Short-circuit the relayoutSubtreeRoot when the child couldn't change dimensions anyway because the …

sky/sdk/lib/rendering/object.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:sky' show Point, Size, Rect, Color, Paint, Path;
 
 import '../base/hit_test.dart';
 import '../base/node.dart';
@@ skipping 19 matching lines @@
 
 class RenderObjectDisplayList extends sky.PictureRecorder {
   RenderObjectDisplayList(double width, double height) : super(width, height);
   void paintChild(RenderObject child, Point position) {
     translate(position.x, position.y);
     child.paint(this);
     translate(-position.x, -position.y);
   }
 }
 
+abstract class Constraints {
+  const Constraints();
+  bool get isTight;
+}
+
 abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   // LAYOUT
 
   // parentData is only for use by the RenderObject that actually lays this
   // node out, and any other nodes who happen to know exactly what
   // kind of node that is.
   dynamic parentData; // TODO(ianh): change the type of this back to ParentData once the analyzer is cleverer
   void setParentData(RenderObject child) {
     // override this to setup .parentData correctly for your class
@@ skipping 22 matching lines @@
     super.dropChild(child);
     markNeedsLayout();
   }
 
   static List<RenderObject> _nodesNeedingLayout = new List<RenderObject>();
   static bool _debugDoingLayout = false;
   static bool get debugDoingLayout => _debugDoingLayout;
   bool _needsLayout = true;
   bool get needsLayout => _needsLayout;
   RenderObject _relayoutSubtreeRoot;
-  dynamic _constraints;
-  dynamic get constraints => _constraints;
+  Constraints _constraints;
+  Constraints get constraints => _constraints;
   bool debugAncestorsAlreadyMarkedNeedsLayout() {
     if (_relayoutSubtreeRoot == null)
       return true; // we haven't yet done layout even once, so there's nothing for us to do
     RenderObject node = this;
     while (node != _relayoutSubtreeRoot) {
       assert(node._relayoutSubtreeRoot == _relayoutSubtreeRoot);
       assert(node.parent != null);
       node = node.parent as RenderObject;
       if (!node._needsLayout)
         return false;
@@ skipping 55 matching lines @@
     try {
       assert(_relayoutSubtreeRoot == this);
       performLayout();
     } catch (e, stack) {
       print('Exception raised during layout of ${this}: ${e}');
       print(stack);
       return;
     }
     _needsLayout = false;
   }
-  void layout(dynamic constraints, { bool parentUsesSize: false }) {
+  void layout(Constraints constraints, { bool parentUsesSize: false }) {
     final parent = this.parent; // TODO(ianh): Remove this once the analyzer is cleverer
     RenderObject relayoutSubtreeRoot;
-    if (!parentUsesSize || sizedByParent || parent is! RenderObject)
+    if (!parentUsesSize || sizedByParent || constraints.isTight || parent is! RenderObject)
       relayoutSubtreeRoot = this;
     else
       relayoutSubtreeRoot = parent._relayoutSubtreeRoot;
     assert(parent == this.parent); // TODO(ianh): Remove this once the analyzer is cleverer
     if (!needsLayout && constraints == _constraints && relayoutSubtreeRoot == _relayoutSubtreeRoot)
       return;
     _constraints = constraints;
     _relayoutSubtreeRoot = relayoutSubtreeRoot;
     if (sizedByParent)
       performResize();
@@ skipping 312 matching lines @@
     int count = 1;
     ChildType child = _firstChild;
     while (child != null) {
       result += '${prefix}child ${count}: ${child.toString(prefix)}';
       count += 1;
       child = child.parentData.nextSibling;
     }
     return result;
   }
 }