Baby steps towards an odeon-like animation system. First victim: Drawer.

sky/sdk/lib/widgets/material.dart

Index: sky/sdk/lib/widgets/material.dart
diff --git a/sky/sdk/lib/widgets/material.dart b/sky/sdk/lib/widgets/material.dart
index 527e055bde83a96951d3e262e55f0aa86ed23c13..9304e9e862af4a1a4151e4f7ba27b56635e8e17b 100644
--- a/sky/sdk/lib/widgets/material.dart
+++ b/sky/sdk/lib/widgets/material.dart
@@ -2,30 +2,52 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import '../base/lerp.dart';
+import '../animation/animated_value.dart';
 import '../painting/box_painter.dart';
 import '../theme/colors.dart' as colors;
 import '../theme/edges.dart';
 import '../theme/shadows.dart';
+import 'animated_component.dart';
 import 'basic.dart';
 import 'default_text_style.dart';
 import 'theme.dart';
 
 export '../theme/edges.dart' show MaterialEdge;
 
-class Material extends Component {
+const double _kAnimateShadowDurationMS = 100.0;
+
+List<BoxShadow> computeShadow(double level) {
+  if (level < 1.0)  // shadows[1] is the first shadow
+    return null;
+
+  int level1 = level.floor();
+  int level2 = level.ceil();
+  double t = level - level1.toDouble();
+
+  List<BoxShadow> shadow = new List<BoxShadow>();
+  for (int i = 0; i < shadows[level1].length; ++i)
+    shadow.add(BoxShadow.lerp(shadows[level1][i], shadows[level2][i], t));
+  return shadow;
+}
+
+class Material extends AnimatedComponent {
 
   Material({
     String key,
     this.child,
     this.edge: MaterialEdge.card,
-    this.level: 0,
+    int level: 0,
     this.color
-  }) : super(key: key);
+  }) : super(key: key) {
+    this.level = new AnimatedValue(level.toDouble());
+    watch(this.level);
+  }
 
-  final Widget child;
-  final int level;
-  final MaterialEdge edge;
-  final Color color;
+  Widget child;
+  MaterialEdge edge;
+  AnimatedValue level;
+  Color color;
 
   Color get backgroundColor {
     if (color != null)
@@ -38,12 +60,23 @@ class Material extends Component {
     }
   }
 
-  // TODO(ianh): we should make this animate level changes and color changes
+  void syncFields(Material source) {
+    child = source.child;
+    edge = source.edge;
+    // TODO(mpcomplete): duration is wrong, because the current level may be
+    // anything. We really want |rate|.
+    if (level.value != source.level.value)
+      level.animateTo(source.level.value.toDouble(), _kAnimateShadowDurationMS);
+    color = source.color;
+    super.syncFields(source);
+  }
+
+  // TODO(mpcomplete): make this animate color changes.
 
   Widget build() {
     return new Container(
       decoration: new BoxDecoration(
-        boxShadow: shadows[level],
+        boxShadow: computeShadow(level.value),
         borderRadius: edges[edge],
         backgroundColor: backgroundColor,
         shape: edge == MaterialEdge.circle ? Shape.circle : Shape.rectangle