AnimatedContainer: generalized Container widget that handles animating values

sky/sdk/lib/widgets/animated_container.dart

Index: sky/sdk/lib/widgets/animated_container.dart
diff --git a/sky/sdk/lib/widgets/animated_container.dart b/sky/sdk/lib/widgets/animated_container.dart
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+++ b/sky/sdk/lib/widgets/animated_container.dart
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+// 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 'dart:async';
+
+import 'package:vector_math/vector_math.dart';
+
+import '../animation/animation_performance.dart';
+import '../animation/curves.dart';
+import '../base/lerp.dart';
+import '../painting/box_painter.dart';
+import '../theme/shadows.dart';
+import 'animated_component.dart';
+import 'basic.dart';
+
+// This class builds a Container object from a collection of optionally-
+// animated properties. Use syncFields to update the Container's properties,
+// which will optionally animate them using an AnimationPerformance.
+class AnimatedContainer {
+  AnimatedType<Point> position;
+  AnimatedType<double> shadow;
+  AnimatedColor backgroundColor;
+
+  Map<AnimatedVariable, AnimationPerformance> _variableToPerformance =
+      new Map<AnimatedVariable, AnimationPerformance>();
+
+  // TODO(mpcomplete): don't rely on AnimatedComponent.
+  AnimatedComponent _component;

[abarth-chromium, 2015/07/09 17:51:46]

We shouldn't have an animated component...

[Matt Perry, 2015/07/09 17:59:17]

OK. It makes the client code a bit more complicated since they have to watch()
each performance. But I'll try to find a better way around that.

+  AnimatedContainer([this._component]);
+
+  AnimationPerformance createPerformance(AnimatedType variable,
+                                         {Duration duration}) {
+    AnimationPerformance performance = new AnimationPerformance()
+      ..duration = duration
+      ..variable = variable;
+    _variableToPerformance[variable] = performance;
+    if (_component != null)
+      _component.watch(performance.timeline);
+    return performance;
+  }
+
+  Widget build(Widget child) {
+    Widget current = child;
+    if (shadow != null || backgroundColor != null) {
+      current = new DecoratedBox(
+        decoration: new BoxDecoration(
+          boxShadow: shadow != null ? _computeShadow(shadow.value) : null,
+          backgroundColor: backgroundColor != null ? backgroundColor.value : null),
+        child: current);
+    }
+
+    if (position != null) {
+      Matrix4 transform = new Matrix4.identity();
+      transform.translate(position.value.x, position.value.y);
+      current = new Transform(transform: transform, child: child);
+    }
+
+    return current;
+  }
+
+  void syncFields(AnimatedContainer source) {
+    _syncField(position, source.position);
+    _syncField(shadow, source.shadow);
+    _syncField(backgroundColor, source.backgroundColor);
+  }
+
+  void _syncField(AnimatedType variable, AnimatedType sourceVariable) {
+    if (variable == null)
+      return;  // TODO(mpcomplete): Should we handle transition from null?
+
+    AnimationPerformance performance = _variableToPerformance[variable];
+    if (performance == null) {
+      // If there's no performance, no need to animate.
+      if (sourceVariable != null)
+        variable.value = sourceVariable.value;
+      return;
+    }
+
+    if (variable.value != sourceVariable.value) {
+      variable
+        ..begin = variable.value
+        ..end = sourceVariable.value;
+      performance
+        ..progress = 0.0
+        ..play();
+    }
+  }
+}
+
+class AnimatedColor extends AnimatedType<Color> {
+  AnimatedColor(Color begin, {Color end, Curve curve: linear})
+      : super(begin, end: end, curve: curve);
+
+  void setFraction(double t) {
+    value = lerpColor(begin, end, t);
+  }
+}
+
+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(lerpBoxShadow(shadows[level1][i], shadows[level2][i], t));
+  return shadow;
+}