AnimatedOpacity

This provides an easy way to do implicit opacity transitions.

packages/flutter/lib/src/widgets/basic.dart

@@ -55,18 +55,26 @@ export 'package:flutter/rendering.dart' show
 /// This class paints its child into an intermediate buffer and then blends the
 /// child back into the scene partially transparent.
 ///
-/// This class is relatively expensive because it requires painting the child
-/// into an intermediate buffer.
+/// For values of opacity other than 0.0 and 1.0, this class is relatively
+/// expensive because it requires painting the child into an intermediate
+/// buffer. For the value 0.0, the child is simply not painted at all. For the
+/// value 1.0, the child is painted immediately without an intermediate buffer.
 class Opacity extends SingleChildRenderObjectWidget {
   Opacity({ Key key, this.opacity, Widget child })
     : super(key: key, child: child) {
-    assert(opacity >= 0.0 && opacity <= 1.0);
+    assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
   }
 
   /// The fraction to scale the child's alpha value.
   ///
   /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
   /// (i.e., invisible).
+  ///
+  /// The opacity must not be null.
+  ///
+  /// Values 1.0 and 0.0 are painted with a fast path. Other values
+  /// require painting the child into an intermediate buffer, which is
+  /// expensive.
   final double opacity;
 
   @override

packages/flutter/lib/src/widgets/implicit_animations.dart

@@ -382,6 +382,23 @@ class AnimatedPositioned extends ImplicitlyAnimatedWidget {
 
   @override
   _AnimatedPositionedState createState() => new _AnimatedPositionedState();
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    if (left != null)
+      description.add('left: $left');
+    if (top != null)
+      description.add('top: $top');
+    if (right != null)
+      description.add('right: $right');
+    if (bottom != null)
+      description.add('bottom: $bottom');
+    if (width != null)
+      description.add('width: $width');
+    if (height != null)
+      description.add('height: $height');
+  }
 }
 
 class _AnimatedPositionedState extends AnimatedWidgetBaseState<AnimatedPositioned> {
@@ -433,3 +450,57 @@ class _AnimatedPositionedState extends AnimatedWidgetBaseState<AnimatedPositione
       description.add('has height');
   }
 }
+
+/// Animated version of [Opacity] which automatically transitions the child's
+/// opacity over a given duration whenever the given opacity changes.
+///
+/// Animating an opacity is relatively expensive.
+class AnimatedOpacity extends ImplicitlyAnimatedWidget {
+  AnimatedOpacity({
+    Key key,
+    this.child,
+    this.opacity,
+    Curve curve: Curves.linear,
+    Duration duration
+  }) : super(key: key, curve: curve, duration: duration) {
+    assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
+  }
+
+  /// The widget below this widget in the tree.
+  final Widget child;
+
+  /// The target opacity.
+  ///
+  /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
+  /// (i.e., invisible).
+  ///
+  /// The opacity must not be null.
+  final double opacity;
+
+  @override
+  _AnimatedOpacityState createState() => new _AnimatedOpacityState();
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('opacity: $opacity');
+  }
+}
+
+class _AnimatedOpacityState extends AnimatedWidgetBaseState<AnimatedOpacity> {
+  Tween<double> _opacity;
+
+  @override
+  void forEachTween(TweenVisitor<dynamic> visitor) {
+    // TODO(ianh): Use constructor tear-offs when it becomes possible
+    _opacity = visitor(_opacity, config.opacity, (dynamic value) => new Tween<double>(begin: value));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Opacity(
+      opacity: _opacity.evaluate(animation),
+      child: config.child
+    );
+  }
+}