentPlatform) {
+      case TargetPlatform.iOS:
+        // On iOS, stop all local animations. CupertinoScrollbar has its own
+        // animations.
+        _fadeoutTimer?.cancel();
+        _fadeoutTimer = null;
+        _fadeoutAnimationController.reset();
+        break;
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        _themeColor = theme.highlightColor.withOpacity(1.0);
+        _textDirection = Directionality.of(context);
+        _materialPainter = _buildMaterialScrollbarPainter();
+        break;
+    }
+  }
+
+  ScrollbarPainter _buildMaterialScrollbarPainter() {
+    return new ScrollbarPainter(
+        color: _themeColor,
+        textDirection: _textDirection,
+        thickness: _kScrollbarThickness,
+        fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
+      );
   }
 
   bool _handleScrollNotification(ScrollNotification notification) {
-    if (notification is ScrollUpdateNotification ||
-        notification is OverscrollNotification)
-      _painter.update(notification.metrics, notification.metrics.axisDirection);
+    // iOS sub-delegates to the CupertinoScrollbar instead and doesn't handle
+    // scroll notifications here.
+    if (_currentPlatform != TargetPlatform.iOS
+        && (notification is ScrollUpdateNotification
+            || notification is OverscrollNotification)) {
+      if (_fadeoutAnimationController.status != AnimationStatus.forward) {
+        _fadeoutAnimationController.forward();
+      }
+
+      _materialPainter.update(notification.metrics, notification.metrics.axisDirection);
+      _fadeoutTimer?.cancel();
+      _fadeoutTimer = new Timer(_kScrollbarTimeToFade, () {
+        _fadeoutAnimationController.reverse();
+        _fadeoutTimer = null;
+      });
+    }
     return false;
   }
 
   @override
   void dispose() {
-    _painter.dispose();
+    _fadeoutAnimationController.dispose();
+    _fadeoutTimer?.cancel();
+    _materialPainter?.dispose();
     super.dispose();
   }
 
