Refactor AnimatedContainer to be reusable.

This will allow AnimatedPositioned to reuse all the same logic.

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

@@ -56,80 +56,31 @@ class AnimatedMatrix4Value extends AnimatedValue<Matrix4> {
   }
 }
 
-/// A container that gradually changes its values over a period of time.
-///
-/// This class is useful for generating simple implicit transitions between
-/// different parameters to [Container]. For more complex animations, you'll
-/// likely want to use a subclass of [Transition] or control a [Performance]
-/// yourself.
-class AnimatedContainer extends StatefulComponent {
-  AnimatedContainer({
+/// An abstract widget for building components that gradually change their
+/// values over a period of time.
+abstract class AnimatedWidgetBase extends StatefulComponent {
+  AnimatedWidgetBase({
     Key key,
-    this.child,
-    this.constraints,
-    this.decoration,
-    this.foregroundDecoration,
-    this.margin,
-    this.padding,
-    this.transform,
-    this.width,
-    this.height,
     this.curve: Curves.linear,
     this.duration
   }) : super(key: key) {
-    assert(margin == null || margin.isNonNegative);
-    assert(padding == null || padding.isNonNegative);
-    assert(decoration == null || decoration.debugAssertValid());
-    assert(foregroundDecoration == null || foregroundDecoration.debugAssertValid());
     assert(curve != null);
     assert(duration != null);
   }
 
-  final Widget child;
-
-  /// Additional constraints to apply to the child.
-  final BoxConstraints constraints;
-
-  /// The decoration to paint behind the child.
-  final Decoration decoration;
-
-  /// The decoration to paint in front of the child.
-  final Decoration foregroundDecoration;
-
-  /// Empty space to surround the decoration.
-  final EdgeDims margin;
-
-  /// Empty space to inscribe inside the decoration.
-  final EdgeDims padding;
-
-  /// The transformation matrix to apply before painting the container.
-  final Matrix4 transform;
-
-  /// If non-null, requires the decoration to have this width.
-  final double width;
-
-  /// If non-null, requires the decoration to have this height.
-  final double height;
-
   /// The curve to apply when animating the parameters of this container.
   final Curve curve;
 
   /// The duration over which to animate the parameters of this container.
   final Duration duration;
 
-  _AnimatedContainerState createState() => new _AnimatedContainerState();
+  AnimatedWidgetBaseState createState();
 }
 
-class _AnimatedContainerState extends State<AnimatedContainer> {
-  AnimatedBoxConstraintsValue _constraints;
-  AnimatedDecorationValue _decoration;
-  AnimatedDecorationValue _foregroundDecoration;
-  AnimatedEdgeDimsValue _margin;
-  AnimatedEdgeDimsValue _padding;
-  AnimatedMatrix4Value _transform;
-  AnimatedValue<double> _width;
-  AnimatedValue<double> _height;
+typedef AnimatedValue<T> VariableConstructor<T>(T targetValue);
+typedef AnimatedValue<T> VariableVisitor<T>(AnimatedValue<T> variable, T targetValue, VariableConstructor<T> constructor);
 
+abstract class AnimatedWidgetBaseState<T extends AnimatedWidgetBase> extends State<T> {
   Performance _performanceController;
   PerformanceView _performance;
 
@@ -143,19 +94,14 @@ class _AnimatedContainerState extends State<AnimatedContainer> {
     _configAllVariables();
   }
 
-  void didUpdateConfig(AnimatedContainer oldConfig) {
+  void didUpdateConfig(T oldConfig) {
     if (config.curve != oldConfig.curve)
       _updateCurve();
     _performanceController.duration = config.duration;
     if (_configAllVariables()) {
-      _updateBeginValue(_constraints);
-      _updateBeginValue(_decoration);
-      _updateBeginValue(_foregroundDecoration);
-      _updateBeginValue(_margin);
-      _updateBeginValue(_padding);
-      _updateBeginValue(_transform);
-      _updateBeginValue(_width);
-      _updateBeginValue(_height);
+      forEachVariable((AnimatedValue variable, dynamic targetValue, VariableConstructor<T> constructor) {
+        _updateBeginValue(variable); return variable;
+      });
       _performanceController.progress = 0.0;
       _performanceController.play();
     }
@@ -182,14 +128,9 @@ class _AnimatedContainerState extends State<AnimatedContainer> {
 
   void _updateAllVariables() {
     setState(() {
-      _updateVariable(_constraints);
-      _updateVariable(_decoration);
-      _updateVariable(_foregroundDecoration);
-      _updateVariable(_margin);
-      _updateVariable(_padding);
-      _updateVariable(_transform);
-      _updateVariable(_width);
-      _updateVariable(_height);
+      forEachVariable((AnimatedValue variable, dynamic targetValue, VariableConstructor<T> constructor) {
+        _updateVariable(variable); return variable;
+      });
     });
   }
 
