Add a basic material slider

The slider in this patch works, but it's missing animations and many subtle
touches.

examples/material_gallery/lib/main.dart

@@ -6,13 +6,15 @@ import 'package:flutter/material.dart';
 
 import 'chip_demo.dart';
 import 'date_picker_demo.dart';
+import 'drop_down_demo.dart';
 import 'gallery_page.dart';
+import 'slider_demo.dart';
 import 'time_picker_demo.dart';
 import 'widget_demo.dart';
-import 'drop_down_demo.dart';
 
 final List<WidgetDemo> _kDemos = <WidgetDemo>[
   kChipDemo,
+  kSliderDemo,
   kDatePickerDemo,
   kTimePickerDemo,
   kDropDownDemo,

examples/material_gallery/lib/slider_demo.dart

+// Copyright 2015 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/material.dart';
+
+import 'widget_demo.dart';
+
+class SliderDemo extends StatefulComponent {
+  _SliderDemoState createState() => new _SliderDemoState();
+}
+
+class _SliderDemoState extends State<SliderDemo> {
+  double _value = 0.25;
+
+  Widget build(BuildContext context) {
+    return new Block([
+      new Container(
+        height: 100.0,
+        child: new Center(
+          child:  new Row([
+            new Slider(
+              value: _value,
+              onChanged: (double value) {
+                setState(() {
+                  _value = value;
+                });
+              }
+            ),
+            new Container(
+              padding: const EdgeDims.symmetric(horizontal: 16.0),
+              child: new Text(_value.toStringAsFixed(2))
+            ),
+          ], justifyContent: FlexJustifyContent.collapse)
+        )
+      )
+    ]);
+  }
+}
+
+final WidgetDemo kSliderDemo = new WidgetDemo(
+  title: 'Sliders',
+  routeName: '/sliders',
+  builder: (_) => new SliderDemo()
+);

packages/flutter/lib/material.dart

@@ -45,6 +45,7 @@ export 'src/material/raised_button.dart';
 export 'src/material/scaffold.dart';
 export 'src/material/scrollbar_painter.dart';
 export 'src/material/shadows.dart';
+export 'src/material/slider.dart';
 export 'src/material/snack_bar.dart';
 export 'src/material/switch.dart';
 export 'src/material/tabs.dart';

