Add an origin parameter to transforms

This parameter makes it easier to do math when you don't want to center your
tranform at (0, 0).

packages/flutter/lib/rendering/proxy_box.dart

@@ -287,13 +287,13 @@ class RenderShrinkWrapHeight extends RenderProxyBox {
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     if (child == null)
       return constraints.constrainWidth(0.0);
-    return child.getMinIntrinsicWidth(_getInnerConstraints(constraints));    
+    return child.getMinIntrinsicWidth(_getInnerConstraints(constraints));
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
     if (child == null)
       return constraints.constrainWidth(0.0);
-    return child.getMaxIntrinsicWidth(_getInnerConstraints(constraints));    
+    return child.getMaxIntrinsicWidth(_getInnerConstraints(constraints));
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
@@ -516,13 +516,24 @@ class RenderDecoratedBox extends RenderProxyBox {
 class RenderTransform extends RenderProxyBox {
   RenderTransform({
     Matrix4 transform,
+    Offset origin,
     RenderBox child
   }) : super(child) {
     assert(transform != null);
     this.transform = transform;
+    this.origin = origin;
   }
 
   Matrix4 _transform;
+  Offset _origin;
+
+  Offset get origin => _origin;
+  void set origin (Offset newOrigin) {
+    if (_origin == newOrigin)
+      return;
+    _origin = newOrigin;
+    markNeedsPaint();
+  }
 
   void set transform(Matrix4 newTransform) {
     assert(newTransform != null);
@@ -562,10 +573,20 @@ class RenderTransform extends RenderProxyBox {
     markNeedsPaint();
   }
 
+  Matrix4 get _effectiveTransform {
+    if (_origin == null)
+      return _transform;
+    return new Matrix4
+      .identity()
+      .translate(_origin.dx, _origin.dy)
+      .multiply(_transform)
+      .translate(-_origin.dx, -_origin.dy);
+  }
+
   bool hitTest(HitTestResult result, { Point position }) {
     Matrix4 inverse = new Matrix4.zero();
     // TODO(abarth): Check the determinant for degeneracy.
-    inverse.copyInverse(_transform);
+    inverse.copyInverse(_effectiveTransform);
 
     Vector3 position3 = new Vector3(position.x, position.y, 0.0);
     Vector3 transformed3 = inverse.transform3(position3);
@@ -575,18 +596,18 @@ class RenderTransform extends RenderProxyBox {
 
   void paint(PaintingContext context, Offset offset) {
     if (child != null)
-      context.paintChildWithTransform(child, offset.toPoint(), _transform);
+      context.paintChildWithTransform(child, offset.toPoint(), _effectiveTransform);
   }
 
   void applyPaintTransform(Matrix4 transform) {
     super.applyPaintTransform(transform);
-    transform.multiply(_transform);
+    transform.multiply(_effectiveTransform);
   }
 
   String debugDescribeSettings(String prefix) {
     List<String> result = _transform.toString().split('\n').map((s) => '$prefix  $s\n').toList();
     result.removeLast();
-    return '${super.debugDescribeSettings(prefix)}${prefix}transform matrix:\n${result.join()}';
+    return '${super.debugDescribeSettings(prefix)}${prefix}transform matrix:\n${result.join()}\n${prefix}origin: ${origin}\n';
   }
 }
 

packages/flutter/lib/widgets/basic.dart

@@ -154,17 +154,19 @@ class ClipOval extends OneChildRenderObjectWrapper {
 // POSITIONING AND SIZING NODES
 
 class Transform extends OneChildRenderObjectWrapper {
-  Transform({ Key key, this.transform, Widget child })
+  Transform({ Key key, this.transform, this.origin, Widget child })
     : super(key: key, child: child);
 
   final Matrix4 transform;
+  final Offset origin;
 
-  RenderTransform createNode() => new RenderTransform(transform: transform);
+  RenderTransform createNode() => new RenderTransform(transform: transform, origin: origin);
   RenderTransform get renderObject => super.renderObject;
 
   void syncRenderObject(Transform old) {
     super.syncRenderObject(old);
     renderObject.transform = transform;
+    renderObject.origin = origin;
   }
 }
 

packages/unit/test/widget/transform_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+  test('Transform origin', () {
+    WidgetTester tester = new WidgetTester();
+
+    bool didReceiveTap = false;
+    tester.pumpFrame(() {
+      return 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, 50.0),
+              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);
+  });
+}

packages/unit/test/widget/widget_tester.dart

@@ -90,7 +90,10 @@ class WidgetTester {
   }
 
   void tap(Widget widget, { int pointer: 1 }) {
-    Point location = getCenter(widget);
+    tapAt(getCenter(widget), pointer: pointer);
+  }
+
+  void tapAt(Point location, { int pointer: 1 }) {
     HitTestResult result = _hitTest(location);
     TestPointer p = new TestPointer(pointer);
     _dispatchEvent(p.down(location), result);