Layout API prototype, for discussion

sky/framework/layout.dart

+library layout;
+
+import 'node.dart';
+
+// VALUE TYPES
+
+class EdgeDims {
+  // used for e.g. padding
+  const EdgeDims(this.top, this.right, this.bottom, this.left);
+  final double top;
+  final double right;
+  final double bottom;
+  final double left;
+  operator ==(EdgeDims other) => (top == other.top) ||
+                                 (right == other.right) ||
+                                 (bottom == other.bottom) ||
+                                 (left == other.left);
+}
+
+class Matrix {
+  external static Matrix get IDENTITY;
+  external void rotate(double cx, double cy, double theta); // rotate theta radians clockwise around cx,cy
+}
+
+
+// PAINTING
+
+class DisplayList {
+  List<RenderNode> _children;
+  void paintChild(RenderNode child, double x, double y) {
+    _addChildToDisplayList(child, x, y);
+    if (_children == null)
+      _children = new List<RenderNode>();
+    assert(!_children.contains(child));
+    _children.add(child);

[ojan, 2015/04/23 20:28:32]

Why add the children to a list instead of calling paint() directly here?

[Hixie, 2015/04/23 20:45:13]

If it's not dirty, you don't need to paint it. We could call _doPaint()
directly, though. I just figured it was best to not actually nest the calls to
paint() if we don't have to. I don't have a strong opinion here.

[ojan, 2015/04/23 21:12:36]

Extra lists == moar slow. So we would only want to do this if it gained us
thing.

+  }
+  external void _addChildToDisplayList(RenderNode child, double x, double y);
+}
+
+
+// ABSTRACT LAYOUT
+
+class ParentData {
+  void detach() {
+    detachSiblings();
+  }
+  void detachSiblings() { } // workaround for lack of inter-class mixins in Dart
+}
+
+class PaintOptions {
+  Matrix transform = Matrix.IDENTITY;
+}
+
+const kLayoutDirections = 4;

[ojan, 2015/04/23 20:28:32]

FYI, this is dead

[Hixie, 2015/04/23 20:45:13]

Acknowledged.

