applyPaintTransform() improvements

Previously, applyPaintTransform() had to know how it was positioned in
its parent, even though that's really the parent's responsibility.

Now, applyPaintTransform() is given a child and applies the transform
that it would give the child during paint.

This makes it possible for applyPaintTransform() to work across
coordinate system boundaries (e.g. box to sector, or view to box --
previously, view to box only worked because we explicitly skipped that
step -- since view doesn't actually apply a transform, it doesn't
really matter).

packages/flutter/lib/src/material/material.dart

@@ -298,17 +298,18 @@ abstract class InkFeature {
     assert(referenceBox.attached);
     assert(!_debugDisposed);
     // find the chain of renderers from us to the feature's referenceBox
-    List<RenderBox> descendants = <RenderBox>[];
+    List<RenderBox> descendants = <RenderBox>[referenceBox];
     RenderBox node = referenceBox;
     while (node != renderer) {
-      descendants.add(node);
       node = node.parent;
       assert(node != null);
+      descendants.add(node);
     }
     // determine the transform that gets our coordinate system to be like theirs
     Matrix4 transform = new Matrix4.identity();
-    for (RenderBox descendant in descendants.reversed)
-      descendant.applyPaintTransform(transform);
+    assert(descendants.length >= 2);
+    for (int index = descendants.length - 1; index > 0; index -= 1)
+      descendants[index].applyPaintTransform(descendants[index - 1], transform);
     paintFeature(canvas, transform);
   }
 

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

@@ -420,12 +420,12 @@ class RenderBlockViewport extends RenderBlockBase {
     context.pushClipRect(needsCompositing, offset, Point.origin & size, _paintContents);
   }
 
-  void applyPaintTransform(Matrix4 transform) {
-    super.applyPaintTransform(transform);
+  void applyPaintTransform(RenderBox child, Matrix4 transform) {
     if (isVertical)
       transform.translate(0.0, startOffset);
     else
       transform.translate(startOffset, 0.0);
+    super.applyPaintTransform(child, transform);
   }
 
   bool hitTestChildren(HitTestResult result, { Point position }) {

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

@@ -609,24 +609,27 @@ abstract class RenderBox extends RenderObject {
   /// visually "on top" (i.e., paints later).
   bool hitTestChildren(HitTestResult result, { Point position }) => false;
 
+  static Point _transformPoint(Matrix4 transform, Point point) {
+    Vector3 position3 = new Vector3(point.x, point.y, 0.0);
+    Vector3 transformed3 = transform.transform3(position3);
+    return new Point(transformed3.x, transformed3.y);
+  }
+
   /// Multiply the transform from the parent's coordinate system to this box's
-  /// coordinate system into the given transform
+  /// coordinate system into the given transform.
   ///
   /// This function is used to convert coordinate systems between boxes.
   /// Subclasses that apply transforms during painting should override this
   /// function to factor those transforms into the calculation.
-  void applyPaintTransform(Matrix4 transform) {
-    if (parentData is BoxParentData) {
-      Point position = (parentData as BoxParentData).position;
+  ///
+  /// The RenderBox implementation takes care of adjusting the matrix for the
+  /// position of the given child.
+  void applyPaintTransform(RenderObject child, Matrix4 transform) {
+    assert(child.parent == this);
+    BoxParentData childParentData = child.parentData;
+    Point position = childParentData.position;
     transform.translate(position.x, position.y);
   }
-  }
-
-  static Point _transformPoint(Matrix4 transform, Point point) {
-    Vector3 position3 = new Vector3(point.x, point.y, 0.0);
-    Vector3 transformed3 = transform.transform3(position3);
-    return new Point(transformed3.x, transformed3.y);
-  }
 
   /// Convert the given point from the global coodinate system to the local
   /// coordinate system for this box
@@ -634,24 +637,25 @@ abstract class RenderBox extends RenderObject {
     assert(attached);
     Matrix4 transform = new Matrix4.identity();
     RenderObject renderer = this;
-    while (renderer != null) {
-      renderer.applyPaintTransform(transform);
-      renderer = renderer.parent;
+    while (renderer.parent is RenderObject) {
+      RenderObject rendererParent = renderer.parent;
+      rendererParent.applyPaintTransform(renderer, transform);
+      renderer = rendererParent;
     }
     /* double det = */ transform.invert();
     // TODO(abarth): Check the determinant for degeneracy.
     return _transformPoint(transform, point);
   }
 
-  /// Convert the given point from the local coordiante system for this box to
-  /// the global coordinate sytem
+  /// Convert the given point from the local coordinate system for this box to
+  /// the global coordinate system.
   Point localToGlobal(Point point) {
     List<RenderObject> renderers = <RenderObject>[];
     for (RenderObject renderer = this; renderer != null; renderer = renderer.parent)
       renderers.add(renderer);
     Matrix4 transform = new Matrix4.identity();
-    for (RenderObject renderer in renderers.reversed)
-      renderer.applyPaintTransform(transform);
+    for (int index = renderers.length - 1; index > 0; index -= 1)
+      renderers[index].applyPaintTransform(renderers[index - 1], transform);
     return _transformPoint(transform, point);
   }
 

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

@@ -1036,12 +1036,14 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// be recorded on separate compositing layers.
   void paint(PaintingContext context, Offset offset) { }
 
-  /// If this render object applies a transform before painting, apply that
-  /// transform to the given matrix
+  /// Applies the transform that would be applied when painting the given child
+  /// to the given matrix.
   ///
   /// Used by coordinate conversion functions to translate coordinates local to
   /// one render object into coordinates local to another render object.
-  void applyPaintTransform(Matrix4 transform) { }
+  void applyPaintTransform(RenderObject child, Matrix4 transform) {
+    assert(child.parent == this);
+  }
 
 
   // EVENTS

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

@@ -974,9 +974,9 @@ class RenderTransform extends RenderProxyBox {
     }
   }
 
-  void applyPaintTransform(Matrix4 transform) {
-    super.applyPaintTransform(transform);
+  void applyPaintTransform(RenderBox child, Matrix4 transform) {
     transform.multiply(_effectiveTransform);
+    super.applyPaintTransform(child, transform);
   }
 
   void debugDescribeSettings(List<String> settings) {

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

@@ -164,9 +164,9 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
     }
   }
 
-  void applyPaintTransform(Matrix4 transform) {
-    super.applyPaintTransform(transform);
+  void applyPaintTransform(RenderBox child, Matrix4 transform) {
     transform.translate(-scrollOffset.dx, -scrollOffset.dy);
+    super.applyPaintTransform(child, transform);
   }
 
   bool hitTestChildren(HitTestResult result, { Point position }) {