Add support for discrete material sliders

Fixes #1541

examples/material_gallery/lib/demo/slider_demo.dart

@@ -11,53 +11,44 @@ class SliderDemo extends StatefulWidget {
 
 class _SliderDemoState extends State<SliderDemo> {
   double _value = 25.0;
+  double _discreteValue = 20.0;
 
   @override
   Widget build(BuildContext context) {
     return new Scaffold(
-      appBar: new AppBar(title: new Text("Sliders")),
-      body: new Block(children: <Widget>[
-        new Container(
-          height: 100.0,
-          child: new Center(
-            child:  new Row(
-              children: <Widget>[
-                new Slider(
-                  value: _value,
-                  min: 0.0,
-                  max: 100.0,
-                  onChanged: (double value) {
-                    setState(() {
-                      _value = value;
-                    });
-                  }
-                ),
-                new Container(
-                  padding: const EdgeInsets.symmetric(horizontal: 16.0),
-                  child: new Text(_value.round().toString().padLeft(3, '0'))
-                ),
-              ],
-              mainAxisAlignment: MainAxisAlignment.collapse
+      appBar: new AppBar(title: new Text('Sliders')),
+      body: new Column(
+        mainAxisAlignment: MainAxisAlignment.spaceAround,
+        children: <Widget>[
+          new Center(
+            child: new Slider(
+              value: _value,
+              min: 0.0,
+              max: 100.0,
+              onChanged: (double value) {
+                setState(() {
+                  _value = value;
+                });
+              }
             )
-          )
-        ),
-        new Container(
-          height: 100.0,
-          child: new Center(
-            child:  new Row(
-              children: <Widget>[
-                // Disabled, but tracking the slider above.
-                new Slider(value: _value / 100.0),
-                new Container(
-                  padding: const EdgeInsets.symmetric(horizontal: 16.0),
-                  child: new Text((_value / 100.0).toStringAsFixed(2))
-                ),
-              ],
-              mainAxisAlignment: MainAxisAlignment.collapse
+          ),
+          new Center(child: new Slider(value: _value / 100.0)),
+          new Center(
+            child: new Slider(
+              value: _discreteValue,
+              min: 0.0,
+              max: 100.0,
+              divisions: 5,
+              label: '${_discreteValue.round()}',
+              onChanged: (double value) {
+                setState(() {
+                  _discreteValue = value;
+                });
+              }
             )
-          )
-        )
-      ])
+          ),
+        ]
+      )
     );
   }
 }

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

@@ -259,16 +259,9 @@ List<TextPainter> _initPainters(List<String> labels) {
   List<TextPainter> painters = new List<TextPainter>(labels.length);
   for (int i = 0; i < painters.length; ++i) {
     String label = labels[i];
-    TextPainter painter = new TextPainter(
+    painters[i] = new TextPainter(
       new TextSpan(style: style, text: label)
-    );
-    painter
-      ..maxWidth = double.INFINITY
-      ..maxHeight = double.INFINITY
-      ..layout()
-      ..maxWidth = painter.maxIntrinsicWidth
-      ..layout();
-    painters[i] = painter;
+    )..layoutToMaxIntrinsicWidth();
   }
   return painters;
 }

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

@@ -243,7 +243,7 @@ class TextPainter {
   }
 
   ui.Paragraph _paragraph;
-  bool _needsLayout = true;
+  bool _needsLayout = false;
 
   TextSpan _text;
   /// The (potentially styled) text to paint.
@@ -253,10 +253,15 @@ class TextPainter {
     if (_text == value)
       return;
     _text = value;
-    ui.ParagraphBuilder builder = new ui.ParagraphBuilder();
-    _text.build(builder);
-    _paragraph = builder.build(_text.style?.paragraphStyle ?? new ui.ParagraphStyle());
-    _needsLayout = true;
+    if (_text != null) {
+      ui.ParagraphBuilder builder = new ui.ParagraphBuilder();
+      _text.build(builder);
+      _paragraph = builder.build(_text.style?.paragraphStyle ?? new ui.ParagraphStyle());
+      _needsLayout = true;
+    } else {
+      _paragraph = null;
+      _needsLayout = false;
+    }
   }
 