+
+double clamp({double min: 0.0, double value: 0.0, double max: double.INFINITY}) {
+  if (value > max)
+    value = max;
+  if (value < min)
+    value = min;
+  return value;
+}
+
+abstract class RenderNode extends Node {
+
+  // LAYOUT
+
+  // pos is only for use by the RenderNode that actually lays this
+  // node out, and any other nodes who happen to know exactly what
+  // kind of node that is.
+  ParentData pos;
+  void setupPos(RenderNode child) {
+    // override this to setup .pos correctly for your class
+    if (child.pos is! ParentData)
+      child.pos = new ParentData();
+  }
+
+  void setAsChild(RenderNode child) { // only for use by subclasses
+    // call this whenever you decide a node is a child
+    assert(child != null);
+    setupPos(child);
+    super.setAsChild(child);
+  }
+  void dropChild(RenderNode child) { // only for use by subclasses
+    assert(child != null);
+    assert(child.pos != null);
+    child.pos.detach();
+    super.dropChild(child);
+  }
+
+  static List<RenderNode> _nodesNeedingLayout = new List<RenderNode>();
+  static bool _debugDoingLayout = false;
+  bool _needsLayout = true;
+  bool get needsLayout => _needsLayout;
+  RenderNode _relayoutSubtreeRoot;
+  void saveRelayoutSubtreeRoot(RenderNode relayoutSubtreeRoot) {
+    assert(_relayoutSubtreeRoot._relayoutSubtreeRoot == null);
+    _relayoutSubtreeRoot = relayoutSubtreeRoot;
+  }
+  void markNeedsLayout() {
+    assert(!_debugDoingLayout);
+    assert(!_debugDoingPaint);
+    if (_needsLayout) return;
+    _needsLayout = true;
+    _nodesNeedingLayout.add(this);
+  }
+  static void flushLayout() {
+    _debugDoingLayout = true;
+    List<RenderNode> dirtyNodes = _nodesNeedingLayout;
+    _nodesNeedingLayout = new List<RenderNode>();
+    dirtyNodes..sort((a, b) => a.order - b.order)..forEach((node) {
+      if (node._needsLayout && node.attached)
+        node._doLayout();
+    });
+    _debugDoingLayout = false;
+  }
+  void _doLayout() {
+    try {
+      if (_relayoutSubtreeRoot != null) {
+        assert(_relayoutSubtreeRoot._relayoutSubtreeRoot == null);
+        _relayoutSubtreeRoot._doLayout();
+      } else {
+        relayout();
+      }
+    } catch (e, stack) {
+      print('Exception raised during layout of ${this}: ${e}');
+      print(stack);
+      return;
+    }
+    assert(!_needsLayout); // check that the relayout() method marked us "not dirty"
+  }
+  /* // this method's signature is subclass-specific, but will exist in
+     // some form in all subclasses:
+     void layout({arguments..., RenderNode relayoutSubtreeRoot}) {
+       if (this node has an opinion about its size, e.g. because it autosizes based on kids, or has an intrinsic dimension) {
+         if (relayoutSubtreeRoot != null) {
+           saveRelayoutSubtreeRoot(relayoutSubtreeRoot);
+           // for each child, if we are going to size ourselves around them:
+           child.layout(... relayoutSubtreeRoot: relayoutSubtreeRoot);
+           width = ...;
+           height = ...;
+         } else {
+           saveRelayoutSubtreeRoot(null); // you can skip this if there's no way you would ever have called saveRelayoutSubtreeRoot() before
+           // we're the root of the relayout subtree
+           // for each child, if we are going to size ourselves around them:
+           child.layout(... relayoutSubtreeRoot: this);
+           width = ...;
+           height = ...;
+         }
+       } else {
+         // we're sizing ourselves exclusively on input from the parent (arguments to this function)
+         // ignore relayoutSubtreeRoot
+         saveRelayoutSubtreeRoot(null); // you can skip this if there's no way you would ever have called saveRelayoutSubtreeRoot() before
+         width = ...; // based on input from arguments only
+         height = ...; // based on input from arguments only
+       }
+       // for each child whose size we'll ignore when deciding ours:
+       child.layout(... relayoutSubtreeRoot: null); // or just omit relayoutSubtreeRoot
