Integrate the linear constraint solver into Sky as a RenderBox subclass.

sky/sdk/lib/rendering/auto_layout.dart

Index: sky/sdk/lib/rendering/auto_layout.dart
diff --git a/sky/sdk/lib/rendering/auto_layout.dart b/sky/sdk/lib/rendering/auto_layout.dart
new file mode 100644
index 0000000000000000000000000000000000000000..04b952d1b0fc5bd5f748bb0d862cda24d9894078
--- /dev/null
+++ b/sky/sdk/lib/rendering/auto_layout.dart
@@ -0,0 +1,225 @@
+// 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
+/// variables. Used as a mixin by layout containers and parent data instances
+/// of render boxes taking part in auto layout
+abstract class _AutoLayoutParamMixin {
+  // Ideally, the edges would all be final, but then they would have to be
+  // initialized before the constructor. Not sure how to do that using a Mixin

[Hixie, 2015/07/09 00:53:14]

Why can't you just use a constructor?

[Hixie, 2015/07/09 22:05:09]

i think you missed this one. :-)

+  al.Param _leftEdge;
+  al.Param _rightEdge;
+  al.Param _topEdge;
+  al.Param _bottomEdge;
+
+  List<al.Constraint> _implicitConstraints;
+
+  al.Param get leftEdge => _leftEdge;
+  al.Param get rightEdge => _rightEdge;
+  al.Param get topEdge => _topEdge;
+  al.Param get bottomEdge => _bottomEdge;
+
+  al.Expression get width => _rightEdge - _leftEdge;
+  al.Expression get height => _bottomEdge - _topEdge;
+
+  al.Expression get horizontalCenter => (_leftEdge + _rightEdge) / al.CM(2.0);
+  al.Expression get verticalCenter => (_topEdge + _bottomEdge) / al.CM(2.0);
+
+  void _setupLayoutParameters(dynamic context) {
+    _leftEdge = new al.Param.withContext(context);
+    _rightEdge = new al.Param.withContext(context);
+    _topEdge = new al.Param.withContext(context);
+    _bottomEdge = new al.Param.withContext(context);
+  }
+
+  void _setupEditVariablesInSolver(al.Solver solver, double priority) {
+    solver.addEditVariables([
+        _leftEdge.variable,
+        _rightEdge.variable,
+        _topEdge.variable,
+        _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);
+  }
+
+  void _applyAutolayoutParameterUpdates();
+  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);

[Hixie, 2015/07/09 00:53:15]

put constructors before fields

[Chinmay, 2015/07/09 21:44:19]

ack

+  }
+
+  @override

[Hixie, 2015/07/09 00:53:14]

remove the @override annotations (or add them across the entire codebase ;-) ).

[Chinmay, 2015/07/09 21:44:20]

ack

+  void _applyAutolayoutParameterUpdates() {

[Hixie, 2015/07/09 00:53:15]

Put a comment here explaining that this is called by the parent, to lay out the
child, etc.

[Chinmay, 2015/07/09 21:44:20]

ack. Since this is part of the abstract class that is used as the mixin, I am
documenting it there.

+    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,
+    ];
+  }
+}

[Hixie, 2015/07/09 00:53:14]

you have a blank line after the class open {, so you should have one before the
class close }.

[Chinmay, 2015/07/09 21:44:19]

ack

+
+class RenderAutoLayout extends RenderBox
+    with ContainerRenderObjectMixin<RenderBox, AutoLayoutParentData>,
+         RenderBoxContainerDefaultsMixin<RenderBox, AutoLayoutParentData>,
+         _AutoLayoutParamMixin {
+
+  final al.Solver _solver = new al.Solver();
+  List<al.Constraint> _explicitConstraints = new List<al.Constraint>();
+
+  RenderAutoLayout({List<RenderBox> children}) {

[Hixie, 2015/07/09 00:53:14]

put spaces inside { }s around parameter arguments.

[Chinmay, 2015/07/09 21:44:20]

ack

+    _setupLayoutParameters(this);
+    _setupEditVariablesInSolver(_solver, al.Priority.required - 1);
+

[Hixie, 2015/07/09 00:53:14]

remove this blank line; it doesn't help readability and makes the constructor's
shape harder to see.

[Chinmay, 2015/07/09 21:44:20]

ack

+    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);
+    }
+

[Hixie, 2015/07/09 00:53:14]

probably don't need the blank lines here either.

[Chinmay, 2015/07/09 21:44:20]

ack

+    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) {
+      child.parentData = new AutoLayoutParentData(child);

[Hixie, 2015/07/09 00:53:14]

no { }s around blocks that are only 1 line long

[Chinmay, 2015/07/09 21:44:20]

ack

+    }
+  }
+
+  @override
+  void performLayout() {
+    // Step 1: Update dimensions of self
+    size = constraints.biggest;

[Hixie, 2015/07/09 00:53:14]

If the size isn't configurable, then you should set sizedByParent and use
performResize to set the size, rather than performLayout.

[Chinmay, 2015/07/09 21:44:20]

This is something I am not completely sure of TBH. In an earlier patch, I had an
override of the size setter and did the edit variable update there. But Adam
indicated that these are never configurable. Need to catch up on this.

[Hixie, 2015/07/09 22:05:09]

As far as I can tell, you don't let the size be set based on the kids currently.
So if that's the case, then you should set sizedByParent.

+    _applyEditsAtSize(_solver, size);
+
+    // Step 2: Resolve solver updates and flush parameters
+
+    // We don't iterate over the children, instead, we ask the solver to tell
+    // us the updated parameters. Attached to the parameters (via the context)
+    // are the _AutoLayoutParamMixin instances.
+    for (_AutoLayoutParamMixin update in _solver.flushUpdates()) {
+      update._applyAutolayoutParameterUpdates();
+    }
+  }
+
+  @override
+  void _applyAutolayoutParameterUpdates() {
+    // Nothing to do since the size update has already been presented to the
+    // solver as an edit variable modification. The invokation of this method
+    // only indicates that the value has been flushed to the variable.
+  }
+
+  @override
+  void hitTestChildren(HitTestResult result, {Point position}) =>

[Hixie, 2015/07/09 00:53:14]

Don't use fat arrow if the body isn't on the same line as the declaration.

[Chinmay, 2015/07/09 21:44:20]

I have been referring to the expression count (instead of the line count) for
the fat arrow syntax. Do you have strong opinions on this? I prefer not to use
the line count as this will eventually depend on what the formatter does to the
code.

[Hixie, 2015/07/09 22:05:09]

our style guide says => is for single-line declarations. If it splits across
lines then it's no better than a block, and consistency with other multiline
constructs says we should just use blocks then.

+      defaultHitTestChildren(result, position: position);
+
+  @override
+  void paint(PaintingCanvas canvas, Offset offset) =>
+      defaultPaint(canvas, offset);
+
+  @override
+  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

[Hixie, 2015/07/09 00:53:14]

it doesn't matter, dropChild doesn't touch parentData.

[Chinmay, 2015/07/09 21:44:20]

ack

+    super.dropChild(child);
+  }
+
+  @override
+  List<al.Constraint> _constructImplicitConstraints() {
+    // Only edits are present on layout containers

[Hixie, 2015/07/09 00:53:14]

What does this mean?

[Chinmay, 2015/07/09 21:44:20]

ack. Elaborated

+    return null;
+  }
+}