   @override
   Widget build(BuildContext context) {
-    return new NotificationListener<ScrollNotification>(
-      onNotification: _handleScrollNotification,
-      // TODO(ianh): Maybe we should try to collapse out these repaint
-      // boundaries when the scroll bars are invisible.
-      child: new RepaintBoundary(
-        child: new CustomPaint(
-          foregroundPainter: _painter,
+    switch (_currentPlatform) {
+      case TargetPlatform.iOS:
+        return new CupertinoScrollbar(
+          child: widget.child,
+        );
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        return new NotificationListener<ScrollNotification>(
+          onNotification: _handleScrollNotification,
           child: new RepaintBoundary(
-            child: widget.child,
+            child: new CustomPaint(
+              foregroundPainter: _materialPainter,
+              child: new RepaintBoundary(
+                child: widget.child,
+              ),
+            ),
           ),
-        ),
-      ),
-    );
+        );
+    }
+    throw new FlutterError('Unknown platform for scrollbar insertion');
   }
 }
-
-class _ScrollbarPainter extends ChangeNotifier implements CustomPainter {
-  _ScrollbarPainter(TickerProvider vsync)
-    : assert(vsync != null) {
-    _fadeController = new AnimationController(duration: _kThumbFadeDuration, vsync: vsync);
-    _opacity = new CurvedAnimation(parent: _fadeController, curve: Curves.fastOutSlowIn)
-      ..addListener(notifyListeners);
-  }
-
-  // animation of the main axis direction
-  AnimationController _fadeController;
-  Animation<double> _opacity;
-
-  // fade-out timer
-  Timer _fadeOut;
-
-  Color get color => _color;
-  Color _color;
-  set color(Color value) {
-    assert(value != null);
-    if (_color == value)
-      return;
-    _color = value;
-    notifyListeners();
-  }
-
-  TextDirection get textDirection => _textDirection;
-  TextDirection _textDirection;
-  set textDirection(TextDirection value) {
-    assert(value != null);
-    if (_textDirection == value)
-      return;
-    _textDirection = value;
-    notifyListeners();
-  }
-
-  @override
-  void dispose() {
-    _fadeOut?.cancel();
-    _fadeController.dispose();
-    super.dispose();
-  }
-
-  ScrollMetrics _lastMetrics;
-  AxisDirection _lastAxisDirection;
-
-  static const double _kMinThumbExtent = 18.0;
-  static const double _kThumbGirth = 6.0;
-  static const Duration _kThumbFadeDuration = const Duration(milliseconds: 300);
-  static const Duration _kFadeOutTimeout = const Duration(milliseconds: 600);
-
-  void update(ScrollMetrics metrics, AxisDirection axisDirection) {
-    _lastMetrics = metrics;
-    _lastAxisDirection = axisDirection;
-    if (_fadeController.status == AnimationStatus.completed) {
-      notifyListeners();
-    } else if (_fadeController.status != AnimationStatus.forward) {
-      _fadeController.forward();
-    }
-    _fadeOut?.cancel();
-    _fadeOut = new Timer(_kFadeOutTimeout, startFadeOut);
-  }
-
-  void startFadeOut() {
-    _fadeOut = null;
-    _fadeController.reverse();
-  }
-
-  Paint get _paint => new Paint()..color = color.withOpacity(_opacity.value);
-
-  double _getThumbX(Size size) {
-    assert(textDirection != null);
-    switch (textDirection) {
-      case TextDirection.rtl:
-        return 0.0;
-      case TextDirection.ltr:
-        return size.width - _kThumbGirth;
-    }
-    return null;
-  }
-
-  void _paintVerticalThumb(Canvas canvas, Size size, double thumbOffset, double thumbExtent) {
-    final Offset thumbOrigin = new Offset(_getThumbX(size), thumbOffset);
-    final Size thumbSize = new Size(_kThumbGirth, thumbExtent);
-    canvas.drawRect(thumbOrigin & thumbSize, _paint);
-  }
-
-  void _paintHorizontalThumb(Canvas canvas, Size size, double thumbOffset, double thumbExtent) {
-    final Offset thumbOrigin = new Offset(thumbOffset, size.height - _kThumbGirth);
-    final Size thumbSize = new Size(thumbExtent, _kThumbGirth);
-    canvas.drawRect(thumbOrigin & thumbSize, _paint);
-  }
-
-  void _paintThumb(double before, double inside, double after, double viewport, Canvas canvas, Size size,
-                   void painter(Canvas canvas, Size size, double thumbOffset, double thumbExtent)) {
-    double thumbExtent = math.min(viewport, _kMinThumbExtent);
-    if (before + inside + after > 0.0)
-      thumbExtent = math.max(thumbExtent, viewport * inside / (before + inside + after));
-
-    final double thumbOffset = (before + after > 0.0) ?
-        before * (viewport - thumbExtent) / (before + after) : 0.0;
-
-    painter(canvas, size, thumbOffset, thumbExtent);
-  }
-
-  @override
-  void paint(Canvas canvas, Size size) {
-    if (_lastAxisDirection == null || _lastMetrics == null || _opacity.value == 0.0)
-      return;
-    switch (_lastAxisDirection) {
-      case AxisDirection.down:
-        _paintThumb(_lastMetrics.extentBefore, _lastMetrics.extentInside, _lastMetrics.extentAfter, size.height, canvas, size, _paintVerticalThumb);
-        break;
-      case AxisDirection.up:
-        _paintThumb(_lastMetrics.extentAfter, _lastMetrics.extentInside, _lastMetrics.extentBefore, size.height, canvas, size, _paintVerticalThumb);
-        break;
-      case AxisDirection.right:
-        _paintThumb(_lastMetrics.extentBefore, _lastMetrics.extentInside, _lastMetrics.extentAfter, size.width, canvas, size, _paintHorizontalThumb);
-        break;
-      case AxisDirection.left:
-        _paintThumb(_lastMetrics.extentAfter, _lastMetrics.extentInside, _lastMetrics.extentBefore, size.width, canvas, size, _paintHorizontalThumb);
-        break;
-    }
-  }
-
-  @override
-  bool hitTest(Offset position) => null;
-
-  @override
-  bool shouldRepaint(_ScrollbarPainter oldDelegate) => false;
-
-  @override
-  bool shouldRebuildSemantics(CustomPainter oldDelegate) => false;
-
-  @override
-  SemanticsBuilderCallback get semanticsBuilder => null;
-}