+       layoutDone();
+       return result;
+     }
+  */
+  void relayout() {
+    // Override this to perform relayout without your parent's
+    // involvement.
+    //
+    // This is what is called after the first layout(), if you mark
+    // yourself dirty and don't have a _relayoutSubtreeRoot set; in
+    // other words, either if your parent doesn't care what size you
+    // are (and thus didn't pass a relayoutSubtreeRoot to your
+    // layout() method) or if you sized yourself entirely based on
+    // what your parents told you, and not based on your children (and
+    // thus you never called saveRelayoutSubtreeRoot()).
+    //
+    // In the former case, you can resize yourself here at will. In
+    // the latter case, just leave your dimensions unchanged.
+    //
+    // If _relayoutSubtreeRoot is set (i.e. you called saveRelayout-
+    // SubtreeRoot() in your layout(), with a relayoutSubtreeRoot
+    // argument that was non-null), then if you mark yourself as dirty
+    // then we'll tell that subtree root instead, and the layout will
+    // occur via the layout() tree rather than starting from this
+    // relayout() method.
+    assert(_relayoutSubtreeRoot == null);
+    layoutDone();
+  }
+  void layoutDone({bool needsPaint: true}) {
+    // make sure to call this at the end of your layout() or relayout()
+    _needsLayout = false;
+    if (needsPaint)
+      markNeedsPaint();
+  }
+
+  // when the parent has rotated (e.g. when the screen has been turned
+  // 90 degrees), immediately prior to layout() being called for the
+  // new dimensions, rotate() is called with the old and new angles.
+  // The next time paint() is called, the coordinate space will have
+  // been rotated N quarter-turns clockwise, where:
+  //    N = newAngle-oldAngle
+  // ...but the rendering is expected to remain the same, pixel for
+  // pixel, on the output device. Then, the layout() method or
+  // equivalent will be invoked.
+
+  void rotate({
+    int oldAngle, // 0..3
+    int newAngle, // 0..3
+    Duration time
+  }) { }
+
+
+  // HIT TESTING  
+
+  void hitTest(double x, double y, List<RenderNode> targets) {
+    // override this if you have children
+    // if any of your children cover x,y, call the top-most such
+    // child's hitTest().
+    // then, call this superclass hitTest().
+    targets.add(this);
+  }
+
+
+  // PAINTING
+
+  PaintOptions paintOptions;
+  DisplayList _cachedPaint;
+  // only set this to a subclass of PaintOptions that this class of RenderNode knows how to deal with
+
+  static List<RenderNode> _nodesNeedingPaint = new List<RenderNode>();
+  static bool _debugDoingPaint = false;
+  bool _needsPaint = true;
+  bool get needsPaint => _needsPaint;
+  void markNeedsPaint() {
+    assert(!_debugDoingPaint);
+    if (_needsPaint) return;
+    _needsPaint = true;
+    _nodesNeedingPaint.add(this);
+  }
+  static void flushPaint() {
+    List<RenderNode> dirtyNodes = _nodesNeedingPaint;
+    _nodesNeedingPaint = new List<RenderNode>();
+    dirtyNodes..sort((a, b) => a.order - b.order)..forEach((node) {
+      if (node._needsPaint && node.attached)
+        node._doPaint();
+    });
+  }
+  void _doPaint() {
+    assert(!_needsLayout);
+    DisplayList newPaint = new DisplayList();
+    _needsPaint = false;
+    try {
+      paint(newPaint);
+    } catch (e, stack) {
+      print('Exception raised during paint of ${this}: ${e}');
+      print(stack);
+      return;
+    }
+    assert(!_needsLayout); // check that the paint() method didn't mark us dirty again
+    assert(!_needsPaint); // check that the paint() method didn't mark us dirty again
+    if (newPaint._children != null)
+      newPaint._children.forEach((node) {
+        assert(node.attached == attached);
+        if (node._needsPaint)
+          node._doPaint();
+      });
+    _cachedPaint = newPaint;
+  }
+
+  void paint(DisplayList canvas) { }
+
+}
+
+
+// GENERIC MIXIN FOR RENDER NODES THAT TAKE A LIST OF CHILDREN
+
