Transform alignment

For those times where you want to spin something around a point relative
to the size of your box, but you don't know the size of your box.

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

@@ -862,15 +862,41 @@ class RenderDecoratedBox extends RenderProxyBox {
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}decoration:\n${_painter.decoration.toString(prefix + "  ")}\n';
 }
 
+/// An offset that's expressed as a fraction of a Size.
+///
+/// FractionalOffset(0.0, 0.0) represents the top left of the Size,
+/// FractionalOffset(1.0, 1.0) represents the bottom right of the Size.
+class FractionalOffset {
+  const FractionalOffset(this.x, this.y);
+  final double x;
+  final double y;
+  bool operator ==(dynamic other) {
+    if (other is! FractionalOffset)
+      return false;
+    final FractionalOffset typedOther = other;
+    return x == typedOther.x &&
+           y == typedOther.y;
+  }
+  int get hashCode {
+    int value = 373;
+    value = 37 * value + x.hashCode;
+    value = 37 * value + y.hashCode;
+    return value;
+  }
+}
+
 /// Applies a transformation before painting its child
 class RenderTransform extends RenderProxyBox {
   RenderTransform({
     Matrix4 transform,
     Offset origin,
+    FractionalOffset alignment,
     RenderBox child
   }) : super(child) {
     assert(transform != null);
+    assert(alignment == null || (alignment.x != null && alignment.y != null));
     this.transform = transform;
+    this.alignment = alignment;
     this.origin = origin;
   }
 
@@ -888,6 +914,20 @@ class RenderTransform extends RenderProxyBox {
     markNeedsPaint();
   }
 
+  /// The alignment of the origin, relative to the size of the box.
+  ///
+  /// This is equivalent to setting an origin based on the size of the box.
+  /// If it is specificed at the same time as an offset, both are applied.
+  FractionalOffset get alignment => _alignment;
+  FractionalOffset _alignment;
+  void set alignment (FractionalOffset newAlignment) {
+    assert(newAlignment == null || (newAlignment.x != null && newAlignment.y != null));
+    if (_alignment == newAlignment)
+      return;
+    _alignment = newAlignment;
+    markNeedsPaint();
+  }
+
   // Note the lack of a getter for transform because Matrix4 is not immutable
   Matrix4 _transform;
 
@@ -937,13 +977,19 @@ class RenderTransform extends RenderProxyBox {
   }
 
   Matrix4 get _effectiveTransform {
-    if (_origin == null)
+    if (_origin == null && _alignment == null)
       return _transform;
-    return new Matrix4
-      .identity()
-      .translate(_origin.dx, _origin.dy)
-      .multiply(_transform)
-      .translate(-_origin.dx, -_origin.dy);
+    Matrix4 result = new Matrix4.identity();
+    if (_origin != null)
+      result.translate(_origin.dx, _origin.dy);
+    if (_alignment != null)
+      result.translate(_alignment.x * size.width, _alignment.y * size.height);
+    result.multiply(_transform);
+    if (_alignment != null)
+      result.translate(-_alignment.x * size.width, -_alignment.y * size.height);
+    if (_origin != null)
+      result.translate(-_origin.dx, -_origin.dy);
+    return result;
   }
 
   bool hitTest(HitTestResult result, { Point position }) {

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

@@ -23,6 +23,7 @@ export 'package:flutter/rendering.dart' show
     FlexAlignItems,
     FlexDirection,
     FlexJustifyContent,
+    FractionalOffset,
     Matrix4,
     Offset,
     Paint,
@@ -171,19 +172,21 @@ class ClipOval extends OneChildRenderObjectWidget {
 // POSITIONING AND SIZING NODES
 
 class Transform extends OneChildRenderObjectWidget {
-  Transform({ Key key, this.transform, this.origin, Widget child })
+  Transform({ Key key, this.transform, this.origin, this.alignment, Widget child })
     : super(key: key, child: child) {
     assert(transform != null);
   }
 
   final Matrix4 transform;
   final Offset origin;
+  final FractionalOffset alignment;
 
-  RenderTransform createRenderObject() => new RenderTransform(transform: transform, origin: origin);
+  RenderTransform createRenderObject() => new RenderTransform(transform: transform, origin: origin, alignment: alignment);
 
   void updateRenderObject(RenderTransform renderObject, Transform oldWidget) {
     renderObject.transform = transform;
     renderObject.origin = origin;
+    renderObject.alignment = alignment;
   }
 }
 

packages/unit/test/widget/transform_test.dart

@@ -48,4 +48,95 @@ void main() {
       expect(didReceiveTap, isTrue);
     });
   });
+
+  test('Transform alignment', () {
+    testWidgets((WidgetTester tester) {
+      bool didReceiveTap = false;
+      tester.pumpWidget(
+        new Stack([
+          new Positioned(
+            top: 100.0,
+            left: 100.0,
+            child: new Container(
+              width: 100.0,
+              height: 100.0,
+              decoration: new BoxDecoration(
+                backgroundColor: new Color(0xFF0000FF)
+              )
+            )
+          ),
+          new Positioned(
+            top: 100.0,
+            left: 100.0,
+            child: new Container(
+              width: 100.0,
+              height: 100.0,
+              child: new Transform(
+                transform: new Matrix4.identity().scale(0.5, 0.5),
+                alignment: new FractionalOffset(1.0, 0.5),
+                child: new GestureDetector(
+                  onTap: () {
+                    didReceiveTap = true;
+                  },
+                  child: new Container()
+                )
+              )
+            )
+          )
+        ])
+      );
+
+      expect(didReceiveTap, isFalse);
+      tester.tapAt(new Point(110.0, 110.0));
+      expect(didReceiveTap, isFalse);
+      tester.tapAt(new Point(190.0, 150.0));
+      expect(didReceiveTap, isTrue);
+    });
+  });
+
+  test('Transform offset + alignment', () {
+    testWidgets((WidgetTester tester) {
+      bool didReceiveTap = false;
+      tester.pumpWidget(
+        new Stack([
+          new Positioned(
+            top: 100.0,
+            left: 100.0,
+            child: new Container(
+              width: 100.0,
+              height: 100.0,
+              decoration: new BoxDecoration(
+                backgroundColor: new Color(0xFF0000FF)
+              )
+            )
+          ),
+          new Positioned(
+            top: 100.0,
+            left: 100.0,
+            child: new Container(
+              width: 100.0,
+              height: 100.0,
+              child: new Transform(
+                transform: new Matrix4.identity().scale(0.5, 0.5),
+                origin: new Offset(100.0, 0.0),
+                alignment: new FractionalOffset(0.0, 0.5),
+                child: new GestureDetector(
+                  onTap: () {
+                    didReceiveTap = true;
+                  },
+                  child: new Container()
+                )
+              )
+            )
+          )
+        ])
+      );
+
+      expect(didReceiveTap, isFalse);
+      tester.tapAt(new Point(110.0, 110.0));
+      expect(didReceiveTap, isFalse);
+      tester.tapAt(new Point(190.0, 150.0));
+      expect(didReceiveTap, isTrue);
+    });
+  });
 }