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

@@ -0,0 +1,222 @@
+// Copyright 2016 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 'package:flutter/animation.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
+
+import 'scroll_metrics.dart';
+
+const double _kMinThumbExtent = 18.0;
+
+/// A [CustomPainter] for painting scrollbars.
+///
+/// Unlike [CustomPainter]s that subclasses [CustomPainter] and only repaint
+/// when [shouldRepaint] returns true (which requires this [CustomPainter] to
+/// be rebuilt), this painter has the added optimization of repainting and not
+/// rebuilding when:
+///
+///  * the scroll position changes; and
+///  * when the scrollbar fades away.
+///
+/// Calling [update] with the new [ScrollMetrics] will repaint the new scrollbar
+/// position.
+///
+/// Updating the value on the provided [fadeoutOpacityAnimation] will repaint
+/// with the new opacity.
+///
+/// You must call [dispose] on this [ScrollbarPainter] when it's no longer used.
+///
+/// See also:
+///
+///  * [Scrollbar] for a widget showing a scrollbar around a [Scrollable] in the
+///    Material Design style.
+///  * [CupertinoScrollbar] for a widget showing a scrollbar around a
+///    [Scrollable] in the iOS style.
+class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
+  /// Creates a scrollbar with customizations given by construction arguments.
+  ScrollbarPainter({
+    @required this.color,
+    @required this.textDirection,
+    @required this.thickness,
+    @required this.fadeoutOpacityAnimation,
+    this.mainAxisMargin: 0.0,
+    this.crossAxisMargin: 0.0,
+    this.radius,
+    this.minLength: _kMinThumbExtent,
+  }) : assert(color != null),
+       assert(textDirection != null),
+       assert(thickness != null),
+       assert(fadeoutOpacityAnimation != null),
+       assert(mainAxisMargin != null),
+       assert(crossAxisMargin != null),
+       assert(minLength != null) {
+    fadeoutOpacityAnimation.addListener(notifyListeners);
+  }
+
+  /// [Color] of the thumb. Mustn't be null.
+  final Color color;
+
+  /// [TextDirection] of the [BuildContext] which dictates the side of the
+  /// screen the scrollbar appears in (the trailing side). Mustn't be null.
+  final TextDirection textDirection;
+
+  /// Thickness of the scrollbar in its cross-axis in pixels. Mustn't be null.
+  final double thickness;
+
+  /// An opacity [Animation] that dictates the opacity of the thumb.
+  /// Changes in value of this [Listenable] will automatically trigger repaints.
+  /// Mustn't be null.
+  final Animation<double> fadeoutOpacityAnimation;
+
+  /// Distance from the scrollbar's start and end to the edge of the viewport in
+  /// pixels. Mustn't be null.
+  final double mainAxisMargin;
+
+  /// Distance from the scrollbar's side to the nearest edge in pixels. Musn't
+  /// be null.
+  final double crossAxisMargin;
+
+  /// [Radius] of corners if the scrollbar should have rounded corners.
+  ///
+  /// Scrollbar will be rectangular if [radius] is null.
+  final Radius radius;
+
+  /// The smallest size the scrollbar can shrink to when the total scrollable
+  /// extent is large and the current visible viewport is small. Mustn't be
+  /// null.
+  final double minLength;
+
+  ScrollMetrics _lastMetrics;
+  AxisDirection _lastAxisDirection;
+
+  /// Update with new [ScrollMetrics]. The scrollbar will show and redraw itself
+  /// based on these new metrics.
+  ///
+  /// The scrollbar will remain on screen.
+  void update(
+    ScrollMetrics metrics,
+    AxisDirection axisDirection,
+  ) {
+    _lastMetrics = metrics;
+    _lastAxisDirection = axisDirection;
+    notifyListeners();
+  }
+
+  Paint get _paint {
+    return new Paint()..color =
+        color.withOpacity(color.opacity * fadeoutOpacityAnimation.value);
+  }
+
+  double _getThumbX(Size size) {
+    assert(textDirection != null);