+abstract class ContainerParentDataMixin<ChildType extends RenderNode> {
+  ChildType previousSibling;
+  ChildType nextSibling;
+  void detachSiblings() {
+    if (previousSibling != null)
+      (previousSibling.pos as ContainerParentDataMixin<ChildType>).nextSibling = nextSibling;
+    if (nextSibling != null)
+      (nextSibling.pos as ContainerParentDataMixin<ChildType>).previousSibling = previousSibling;
+    previousSibling = null;
+    nextSibling = null;
+  }
+}
+
+abstract class ContainerRenderNodeMixin<ChildType extends RenderNode, ParentDataType extends ContainerParentDataMixin<ChildType>> {
+  // abstract class that has only InlineNode children
+
+  bool _debugUltimatePreviousSiblingOf(ChildType child, { ChildType equals }) {
+    assert(child.pos is ParentDataType);
+    while ((child.pos as ParentDataType).previousSibling != null) {
+      child = (child.pos as ParentDataType).previousSibling;
+      assert(child.pos is ParentDataType);
+    }
+    return child == equals;
+  }
+  bool _debugUltimateNextSiblingOf(ChildType child, { ChildType equals }) {
+    assert(child.pos is ParentDataType);
+    while ((child.pos as ParentDataType).nextSibling != null) {
+      child = (child.pos as ParentDataType).nextSibling;
+      assert(child.pos is ParentDataType);
+    }
+    return child == equals;
+  }
+
+  ChildType _firstChild;
+  ChildType _lastChild;
+  void add(ChildType child, { ChildType before }) {
+    setAsChild(child);
+    if (before == null) {
+      // append at the end (_lastChild)
+      (child.pos as ParentDataType).previousSibling = _lastChild;
+      if (_lastChild != null)
+        (_lastChild.pos as ParentDataType).nextSibling = child;
+      (child.pos as ParentDataType).previousSibling = _lastChild;
+      _lastChild = child;
+      if (_firstChild == null)
+        _firstChild = child;
+    } else {
+      assert(_firstChild != null);
+      assert(_lastChild != null);
+      assert(_debugUltimatePreviousSiblingOf(before, equals: _firstChild));
+      assert(_debugUltimateNextSiblingOf(before, equals: _lastChild));
+      if ((before.pos as ParentDataType).previousSibling == null) {
+        // insert at the start (_firstChild); there's two or more children
+        assert(before == _firstChild);
+        (child.pos as ParentDataType).nextSibling = before;
+        (before.pos as ParentDataType).previousSibling = child;
+        _firstChild = child;
+      } else {
+        // insert in the middle; there's two or more children
+        // set up links from child to siblings
+        (child.pos as ParentDataType).previousSibling = (before.pos as ParentDataType).previousSibling;
+        (child.pos as ParentDataType).nextSibling = before;
+        // set up links from siblings to child
+        ((child.pos as ParentDataType).previousSibling.pos as ParentDataType).nextSibling = child;
+        ((child.pos as ParentDataType).nextSibling.pos as ParentDataType).previousSibling = child;
+      }
+    }
+    markNeedsLayout();
+  }
+  void remove(ChildType child) {
+    assert(child.pos is ParentDataType);
+    assert(_debugUltimatePreviousSiblingOf(child, equals: _firstChild));
+    assert(_debugUltimateNextSiblingOf(child, equals: _lastChild));
+    if ((child.pos as ParentDataType).previousSibling == null)
+      _firstChild = (child.pos as ParentDataType).nextSibling;
+    else
+      ((child.pos as ParentDataType).previousSibling.pos as ParentDataType).nextSibling = (child.pos as ParentDataType).nextSibling;
+    if ((child.pos as ParentDataType).nextSibling == null)
+      _lastChild = (child.pos as ParentDataType).previousSibling;
+    else
+      ((child.pos as ParentDataType).nextSibling.pos as ParentDataType).previousSibling = (child.pos as ParentDataType).previousSibling;
+    dropChild(child);
+    markNeedsLayout();
+  }
+  void reorderChildren() {
+    ChildType child = _firstChild;
+    while (child != null) {
+      reorderChild(child);
+      child = (child.pos as ParentDataType).nextSibling;
+    }
+  }
+  void attachChildren() {
