Slider Improvements

Have the widget support min/max arguments to make the widget easier to
use. Also, fix the dragging so it's actually where the knob is.

examples/material_gallery/lib/demo/slider_demo.dart

@@ -11,13 +11,9 @@ class SliderDemo extends StatefulComponent {
 }
 
 class _SliderDemoState extends State<SliderDemo> {
-  double _value = 0.25;
+  double _value = 25.0;
 
   Widget build(BuildContext context) {
-    Widget label = new Container(
-      padding: const EdgeDims.symmetric(horizontal: 16.0),
-      child: new Text(_value.toStringAsFixed(2))
-    );
     return new Block([
       new Container(
         height: 100.0,
@@ -25,13 +21,18 @@ class _SliderDemoState extends State<SliderDemo> {
           child:  new Row([
             new Slider(
               value: _value,
+              min: 0.0,
+              max: 100.0,
               onChanged: (double value) {
                 setState(() {
                   _value = value;
                 });
               }
             ),
-            label,
+            new Container(
+              padding: const EdgeDims.symmetric(horizontal: 16.0),
+              child: new Text(_value.round().toString().padLeft(3, '0'))
+            ),
           ], justifyContent: FlexJustifyContent.collapse)
         )
       ),
@@ -40,8 +41,11 @@ class _SliderDemoState extends State<SliderDemo> {
         child: new Center(
           child:  new Row([
             // Disabled, but tracking the slider above.
-            new Slider(value: _value),
-            label,
+            new Slider(value: _value / 100.0),
+            new Container(
+              padding: const EdgeDims.symmetric(horizontal: 16.0),
+              child: new Text((_value / 100.0).toStringAsFixed(2))
+            ),
           ], justifyContent: FlexJustifyContent.collapse)
         )
       )

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

@@ -13,17 +13,34 @@ import 'constants.dart';
 import 'theme.dart';
 
 class Slider extends StatelessComponent {
-  Slider({ Key key, this.value, this.onChanged })
-      : super(key: key);
+  Slider({
+    Key key,
+    this.value,
+    this.min: 0.0,
+    this.max: 1.0,
+    this.onChanged
+  }) : super(key: key) {
+    assert(value != null);
+    assert(min != null);
+    assert(max != null);
+    assert(value >= min && value <= max);
+  }
 
   final double value;
+  final double min;
+  final double max;
   final ValueChanged<double> onChanged;
 
+  void _handleChanged(double value) {
+    assert(onChanged != null);
+    onChanged(value * (max - min) + min);
+  }
+
   Widget build(BuildContext context) {
     return new _SliderRenderObjectWidget(
-      value: value,
+      value: (value - min) / (max - min),
       primaryColor: Theme.of(context).accentColor,
-      onChanged: onChanged
+      onChanged: onChanged != null ? _handleChanged : null
     );
   }
 }
@@ -106,7 +123,7 @@ class _RenderSlider extends RenderConstrainedBox {
   void _handleDragStart(Point globalPosition) {
     if (onChanged != null) {
       _active = true;
-      _currentDragValue = globalToLocal(globalPosition).x / _trackLength;
+      _currentDragValue = (globalToLocal(globalPosition).x - _kReactionRadius) / _trackLength;
       onChanged(_currentDragValue.clamp(0.0, 1.0));
       _reaction.forward();
       markNeedsPaint();