+    switch (textDirection) {
+      case TextDirection.rtl:
+        return crossAxisMargin;
+      case TextDirection.ltr:
+        return size.width - thickness - crossAxisMargin;
+    }
+    return null;
+  }
+
+  void _paintVerticalThumb(Canvas canvas, Size size, double thumbOffset, double thumbExtent) {
+    final Offset thumbOrigin = new Offset(_getThumbX(size), thumbOffset);
+    final Size thumbSize = new Size(thickness, thumbExtent);
+    final Rect thumbRect = thumbOrigin & thumbSize;
+    if (radius == null)
+      canvas.drawRect(thumbRect, _paint);
+    else
+      canvas.drawRRect(new RRect.fromRectAndRadius(thumbRect, radius), _paint);
+  }
+
+  void _paintHorizontalThumb(Canvas canvas, Size size, double thumbOffset, double thumbExtent) {
+    final Offset thumbOrigin = new Offset(thumbOffset, size.height - thickness);
+    final Size thumbSize = new Size(thumbExtent, thickness);
+    final Rect thumbRect = thumbOrigin & thumbSize;
+    if (radius == null)
+      canvas.drawRect(thumbRect, _paint);
+    else
+      canvas.drawRRect(new RRect.fromRectAndRadius(thumbRect, radius), _paint);
+  }
+
+  void _paintThumb(
+    double before,
+    double inside,
+    double after,
+    double viewport,
+    Canvas canvas,
+    Size size,
+    void painter(Canvas canvas, Size size, double thumbOffset, double thumbExtent),
+  ) {
+    // Establish the minimum size possible.
+    double thumbExtent = math.min(viewport, minLength);
+    if (before + inside + after > 0.0) {
+      final double fractionVisible = inside / (before + inside + after);
+      thumbExtent = math.max(
+        thumbExtent,
+        viewport * fractionVisible - 2 * mainAxisMargin,
+      );
+    }
+
+    final double fractionPast = before / (before + after);
+    final double thumbOffset = (before + after > 0.0)
+        ? fractionPast * (viewport - thumbExtent - 2 * mainAxisMargin) + mainAxisMargin
+        : mainAxisMargin;
+
+    painter(canvas, size, thumbOffset, thumbExtent);
+  }
+
+  @override
+  void dispose() {
+    fadeoutOpacityAnimation.removeListener(notifyListeners);
+    super.dispose();
+  }
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    if (_lastAxisDirection == null
+        || _lastMetrics == null
+        || fadeoutOpacityAnimation.value == 0.0)
+      return;
+    switch (_lastAxisDirection) {
+      case AxisDirection.down:
+        _paintThumb(_lastMetrics.extentBefore, _lastMetrics.extentInside, _lastMetrics.extentAfter, size.height, canvas, size, _paintVerticalThumb);
+        break;
+      case AxisDirection.up:
+        _paintThumb(_lastMetrics.extentAfter, _lastMetrics.extentInside, _lastMetrics.extentBefore, size.height, canvas, size, _paintVerticalThumb);
+        break;
+      case AxisDirection.right:
+        _paintThumb(_lastMetrics.extentBefore, _lastMetrics.extentInside, _lastMetrics.extentAfter, size.width, canvas, size, _paintHorizontalThumb);
+        break;
+      case AxisDirection.left:
+        _paintThumb(_lastMetrics.extentAfter, _lastMetrics.extentInside, _lastMetrics.extentBefore, size.width, canvas, size, _paintHorizontalThumb);
+        break;
+    }
+  }
+
+  // Scrollbars are (currently) not interactive.
+  @override
+  bool hitTest(Offset position) => null;
+
+  @override
+  bool shouldRepaint(ScrollbarPainter old) {
+    // Should repaint if any properties changed.
+    return color != old.color
+        || textDirection != old.textDirection
+        || thickness != old.thickness
+        || fadeoutOpacityAnimation != old.fadeoutOpacityAnimation
+        || mainAxisMargin != old.mainAxisMargin
+        || crossAxisMargin != old.crossAxisMargin
+        || radius != old.radius
+        || minLength != old.minLength;
+  }
+
+  @override
+  bool shouldRebuildSemantics(CustomPainter oldDelegate) => false;
+
+  @override
+  SemanticsBuilderCallback get semanticsBuilder => null;
+}

