Add a Positioned constructor from a Rect

A common pattern is to use a Positioned with a Sized box to give both an offset
from the edge as well as a fixed size to the child. This patch rolls into into
the Stack layout algorithm for simplicity.

Fixes #250

examples/widgets/overlay_geometry.dart

@@ -54,12 +54,10 @@ class Marker extends StatelessComponent {
     return new Positioned(
       left: position.x - size / 2.0,
       top: position.y - size / 2.0,
+      width: size,
+      height: size,
       child: new IgnorePointer(
-        child: new Container(
-          width: size,
-          height: size,
-          child: new CustomPaint(onPaint: paintMarker)
-        )
+        child: new CustomPaint(onPaint: paintMarker)
       )
     );
   }

packages/flutter/lib/src/rendering/stack.dart

@@ -137,18 +137,28 @@ class RelativeRect {
 
 /// Parent data for use with [RenderStack]
 class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
-  /// The offset of the child's top edge from the top of the stack
+  /// The offset of the child's top edge from the top of the stack.
   double top;
 
-  /// The offset of the child's right edge from the right of the stack
+  /// The offset of the child's right edge from the right of the stack.
   double right;
 
-  /// The offset of the child's bottom edge from the bottom of the stack
+  /// The offset of the child's bottom edge from the bottom of the stack.
   double bottom;
 
-  /// The offset of the child's left edge from the left of the stack
+  /// The offset of the child's left edge from the left of the stack.
   double left;
 
+  /// The child's width.
+  ///
+  /// Ignored if both left and right are non-null.
+  double width;
+
+  /// The child's height.
+  ///
+  /// Ignored if both top and bottom are non-null.
+  double height;
+
   /// Get or set the current values in terms of a RelativeRect object.
   RelativeRect get rect => new RelativeRect.fromLTRB(left, top, right, bottom);
   void set rect(RelativeRect value) {
@@ -167,6 +177,10 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
       bottom = other.bottom;
     if (other.left != null)
       left = other.left;
+    if (other.width != null)
+      width = other.width;
+    if (other.height != null)
+      height = other.height;
     super.merge(other);
   }
 
@@ -176,7 +190,7 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
   /// are non-null. Positioned children do not factor into determining the size
   /// of the stack but are instead placed relative to the non-positioned
   /// children in the stack.
-  bool get isPositioned => top != null || right != null || bottom != null || left != null;
+  bool get isPositioned => top != null || right != null || bottom != null || left != null || width != null || height != null;
 
   String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
 }
@@ -325,9 +339,13 @@ abstract class RenderStackBase extends RenderBox
 
         if (childParentData.left != null && childParentData.right != null)
           childConstraints = childConstraints.tightenWidth(size.width - childParentData.right - childParentData.left);
+        else if (childParentData.width != null)
+          childConstraints = childConstraints.tightenWidth(childParentData.width);
 
         if (childParentData.top != null && childParentData.bottom != null)
           childConstraints = childConstraints.tightenHeight(size.height - childParentData.bottom - childParentData.top);
+        else if (childParentData.height != null)
+          childConstraints = childConstraints.tightenHeight(childParentData.height);
 
         child.layout(childConstraints, parentUsesSize: true);
 

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

@@ -733,14 +733,32 @@ class Positioned extends ParentDataWidget {
     this.top,
     this.right,
     this.bottom,
-    this.left
-  }) : super(key: key, child: child);
+    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);
+  }
+
+  Positioned.fromRect({
+    Key key,
+    Widget child,
+    Rect rect
+  }) : left = rect.left,
+       top = rect.top,
+       width = rect.width,
+       height = rect.height,
+       super(key: key, child: child);
 
   final double top;
   final double right;
   final double bottom;
   final double left;
 
+  final double width;
+  final double height;
+
   void debugValidateAncestor(Widget ancestor) {
     assert(() {
       'Positioned must placed inside a Stack';
@@ -773,6 +791,16 @@ class Positioned extends ParentDataWidget {
       needsLayout = true;
     }
 