   /// The minimum width at which to layout the text.
@@ -343,12 +348,33 @@ class TextPainter {
     }
   }
 
+  bool _lastLayoutWasToMaxIntrinsicWidth = false;
+
   /// Computes the visual position of the glyphs for painting the text.
   void layout() {
     if (!_needsLayout)
       return;
     _paragraph.layout();
     _needsLayout = false;
+    _lastLayoutWasToMaxIntrinsicWidth = false;
+  }
+
+  /// Computes the visual position of the glyphs using the unconstrainted max intrinsic width.
+  void layoutToMaxIntrinsicWidth() {
+    if (!_needsLayout && _lastLayoutWasToMaxIntrinsicWidth && width == maxIntrinsicWidth)
+      return;
+    _needsLayout = false;
+    _lastLayoutWasToMaxIntrinsicWidth = true;
+    _paragraph
+      ..minWidth = 0.0
+      ..maxWidth = double.INFINITY
+      ..layout();
+    final double newMaxIntrinsicWidth = maxIntrinsicWidth;
+    _paragraph
+      ..minWidth = newMaxIntrinsicWidth
+      ..maxWidth = newMaxIntrinsicWidth
+      ..layout();
+    assert(width == maxIntrinsicWidth);
   }
 
   /// Paints the text onto the given canvas at the given offset.

packages/flutter/test/material/slider_test.dart

+// 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 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Slider can move when tapped', () {
+    testWidgets((WidgetTester tester) {
+      Key sliderKey = new UniqueKey();
+      double value = 0.0;
+
+      tester.pumpWidget(
+        new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Material(
+              child: new Center(
+                child: new Slider(
+                  key: sliderKey,
+                  value: value,
+                  onChanged: (double newValue) {
+                    setState(() {
+                      value = newValue;
+                    });
+                  }
+                )
+              )
+            );
+          }
+        )
+      );
+
+      expect(value, equals(0.0));
+      tester.tap(tester.findElementByKey(sliderKey));
+      expect(value, equals(0.5));
+      tester.pump(); // No animation should start.
+      expect(Scheduler.instance.transientCallbackCount, equals(0));
+    });
+  });
+
+  test('Slider take on discrete values', () {
+    testWidgets((WidgetTester tester) {
+      Key sliderKey = new UniqueKey();
+      double value = 0.0;
+
+      tester.pumpWidget(
+        new StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return new Material(
+              child: new Center(
+                child: new Slider(
+                  key: sliderKey,
+                  min: 0.0,
+                  max: 100.0,
+                  divisions: 10,
+                  value: value,
+                  onChanged: (double newValue) {
+                    setState(() {
+                      value = newValue;
+                    });
+                  }
+                )
+              )
+            );
+          }
+        )
+      );
+
+      expect(value, equals(0.0));
+      tester.tap(tester.findElementByKey(sliderKey));
+      expect(value, equals(50.0));
+      tester.scroll(tester.findElementByKey(sliderKey), const Offset(5.0, 0.0));
+      expect(value, equals(50.0));
+      tester.scroll(tester.findElementByKey(sliderKey), const Offset(40.0, 0.0));
+      expect(value, equals(80.0));
+
+      tester.pump(); // Starts animation.
+      expect(Scheduler.instance.transientCallbackCount, greaterThan(0));
+      tester.pump(const Duration(milliseconds: 200));
+      tester.pump(const Duration(milliseconds: 200));
+      tester.pump(const Duration(milliseconds: 200));
+      tester.pump(const Duration(milliseconds: 200));
+      // Animation complete.
+      expect(Scheduler.instance.transientCallbackCount, equals(0));
+    });
+  });
+}