packages/flutter/lib/widgets.dart

@@ -77,6 +77,7 @@ export 'src/widgets/scroll_position_with_single_context.dart';
 export 'src/widgets/scroll_simulation.dart';
 export 'src/widgets/scroll_view.dart';
 export 'src/widgets/scrollable.dart';
+export 'src/widgets/scrollbar.dart';
 export 'src/widgets/semantics_debugger.dart';
 export 'src/widgets/single_child_scroll_view.dart';
 export 'src/widgets/size_changed_layout_notifier.dart';

packages/flutter/test/cupertino/scrollbar_paint_test.dart

@@ -0,0 +1,41 @@
+// Copyright 2017 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 'package:flutter/cupertino.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../rendering/mock_canvas.dart';
+
+void main() {
+  testWidgets('Paints iOS spec', (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new CupertinoScrollbar(
+        child: new SingleChildScrollView(
+          child: const SizedBox(width: 4000.0, height: 4000.0),
+        ),
+      ),
+    ));
+    expect(find.byType(CupertinoScrollbar), isNot(paints..rrect()));
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(SingleChildScrollView)));
+    await gesture.moveBy(const Offset(0.0, -10.0));
+    // Move back to original position.
+    await gesture.moveBy(const Offset(0.0, 10.0));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: const Color(0x99777777),
+      rrect: new RRect.fromRectAndRadius(
+        new Rect.fromLTWH(
+          800.0 - 2.5 - 2.5, // Screen width - margin - thickness.
+          4.0, // Initial position is the top margin.
+          2.5, // Thickness.
+          // Fraction in viewport * scrollbar height - top, bottom margin.
+          600.0 / 4000.0 * 600.0 - 4.0 - 4.0,
+        ),
+        const Radius.circular(1.25),
+      ),
+    ));
+  });
+}

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -0,0 +1,45 @@
+// Copyright 2017 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 'package:flutter/cupertino.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../rendering/mock_canvas.dart';
+
+void main() {
+  testWidgets('Scrollbar never goes away until finger lift', (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new CupertinoScrollbar(
+        child: new SingleChildScrollView(
+          child: const SizedBox(width: 4000.0, height: 4000.0),
+        ),
+      ),
+    ));
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(SingleChildScrollView)));
+    await gesture.moveBy(const Offset(0.0, -10.0));
+    await tester.pump();
+    // Scrollbar fully showing
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: const Color(0x99777777),
+    ));
+
+    await tester.pump(const Duration(seconds: 3));
+    await tester.pump(const Duration(seconds: 3));
+    // Still there.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: const Color(0x99777777),
+    ));
+
+    await gesture.up();
+    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 200));
+
+    // Opacity going down now.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: const Color(0x15777777),
+    ));
+  });
+}

packages/flutter/test/material/scrollbar_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../rendering/mock_canvas.dart';
+
 class TestCanvas implements Canvas {
   TestCanvas([this.invocations]);
 
@@ -77,8 +79,16 @@ void main() {
       ),
     ));
 
-    final CustomPaint custom = tester.widget(find.descendant(of: find.byType(Scrollbar), matching: find.byType(CustomPaint)).first);
+    final CustomPaint custom = tester.widget(find.descendant(
+      of: find.byType(Scrollbar),
+      matching: find.byType(CustomPaint)).first
+    );
     final dynamic scrollPainter = custom.foregroundPainter;
+    // Dragging makes the scrollbar first appear.
+    await tester.drag(find.text('0'), const Offset(0.0, -10.0));
+    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 200));
+
     final ScrollMetrics metrics = new FixedScrollMetrics(
       minScrollExtent: 0.0,
       maxScrollExtent: 0.0,
@@ -87,8 +97,6 @@ void main() {
       axisDirection: AxisDirection.down
     );
     scrollPainter.update(metrics, AxisDirection.down);
-    await tester.pump(const Duration(milliseconds: 200));
-    await tester.pump(const Duration(milliseconds: 200));
 
     final List<Invocation> invocations = <Invocation>[];
     final TestCanvas canvas = new TestCanvas(invocations);
@@ -96,4 +104,39 @@ void main() {
     final Rect thumbRect = invocations.single.positionalArguments[0];
     expect(thumbRect.isFinite, isTrue);
   });
+
+  testWidgets('Adaptive scrollbar', (WidgetTester tester) async {
+    Widget viewWithScroll(TargetPlatform platform) {
+      return new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Theme(
+          data: new ThemeData(
+            platform: platform
+          ),
+          child: new Scrollbar(
+            child: new SingleChildScrollView(
+              child: const SizedBox(width: 4000.0, height: 4000.0),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(viewWithScroll(TargetPlatform.android));
+    await tester.drag(find.byType(SingleChildScrollView), const Offset(0.0, -10.0));
+    await tester.pump();
+    // Scrollbar fully showing
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(find.byType(Scrollbar), paints..rect());
+
+    await tester.pumpWidget(viewWithScroll(TargetPlatform.iOS));
+    final TestGesture gesture = await tester.startGesture(
+      tester.getCenter(find.byType(SingleChildScrollView))
+    );
+    await gesture.moveBy(const Offset(0.0, -10.0));
+    await tester.drag(find.byType(SingleChildScrollView), const Offset(0.0, -10.0));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 200));
+    expect(find.byType(Scrollbar), paints..rrect());
+  });
 }