[AutoLayout] Address further concerns raised in https://codereview.chromium.org/1230583003

sky/sdk/lib/rendering/auto_layout.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 'box.dart';
 import 'object.dart';
 import 'package:cassowary/cassowary.dart' as al;
 
 /// Hosts the edge parameters and vends useful methods to construct expressions
 /// for constraints. Also sets up and manages implicit constraints and edit
@@ skipping 35 matching lines @@
         _bottomEdge.variable], priority);
   }
 
   void _applyEditsAtSize(al.Solver solver, Size size) {
     solver.suggestValueForVariable(_leftEdge.variable, 0.0);
     solver.suggestValueForVariable(_topEdge.variable, 0.0);
     solver.suggestValueForVariable(_bottomEdge.variable, size.height);
     solver.suggestValueForVariable(_rightEdge.variable, size.width);
   }
 
+  /// Called when the solver has updated at least one of the layout parameters
+  /// of this object. The object is now responsible for applying this update to
+  /// it other properties (if necessary)

[Hixie, 2015/07/09 21:52:07]

s/it/the/, I assume?

   void _applyAutolayoutParameterUpdates();
+
+  /// Returns the set of implicit constraints that need to be applied to all
+  /// instances of this class when they are moved into a render object with an
+  /// active solver. If no implicit constraints needs to be applied, the object
+  /// may return null.

[Hixie, 2015/07/09 21:52:07]

If it's for all instances, why isn't it static?

   List<al.Constraint> _constructImplicitConstraints();
 
   void _setupImplicitConstraints(al.Solver solver) {
     List<al.Constraint> implicit = _constructImplicitConstraints();
 
     if (implicit == null || implicit.length == 0) {
       return;
     }
 
     al.Result result = solver.addConstraints(implicit);
     assert(result == al.Result.success);
 
     _implicitConstraints = implicit;
   }
 
   void _collectImplicitConstraints(al.Solver solver) {
     if (_implicitConstraints == null || _implicitConstraints.length == 0) {
       return;
     }
 
     al.Result result = solver.removeConstraints(_implicitConstraints);
     assert(result == al.Result.success);
 
     _implicitConstraints = null;
   }
+
 }
 
 class AutoLayoutParentData extends BoxParentData
     with ContainerParentDataMixin<RenderBox>, _AutoLayoutParamMixin {
 
-  final RenderBox _renderBox;
-
   AutoLayoutParentData(this._renderBox) {
     _setupLayoutParameters(this);
   }
 
+  final RenderBox _renderBox;
+
   @override
   void _applyAutolayoutParameterUpdates() {
     BoxConstraints box = new BoxConstraints.tightFor(
         width: _rightEdge.value - _leftEdge.value,
         height: _bottomEdge.value - _topEdge.value);
 
     _renderBox.layout(box, parentUsesSize: false);
     position = new Point(_leftEdge.value, _topEdge.value);
   }
 
   @override
   List<al.Constraint> _constructImplicitConstraints() {
     return [
       // The left edge must be positive
       _leftEdge >= al.cm(0.0),
 
       // Width must be positive
       _rightEdge >= _leftEdge,
     ];
   }
 }
 
 class RenderAutoLayout extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, AutoLayoutParentData>,
          RenderBoxContainerDefaultsMixin<RenderBox, AutoLayoutParentData>,
          _AutoLayoutParamMixin {
 
+  RenderAutoLayout({ List<RenderBox> children }) {
+    _setupLayoutParameters(this);
+    _setupEditVariablesInSolver(_solver, al.Priority.required - 1);
+    addAll(children);
+  }
+
   final al.Solver _solver = new al.Solver();
   List<al.Constraint> _explicitConstraints = new List<al.Constraint>();
 
-  RenderAutoLayout({List<RenderBox> children}) {
-    _setupLayoutParameters(this);
-    _setupEditVariablesInSolver(_solver, al.Priority.required - 1);
-
-    addAll(children);
-  }
-
   /// Adds all the given constraints to the solver. Either all constraints are
   /// added or none
   al.Result addConstraints(List<al.Constraint> constraints) {
     al.Result result = _solver.addConstraints(constraints);
-
     if (result == al.Result.success) {
       markNeedsLayout();
       _explicitConstraints.addAll(constraints);
     }
-
     return result;
   }
 
   /// Add the given constraint to the solver.
   al.Result addConstraint(al.Constraint constraint) {
     al.Result result = _solver.addConstraint(constraint);
 
     if (result == al.Result.success) {
       markNeedsLayout();
       _explicitConstraints.add(constraint);
     }
 
     return result;
   }
 
   /// Removes all explicitly added constraints.
   al.Result clearAllConstraints() {
     al.Result result = _solver.removeConstraints(_explicitConstraints);
 
     if (result == al.Result.success) {
       markNeedsLayout();
       _explicitConstraints = new List<al.Constraint>();
     }
 
     return result;
   }
 
   @override
   void setupParentData(RenderObject child) {
-    if (child.parentData is! AutoLayoutParentData) {
+    if (child.parentData is! AutoLayoutParentData)
       child.parentData = new AutoLayoutParentData(child);
-    }
   }
 
   @override
   void performLayout() {
     // Step 1: Update dimensions of self
     size = constraints.biggest;
     _applyEditsAtSize(_solver, size);
 
     // Step 2: Resolve solver updates and flush parameters
 
@@ skipping 24 matching lines @@
   void adoptChild(RenderObject child) {
     // Make sure to call super first to setup the parent data
     super.adoptChild(child);
     child.parentData._setupImplicitConstraints(_solver);
   }
 
   @override
   void dropChild(RenderObject child) {
     child.parentData._collectImplicitConstraints(_solver);
 
-    // Call super last as this collects parent data
     super.dropChild(child);
   }
 
   @override
   List<al.Constraint> _constructImplicitConstraints() {
-    // Only edits are present on layout containers
+    // Only edits variables are present on layout containers. If, in the future,
+    // implicit constraints (for say margins, padding, etc.) need to be added,
+    // they must be returned from here.
     return null;
   }
 }