+    ChildType child = _firstChild;
+    while (child != null) {
+      child.attach();
+      child = (child.pos as ParentDataType).nextSibling;
+    }
+  }
+  void detachChildren() {
+    ChildType child = _firstChild;
+    while (child != null) {
+      child.detach();
+      child = (child.pos as ParentDataType).nextSibling;
+    }
+  }
+
+}
+
+
+// GENERIC BOX RENDERING
+// Anything that has a concept of x, y, width, height is going to derive from this
+
+class BoxDimensions {
+  const BoxDimensions({this.width, this.height});
+  final double width;
+  final double height;
+}
+
+class BoxParentData extends ParentData {
+  double x = 0.0;
+  double y = 0.0;
+}
+
+abstract class RenderBox extends RenderNode {
+
+  void setupPos(RenderNode child) {
+    if (child.pos is! BoxParentData)
+      child.pos = new BoxParentData();
+  }
+
+  // override this to report what dimensions you would have if you
+  // were laid out with the given constraints this can walk the tree
+  // if it must, but it should be as cheap as possible; just get the
+  // dimensions and nothing else (e.g. don't calculate hypothetical
+  // child positions if they're not needed to determine dimensions)
+  BoxDimensions getIntrinsicDimensions({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY
+  }) {
+    return new BoxDimensions(
+      width: clamp(min: minWidth, max: maxWidth),
+      height: clamp(min: minHeight, max: maxHeight)
+    );
+  }
+
+  void layout({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY,
+    RenderNode relayoutSubtreeRoot
+  }) {
+    width = clamp(min: minWidth, max: maxWidth);
+    height = clamp(min: minHeight, max: maxHeight);
+    layoutDone();
+  }
+
+  double width;
+  double height;
+
+  void rotate({
+    int oldAngle, // 0..3
+    int newAngle, // 0..3
+    Duration time
+  }) { }
+
+}
+
+
+// SCREEN LAYOUT MANAGER
+
+class Screen extends RenderNode {
+
+  Screen({
+    RenderBox root,
+    this.timeForRotation: const Duration(microseconds: 83333)
+  }) {
+    assert(root != null);
+    this.root = root;
+  }
+
+  double _width;
+  double get width => _width;
+  double _height;
+  double get height => _height;
+
+  int _orientation; // 0..3
+  int get orientation => _orientation;
+  Duration timeForRotation;
+
+  RenderBox _root;
+  RenderBox get root => _root;
+  void set root (RenderBox value) {
+    assert(root != null);
+    _root = value;
+    setAsChild(_root);
+    markNeedsLayout();
+  }
+
+  void layout({
+    double newWidth,
+    double newHeight,
+    int newOrientation
+  }) {
+    assert(root != null);
+    if (newOrientation != orientation) {
+      if (orientation != null)
+        root.rotate(oldAngle: orientation, newAngle: newOrientation, time: timeForRotation);
+      _orientation = newOrientation;
+    }
+    if ((newWidth != width) || (newHeight != height)) {
+      _width = newWidth;
+      _height = newHeight;
+      relayout();
+    }
+  }
+
+  void relayout() {
+    assert(root != null);
+    root.layout(
+      minWidth: width,
+      maxWidth: width,
+      minHeight: height,
+      maxHeight: height
+    );
+    assert(root.width == width);
+    assert(root.height == height);
+  }
+
+  void rotate({ int oldAngle, int newAngle, Duration time }) {
+    assert(false); // nobody tells the screen to rotate, the whole rotate() dance is started from our layout()
+  }
+
+  void paint(DisplayList canvas) {
+    canvas.paintChild(root, 0, 0);
+  }
+
+}
+
+
+// BLOCK LAYOUT MANAGER
+
+class BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
+
+class BlockBox extends RenderBox with ContainerRenderNodeMixin<RenderBox, BlockParentData> {
+  // lays out RenderBox children in a vertical stack
+  // uses the maximum width provided by the parent
+  // sizes itself to the height of its child stack
+
+  BlockBox({
+    EdgeDims padding: const EdgeDims(0.0, 0.0, 0.0, 0.0)
+  }) {
+    _padding = padding;
+  }
+
+  EdgeDims _padding;
+  EdgeDims get padding => _padding;
+  void set padding(EdgeDims value) {