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

+// Copyright 2015 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/gestures.dart';
+import 'package:flutter/painting.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+
+import 'colors.dart';
+import 'theme.dart';
+
+const double _kThumbRadius = 6.0;
+const double _kReactionRadius = 16.0;
+const double _kTrackWidth = 144.0;
+const int _kReactionAlpha = 0x33;
+
+class Slider extends StatelessComponent {
+  Slider({ Key key, this.value, this.onChanged })
+      : super(key: key);
+
+  final double value;
+  final ValueChanged<double> onChanged;
+
+  Widget build(BuildContext context) {
+    return new _SliderRenderObjectWidget(
+      value: value,
+      primaryColor: Theme.of(context).accentColor,
+      onChanged: onChanged
+    );
+  }
+}
+
+class _SliderRenderObjectWidget extends LeafRenderObjectWidget {
+  _SliderRenderObjectWidget({ Key key, this.value, this.primaryColor, this.onChanged })
+      : super(key: key);
+
+  final double value;
+  final Color primaryColor;
+  final ValueChanged<double> onChanged;
+
+  _RenderSlider createRenderObject() => new _RenderSlider(
+    value: value,
+    primaryColor: primaryColor,
+    onChanged: onChanged
+  );
+
+  void updateRenderObject(_RenderSlider renderObject, _SliderRenderObjectWidget oldWidget) {
+    renderObject.value = value;
+    renderObject.primaryColor = primaryColor;
+    renderObject.onChanged = onChanged;
+  }
+}
+
+final Color _kInactiveTrackColor = Colors.grey[400];
+final Color _kActiveTrackColor = Colors.grey[500];
+
+class _RenderSlider extends RenderConstrainedBox {
+  _RenderSlider({
+    double value,
+    Color primaryColor,
+    this.onChanged
+  }) : _value = value,
+       _primaryColor = primaryColor,
+        super(additionalConstraints: const BoxConstraints.tightFor(width: _kTrackWidth + 2 * _kReactionRadius, height: 2 * _kReactionRadius)) {
+    assert(value != null && value >= 0.0 && value <= 1.0);
+    _drag = new HorizontalDragGestureRecognizer(router: FlutterBinding.instance.pointerRouter)
+      ..onStart = _handleDragStart
+      ..onUpdate = _handleDragUpdate
+      ..onEnd = _handleDragEnd;
+  }
+
+  double get value => _value;
+  double _value;
+  void set value(double newValue) {
+    assert(newValue != null && newValue >= 0.0 && newValue <= 1.0);
+    if (newValue == _value)
+      return;
+    _value = newValue;
+    markNeedsPaint();
+  }
+
+  Color get primaryColor => _primaryColor;
+  Color _primaryColor;
+  void set primaryColor(Color value) {
+    if (value == _primaryColor)
+      return;
+    _primaryColor = value;
+    markNeedsPaint();
+  }
+
+  ValueChanged<double> onChanged;
+
+  double get _trackLength => size.width - 2.0 * _kReactionRadius;
+
+  HorizontalDragGestureRecognizer _drag;
+  bool _active = false;
+  double _currentDragValue = 0.0;
+
+  void _handleDragStart(Point globalPosition) {
+    if (onChanged != null) {
+      _active = true;
+      _currentDragValue = globalToLocal(globalPosition).x / _trackLength;
+      onChanged(_currentDragValue.clamp(0.0, 1.0));
+      markNeedsPaint();
+    }
+  }
+
+  void _handleDragUpdate(double delta) {
+    if (onChanged != null) {
+      _currentDragValue += delta / _trackLength;
+      onChanged(_currentDragValue.clamp(0.0, 1.0));
+    }
+  }
+
+  void _handleDragEnd(Offset velocity) {
+    if (_active) {
+      _active = false;
+      _currentDragValue = 0.0;
+      markNeedsPaint();
+    }
+  }
+
+  bool hitTestSelf(Point position) => true;
+
+  void handleEvent(InputEvent event, BoxHitTestEntry entry) {
+    if (event.type == 'pointerdown' && onChanged != null)
+      _drag.addPointer(event);
+  }
+
+  void paint(PaintingContext context, Offset offset) {
+    final Canvas canvas = context.canvas;
+
+    final double trackLength = _trackLength;
+
+    double trackCenter = offset.dy + size.height / 2.0;
+    double trackLeft = offset.dx + _kReactionRadius;
+    double trackTop = trackCenter - 1.0;
+    double trackBottom = trackCenter + 1.0;
+    double trackRight = trackLeft + trackLength;
+    double trackActive = trackLeft + trackLength * value;
+
+    Paint primaryPaint = new Paint()..color = _primaryColor;
+    Paint trackPaint = new Paint()..color = _active ? _kActiveTrackColor : _kInactiveTrackColor;
+
+    canvas.drawRect(new Rect.fromLTRB(trackLeft, trackTop, trackRight, trackBottom), trackPaint);
+    canvas.drawRect(new Rect.fromLTRB(trackLeft, trackTop, trackActive, trackBottom), primaryPaint);
+
+    Point activeLocation = new Point(trackActive, trackCenter);
+    if (_active) {
+      Paint reactionPaint = new Paint()..color = _primaryColor.withAlpha(_kReactionAlpha);
+      canvas.drawCircle(activeLocation, _kReactionRadius, reactionPaint);
+    }
+    canvas.drawCircle(activeLocation, _kThumbRadius, trackPaint);
+  }
+}