@@ -206,71 +147,113 @@ class _AnimatedContainerState extends State<AnimatedContainer> {
 
   bool _configAllVariables() {
     bool startAnimation = false;
-    if (config.constraints != null) {
-      _constraints ??= new AnimatedBoxConstraintsValue(config.constraints);
-      if (_updateEndValue(_constraints, config.constraints))
-        startAnimation = true;
-    } else {
-      _constraints = null;
-    }
+    forEachVariable((AnimatedValue variable, dynamic targetValue, VariableConstructor<T> constructor) { 
+      if (targetValue != null) {
+        variable ??= constructor(targetValue);
+        if (_updateEndValue(variable, targetValue))
+          startAnimation = true;
+      } else {
+        variable = null;
+      }
+      return variable;
+    });
+    return startAnimation;
+  }
 
-    if (config.decoration != null) {
-      _decoration ??= new AnimatedDecorationValue(config.decoration);
-      if (_updateEndValue(_decoration, config.decoration))
-        startAnimation = true;
-    } else {
-      _decoration = null;
-    }
+  /// Subclasses must implement this function by running through the following
+  /// steps for for each animatable facet in the class:
+  ///
+  /// 1. Call the visitor callback with three arguments, the first argument
+  /// being the current value of the AnimatedValue<T> object that represents the
+  /// variable (initially null), the second argument, of type T, being the value
+  /// on the Widget (config) that represents the current target value of the
+  /// variable, and the third being a callback that takes a value T (which will
+  /// be the second argument to the visitor callback), and that returns an
+  /// AnimatedValue<T> object for the variable, configured with the given value
+  /// as the begin value.
+  ///
+  /// 2. Take the value returned from the callback, and store it. This is the
+  /// value to use as the current value the next time that the forEachVariable()
+  /// method is called.
+  void forEachVariable(VariableVisitor visitor);
+}
 
-    if (config.foregroundDecoration != null) {
-      _foregroundDecoration ??= new AnimatedDecorationValue(config.foregroundDecoration);
-      if (_updateEndValue(_foregroundDecoration, config.foregroundDecoration))
-        startAnimation = true;
-    } else {
-      _foregroundDecoration = null;
-    }
+/// A container that gradually changes its values over a period of time.
+///
+/// This class is useful for generating simple implicit transitions between
+/// different parameters to [Container]. For more complex animations, you'll
+/// likely want to use a subclass of [Transition] or control a [Performance]
+/// yourself.
+class AnimatedContainer extends AnimatedWidgetBase {
+  AnimatedContainer({
+    Key key,
+    this.child,
+    this.constraints,
+    this.decoration,
+    this.foregroundDecoration,
+    this.margin,
+    this.padding,
+    this.transform,
+    this.width,
+    this.height,
+    Curve curve: Curves.linear,
+    Duration duration
+  }) : super(key: key, curve: curve, duration: duration) {
+    assert(decoration == null || decoration.debugAssertValid());
+    assert(foregroundDecoration == null || foregroundDecoration.debugAssertValid());
+    assert(margin == null || margin.isNonNegative);
+    assert(padding == null || padding.isNonNegative);
+  }
 
-    if (config.margin != null) {
-      _margin ??= new AnimatedEdgeDimsValue(config.margin);
-      if (_updateEndValue(_margin, config.margin))
-        startAnimation = true;
-    } else {
-      _margin = null;
-    }
+  final Widget child;
 
-    if (config.padding != null) {
-      _padding ??= new AnimatedEdgeDimsValue(config.padding);
-      if (_updateEndValue(_padding, config.padding))
-        startAnimation = true;
-    } else {
-      _padding = null;
-    }
+  /// Additional constraints to apply to the child.
+  final BoxConstraints constraints;
 
-    if (config.transform != null) {
-      _transform ??= new AnimatedMatrix4Value(config.transform);
-      if (_updateEndValue(_transform, config.transform))
-        startAnimation = true;
-    } else {
-      _transform = null;
-    }
+  /// The decoration to paint behind the child.
+  final Decoration decoration;
 
-    if (config.width != null) {
-      _width ??= new AnimatedValue<double>(config.width);
-      if (_updateEndValue(_width, config.width))
-        startAnimation = true;
-    } else {
-      _width = null;
-    }
+  /// The decoration to paint in front of the child.
+  final Decoration foregroundDecoration;
 
-    if (config.height != null) {
-      _height ??= new AnimatedValue<double>(config.height);
-      if (_updateEndValue(_height, config.height))
-        startAnimation = true;
-    } else {
-      _height = null;
-    }
+  /// Empty space to surround the decoration.
+  final EdgeDims margin;
 