+    assert(value != null);
+    if (_padding != value) {
+      _padding = value;
+      markNeedsLayout();
+    }
+  }
+
+  void setupPos(RenderBox child) {
+    if (child.pos is! BlockParentData)
+      child.pos = new BlockParentData();
+  }
+
+  // override this to report what dimensions you would have if you
+  // were laid out with the given constraints this can walk the tree
+  // if it must, but it should be as cheap as possible; just get the
+  // dimensions and nothing else (e.g. don't calculate hypothetical
+  // child positions if they're not needed to determine dimensions)
+  BoxDimensions getIntrinsicDimensions({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY
+  }) {
+    double outerHeight = _padding.top + _padding.bottom;
+    double outerWidth = clamp(min: minWidth, max: maxWidth);
+    double innerWidth = outerWidth - (_padding.left + _padding.right);
+    RenderBox child = _firstChild;
+    while (child != null) {
+      outerHeight += child.getIntrinsicDimensions(minWidth: innerWidth, maxWidth: innerWidth).height;
+      child = (child.pos as BlockParentData).nextSibling;
+    }
+    return new BoxDimensions(
+      width: outerWidth,
+      height: clamp(min: minHeight, max: maxHeight, value: outerHeight)
+    );
+  }
+
+  double _minHeight; // value cached from parent for relayout call
+  double _maxHeight; // value cached from parent for relayout call
+  void layout({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY,
+    RenderNode relayoutSubtreeRoot
+  }) {
+    if (relayoutSubtreeRoot != null)
+      saveRelayoutSubtreeRoot(relayoutSubtreeRoot);
+    relayoutSubtreeRoot = relayoutSubtreeRoot == null ? this : relayoutSubtreeRoot;
+    width = clamp(min: minWidth, max: maxWidth);
+    _minHeight = minHeight;
+    _maxHeight = maxHeight;
+    internalLayout(relayoutSubtreeRoot);
+  }
+
+  void relayout() {
+    internalLayout(this);
+  }
+
+  void internalLayout(RenderNode relayoutSubtreeRoot) {
+    assert(_minHeight != null);
+    assert(_maxHeight != null);
+    double y = _padding.top;
+    double innerWidth = width - (_padding.left + _padding.right);
+    RenderBox child = _firstChild;
+    while (child != null) {
+      child.layout(minWidth: innerWidth, maxWidth: innerWidth, relayoutSubtreeRoot: relayoutSubtreeRoot);
+      assert(child.pos is BlockParentData);
+      (child.pos as BlockParentData).x = 0.0;
+      (child.pos as BlockParentData).y = y;
+      y += child.height;
+      child = (child.pos as BlockParentData).nextSibling;
+    }
+    height = clamp(min: _minHeight, value: y + _padding.bottom, max: _maxHeight);
+    layoutDone();
+  }
+
+  void hitTest(double x, double y, List<RenderNode> targets) {
+    RenderBox child = _lastChild;
+    while (child != null) {
+      if ((x >= (child.pos as BlockParentData).x) && (x < (child.pos as BlockParentData).x + child.width) &&
+          (y >= (child.pos as BlockParentData).y) && (y < (child.pos as BlockParentData).y + child.height)) {
+        child.hitTest(x, y, targets);
+        break;
+      }
+      child = (child.pos as BlockParentData).previousSibling;
+    }
+    super.hitTest(x, y, targets);
+  }
+
+  void paint(DisplayList canvas) {
+    RenderBox child = _firstChild;
+    while (child != null) {
+      canvas.paintChild(child, (child.pos as BlockParentData).x, (child.pos as BlockParentData).y);
+      child = (child.pos as BlockParentData).nextSibling;
+    }
+  }
+
+}
+
+
+// PARAGRAPH LAYOUT MANAGER
+
+class InlineParentData extends BoxParentData with ContainerParentDataMixin<InlineNode> { }
+
+class ParagraphBox extends RenderBox with ContainerRenderNodeMixin<InlineNode, InlineParentData> {
+
+  void setupPos(InlineNode child) {
+    if (child.pos is! InlineParentData)
+      child.pos = new InlineParentData();
+  }
+
+  BoxDimensions getIntrinsicDimensions({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY
+  }) {
+    // ...compute intrinsic dimension given these constraints...
+  }
+
+  double _minWidth; // value cached from parent for relayout call