+    if (parentData.width != width) {
+      parentData.width = width;
+      needsLayout = true;
+    }
+
+    if (parentData.height != height) {
+      parentData.height = height;
+      needsLayout = true;
+    }
+
     if (needsLayout) {
       AbstractNode targetParent = renderObject.parent;
       if (targetParent is RenderObject)
@@ -790,6 +818,10 @@ class Positioned extends ParentDataWidget {
       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');
   }
 }
 

packages/unit/test/widget/stack_test.dart

@@ -47,6 +47,8 @@ void main() {
       expect(parentData.right, isNull);
       expect(parentData.bottom, isNull);
       expect(parentData.left, equals(10.0));
+      expect(parentData.width, isNull);
+      expect(parentData.height, isNull);
 
       tester.pumpWidget(
         new Stack(<Widget>[
@@ -67,6 +69,8 @@ void main() {
       expect(parentData.right, equals(10.0));
       expect(parentData.bottom, isNull);
       expect(parentData.left, isNull);
+      expect(parentData.width, isNull);
+      expect(parentData.height, isNull);
     });
   });
 
@@ -84,6 +88,8 @@ void main() {
       expect(parentData.right, isNull);
       expect(parentData.bottom, isNull);
       expect(parentData.left, equals(10.0));
+      expect(parentData.width, isNull);
+      expect(parentData.height, isNull);
 
       tester.pumpWidget(new Stack(<Widget>[ container ]));
       containerElement = tester.findElementByKey(key);
@@ -93,6 +99,8 @@ void main() {
       expect(parentData.right, isNull);
       expect(parentData.bottom, isNull);
       expect(parentData.left, isNull);
+      expect(parentData.width, isNull);
+      expect(parentData.height, isNull);
     });
   });
 
@@ -187,4 +195,68 @@ void main() {
     });
   });
 
+  test('Can set width and height', () {
+    testWidgets((WidgetTester tester) {
+      Key key = new Key('container');
+
+      BoxDecoration kBoxDecoration = new BoxDecoration(
+        backgroundColor: new Color(0xFF00FF00)
+      );
+
+      tester.pumpWidget(
+        new Stack(<Widget>[
+          new Positioned(
+            left: 10.0,
+            width: 11.0,
+            height: 12.0,
+            child: new DecoratedBox(key: key, decoration: kBoxDecoration)
+          )
+        ])
+      );
+
+      Element box;
+      RenderBox renderBox;
+      StackParentData parentData;
+
+      box = tester.findElementByKey(key);
+      renderBox = box.renderObject;
+      parentData = renderBox.parentData;
+      expect(parentData.top, isNull);
+      expect(parentData.right, isNull);
+      expect(parentData.bottom, isNull);
+      expect(parentData.left, equals(10.0));
+      expect(parentData.width, equals(11.0));
+      expect(parentData.height, equals(12.0));
+      expect(parentData.position.x, equals(10.0));
+      expect(parentData.position.y, equals(0.0));
+      expect(renderBox.size.width, equals(11.0));
+      expect(renderBox.size.height, equals(12.0));
+
+      tester.pumpWidget(
+        new Stack(<Widget>[
+          new Positioned(
+            right: 10.0,
+            width: 11.0,
+            height: 12.0,
+            child: new DecoratedBox(key: key, decoration: kBoxDecoration)
+          )
+        ])
+      );
+
+      box = tester.findElementByKey(key);
+      renderBox = box.renderObject;
+      parentData = renderBox.parentData;
+      expect(parentData.top, isNull);
+      expect(parentData.right, equals(10.0));
+      expect(parentData.bottom, isNull);
+      expect(parentData.left, isNull);
+      expect(parentData.width, equals(11.0));
+      expect(parentData.height, equals(12.0));
+      expect(parentData.position.x, equals(779.0));
+      expect(parentData.position.y, equals(0.0));
+      expect(renderBox.size.width, equals(11.0));
+      expect(renderBox.size.height, equals(12.0));
+    });
+  });
+
 }