-    return startAnimation;
+  /// Empty space to inscribe inside the decoration.
+  final EdgeDims padding;
+
+  /// The transformation matrix to apply before painting the container.
+  final Matrix4 transform;
+
+  /// If non-null, requires the decoration to have this width.
+  final double width;
+
+  /// If non-null, requires the decoration to have this height.
+  final double height;
+
+  _AnimatedContainerState createState() => new _AnimatedContainerState();
+}
+
+class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer> {
+  AnimatedBoxConstraintsValue _constraints;
+  AnimatedDecorationValue _decoration;
+  AnimatedDecorationValue _foregroundDecoration;
+  AnimatedEdgeDimsValue _margin;
+  AnimatedEdgeDimsValue _padding;
+  AnimatedMatrix4Value _transform;
+  AnimatedValue<double> _width;
+  AnimatedValue<double> _height;
+
+  void forEachVariable(VariableVisitor visitor) {
+    // TODO(ianh): Use constructor tear-offs when it becomes possible
+    _constraints = visitor(_constraints, config.constraints, (dynamic value) => new AnimatedBoxConstraintsValue(value));
+    _decoration = visitor(_decoration, config.decoration, (dynamic value) => new AnimatedDecorationValue(value));
+    _foregroundDecoration = visitor(_foregroundDecoration, config.foregroundDecoration, (dynamic value) => new AnimatedDecorationValue(value));
+    _margin = visitor(_margin, config.margin, (dynamic value) => new AnimatedEdgeDimsValue(value));
+    _padding = visitor(_padding, config.padding, (dynamic value) => new AnimatedEdgeDimsValue(value));
+    _transform = visitor(_transform, config.transform, (dynamic value) => new AnimatedMatrix4Value(value));
+    _width = visitor(_width, config.width, (dynamic value) => new AnimatedValue<double>(value));
+    _height = visitor(_height, config.height, (dynamic value) => new AnimatedValue<double>(value));
   }
 
   Widget build(BuildContext context) {

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

@@ -1023,15 +1023,15 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
   Positioned({
     Key key,
     Widget child,
+    this.left,
     this.top,
     this.right,
     this.bottom,
-    this.left,
     this.width,
     this.height
   }) : super(key: key, child: child) {
-    assert(top == null || bottom == null || height == null);
     assert(left == null || right == null || width == null);
+    assert(top == null || bottom == null || height == null);
   }
 
   Positioned.fromRect({
@@ -1046,6 +1046,9 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
        bottom = null,
        super(key: key, child: child);
 
+  /// The offset of the child's left edge from the left of the stack.
+  final double left;
+
   /// The offset of the child's top edge from the top of the stack.
   final double top;
 
@@ -1055,17 +1058,16 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
   /// The offset of the child's bottom edge from the bottom of the stack.
   final double bottom;
 
-  /// The offset of the child's left edge from the left of the stack.
-  final double left;
-
   /// The child's width.
   ///
-  /// Ignored if both left and right are non-null.
+  /// Only two out of the three horizontal values (left, right, width) can be
+  /// set. The third must be null.
   final double width;
 
   /// The child's height.
   ///
-  /// Ignored if both top and bottom are non-null.
+  /// Only two out of the three vertical values (top, bottom, height) can be
+  /// set. The third must be null.
   final double height;
 
   void applyParentData(RenderObject renderObject) {
@@ -1073,6 +1075,11 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
     final StackParentData parentData = renderObject.parentData;
     bool needsLayout = false;
 
+    if (parentData.left != left) {
+      parentData.left = left;
+      needsLayout = true;
+    }
+
     if (parentData.top != top) {
       parentData.top = top;
       needsLayout = true;
@@ -1088,11 +1095,6 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
       needsLayout = true;
     }
 
-    if (parentData.left != left) {
-      parentData.left = left;
-      needsLayout = true;
-    }
-
     if (parentData.width != width) {
       parentData.width = width;
       needsLayout = true;

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

@@ -226,7 +226,11 @@ class AnimatedRelativeRectValue extends AnimatedValue<RelativeRect> {
   RelativeRect lerp(double t) => RelativeRect.lerp(begin, end, t);
 }
 
-/// Animated version of [Positioned].
+/// Animated version of [Positioned] which takes a specific
+/// [AnimatedRelativeRectValue] and a [PerformanceView] to transition the
+/// child's position from a start position to and end position over the lifetime
+/// of the performance.
+///
 /// Only works if it's the child of a [Stack].
 class PositionedTransition extends TransitionWithChild {
   PositionedTransition({