+  double _maxWidth; // value cached from parent for relayout call
+  double _minHeight; // value cached from parent for relayout call
+  double _maxHeight; // value cached from parent for relayout call
+  void layout({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY,
+    RenderNode relayoutSubtreeRoot
+  }) {
+    if (relayoutSubtreeRoot != null)
+      saveRelayoutSubtreeRoot(relayoutSubtreeRoot);
+    relayoutSubtreeRoot = relayoutSubtreeRoot == null ? this : relayoutSubtreeRoot;
+    _minWidth = minWidth;
+    _maxWidth = maxWidth;
+    _minHeight = minHeight;
+    _maxHeight = maxHeight;
+    internalLayout(relayoutSubtreeRoot);
+    layoutDone();
+  }
+
+  void relayout() {
+    internalLayout(this);
+    layoutDone();
+  }
+
+  external void internalLayout(RenderNode relayoutSubtreeRoot);
+  external void hitTest(double x, double y, List<RenderNode> targets);
+  external void paint(DisplayList canvas);
+
+}
+
+class InlineNode extends RenderNode with ContainerRenderNodeMixin<InlineNode, InlineParentData> {
+  // ...
+}
+
+
+// InlineBox wraps a RenderBox, i.e. it's a box that fits into inline
+// layout without breaking into multiple lines. This allows you to put
+// images, form controls, inline blocks, etc, into paragraphs
+class InlineBox extends InlineNode {
+  InlineBox(this.child);
+  final RenderBox child;
+
+  // ...
+}
+
+
+// SCAFFOLD LAYOUT MANAGER
+
+// a sample special-purpose layout manager
+
+class ScaffoldBox extends RenderBox {
+
+  ScaffoldBox(this.toolbar, this.body, this.statusbar, this.drawer) {
+    assert(body != null);
+  }
+
+  final RenderBox toolbar;
+  final RenderBox body;
+  final RenderBox statusbar;
+  final RenderBox drawer;
+
+  void layout({
+    double minWidth: 0.0,
+    double maxWidth: double.INFINITY,
+    double minHeight: 0.0,
+    double maxHeight: double.INFINITY,
+    RenderNode relayoutSubtreeRoot
+  }) {
+    width = clamp(min: minWidth, max: maxWidth);
+    height = clamp(min: minHeight, max: maxHeight);
+    relayout();
+  }
+
+  static const kToolbarHeight = 100.0;
+  static const kStatusbarHeight = 50.0;
+
+  void relayout() {
+    double bodyHeight = height;
+    if (toolbar != null) {
+      toolbar.layout(minWidth: width, maxWidth: width, minHeight: kToolbarHeight, maxHeight: kToolbarHeight);
+      (toolbar.pos as BoxParentData).x = 0.0;
+      (toolbar.pos as BoxParentData).y = 0.0;
+      bodyHeight -= kToolbarHeight;
+    }
+    if (statusbar != null) {
+      statusbar.layout(minWidth: width, maxWidth: width, minHeight: kStatusbarHeight, maxHeight: kStatusbarHeight);
+      (statusbar.pos as BoxParentData).x = 0.0;
+      (statusbar.pos as BoxParentData).y = height - kStatusbarHeight;
+      bodyHeight -= kStatusbarHeight;
+    }
+    body.layout(minWidth: width, maxWidth: width, minHeight: bodyHeight, maxHeight: bodyHeight);
+    if (drawer != null)
+      drawer.layout(minWidth: 0.0, maxWidth: width, minHeight: height, maxHeight: height);
+    layoutDone();
+  }
+
+  void hitTest(double x, double y, List<RenderNode> targets) {
+    if ((drawer != null) && (x < drawer.width)) {
+      drawer.hitTest(x, y, targets);
+    } else if ((toolbar != null) && (y < toolbar.height)) {
+      toolbar.hitTest(x, y, targets);
+    } else if ((statusbar != null) && (y > (statusbar.pos as BoxParentData).y)) {
+      statusbar.hitTest(x, y, targets);
+    } else {
+      body.hitTest(x, y, targets);
+    }
+    super.hitTest(x, y, targets);
+  }
+
+  void paint(DisplayList canvas) {
+    canvas.paintChild(body, (body.pos as BoxParentData).x, (body.pos as BoxParentData).y);
+    if (statusbar != null)
+      canvas.paintChild(statusbar, (statusbar.pos as BoxParentData).x, (statusbar.pos as BoxParentData).y);
+    if (toolbar != null)
+      canvas.paintChild(toolbar, (toolbar.pos as BoxParentData).x, (toolbar.pos as BoxParentData).y);
+    if (drawer != null)
+      canvas.paintChild(drawer, (drawer.pos as BoxParentData).x, (drawer.pos as BoxParentData).y);
+  }
+
+}