Add more dartdoc to rendering.dart (#4232)

There are still many more left before rendering.dart is complete.

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

@@ -11,29 +11,38 @@ import 'object.dart';
 /// for constraints. Also sets up and manages implicit constraints and edit
 /// variables.
 class AutoLayoutRect {
-  AutoLayoutRect() {
-    _left = new al.Param();
-    _right = new al.Param();
-    _top = new al.Param();
-    _bottom = new al.Param();
-  }
+  /// Creates parameters for a rectangle for use with auto layout.
+  AutoLayoutRect()
+    : left = new al.Param(),
+      right = new al.Param(),
+      top = new al.Param(),
+      bottom = new al.Param();
+
+  /// A parameter that represents the left edge of the rectangle.
+  final al.Param left;
+
+  /// A parameter that represents the right edge of the rectangle.
+  final al.Param right;
+
+  /// A parameter that represents the top edge of the rectangle.
+  final al.Param top;
 
-  al.Param _left;
-  al.Param _right;
-  al.Param _top;
-  al.Param _bottom;
+  /// A parameter that represents the bottom edge of the rectangle.
+  final al.Param bottom;
 
-  al.Param get left => _left;
-  al.Param get right => _right;
-  al.Param get top => _top;
-  al.Param get bottom => _bottom;
+  /// An expression that represents the horizontal extent of the rectangle.
+  al.Expression get width => right - left;
 
-  al.Expression get width => _right - _left;
-  al.Expression get height => _bottom - _top;
+  /// An expression that represents the vertical extent of the rectangle.
+  al.Expression get height => bottom - top;
 
-  al.Expression get horizontalCenter => (_left + _right) / al.cm(2.0);
-  al.Expression get verticalCenter => (_top + _bottom) / al.cm(2.0);
+  /// An expression that represents halfway between the left and right edges of the rectangle.
+  al.Expression get horizontalCenter => (left + right) / al.cm(2.0);
 
+  /// An expression that represents halfway between the top and bottom edges of the rectangle.
+  al.Expression get verticalCenter => (top + bottom) / al.cm(2.0);
+
+  /// Constraints that require that this rect contains the given rect.
   List<al.Constraint> contains(AutoLayoutRect other) {
     return <al.Constraint>[
       other.left >= left,
@@ -44,11 +53,14 @@ class AutoLayoutRect {
   }
 }
 
+/// Parent data for use with [RenderAutoLayout].
 class AutoLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
+  /// Creates parent data associated with the given render box.
   AutoLayoutParentData(this._renderBox);
 
   final RenderBox _renderBox;
 
+  /// Parameters that represent the size and position of the render box.
   AutoLayoutRect get rect => _rect;
   AutoLayoutRect _rect;
   set rect(AutoLayoutRect value) {
@@ -63,13 +75,13 @@ class AutoLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
 
   BoxConstraints get _constraintsFromSolver {
     return new BoxConstraints.tightFor(
-      width: _rect._right.value - _rect._left.value,
-      height: _rect._bottom.value - _rect._top.value
+      width: _rect.right.value - _rect.left.value,
+      height: _rect.bottom.value - _rect.top.value
     );
   }
 
   Offset get _offsetFromSolver {
-    return new Offset(_rect._left.value, _rect._top.value);
+    return new Offset(_rect.left.value, _rect.top.value);
   }
 
   List<al.Constraint> _implicitConstraints;
@@ -106,40 +118,66 @@ class AutoLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
   /// may return null.
   List<al.Constraint> _constructImplicitConstraints() {
     return <al.Constraint>[
-      _rect._left >= al.cm(0.0), // The left edge must be positive.
-      _rect._right >= _rect._left, // Width must be positive.
-      // Why don't we need something similar for the top and the bottom?
+      _rect.left >= al.cm(0.0), // The left edge must be positive.
+      _rect.right >= _rect.left, // Width must be positive.
+      // TODO(chinmay): Check whether we need something similar for the top and
+      // bottom.
     ];
   }
 }
 
+/// Subclass to control the layout of a [RenderAutoLayout].
 abstract class AutoLayoutDelegate {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
   const AutoLayoutDelegate();
 
+  /// Returns the constraints to use when computing layout.
+  ///
+  /// The `parent` argument contains the parameters for the parent's position
+  /// and size. Typical implementations will return constraints that determine
+  /// the size and position of each child.
+  ///
+  /// The delegate interface does not provide a mechanism for obtaining the
+  /// parameters for children. Subclasses are expected to obtain those
+  /// parameters through some other mechanism.
   List<al.Constraint> getConstraints(AutoLayoutRect parent);
+
+  /// Override this method to return true when new constraints need to be generated.
   bool shouldUpdateConstraints(AutoLayoutDelegate oldDelegate);
 }
 
+/// Uses the cassowary constraint solver to automatically size and position children.
 class RenderAutoLayout extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, AutoLayoutParentData>,
          RenderBoxContainerDefaultsMixin<RenderBox, AutoLayoutParentData> {
-
+  /// Creates a render box that automatically sizes and positions its children.
   RenderAutoLayout({
     AutoLayoutDelegate delegate,
     List<RenderBox> children
   }) : _delegate = delegate, _needToUpdateConstraints = (delegate != null) {
     _solver.addEditVariables(<al.Variable>[
-        _rect._left.variable,
-        _rect._right.variable,
-        _rect._top.variable,
-        _rect._bottom.variable
+        _rect.left.variable,
+        _rect.right.variable,
+        _rect.top.variable,
+        _rect.bottom.variable
       ], al.Priority.required - 1);
 
     addAll(children);
   }
 
+  /// The delegate that generates constraints for the layout.
+  ///
+  /// If the new delegate is the same as the previous one, this does nothing.
+  ///
+  /// If the new delegate is the same class as the previous one, then the new
+  /// delegate has its [AutoLayoutDelegate.shouldUpdateConstraints] called; if
+  /// the result is `true`, then the delegate will be called.
+  ///
+  /// If the new delegate is a different class than the previous one, then the
+  /// delegate will be called.
+  ///
+  /// If the delgate is null, the layout is unconstrained.
   AutoLayoutDelegate get delegate => _delegate;
   AutoLayoutDelegate _delegate;
   set delegate(AutoLayoutDelegate newDelegate) {
@@ -228,10 +266,10 @@ class RenderAutoLayout extends RenderBox
 
     if (size != _previousSize) {
       _solver
-        ..suggestValueForVariable(_rect._left.variable, 0.0)
-        ..suggestValueForVariable(_rect._top.variable, 0.0)
-        ..suggestValueForVariable(_rect._bottom.variable, size.height)
-        ..suggestValueForVariable(_rect._right.variable, size.width);
+        ..suggestValueForVariable(_rect.left.variable, 0.0)
+        ..suggestValueForVariable(_rect.top.variable, 0.0)
+        ..suggestValueForVariable(_rect.bottom.variable, size.height)
+        ..suggestValueForVariable(_rect.right.variable, size.width);
       _previousSize = size;
       needToFlushUpdates = true;
     }

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

@@ -209,6 +209,10 @@ void debugDumpSemanticsTree() {
 ///
 /// See also [BindingBase].
 class RenderingFlutterBinding extends BindingBase with SchedulerBinding, GestureBinding, ServicesBinding, RendererBinding {
+  /// Creates a binding for the rendering layer.
+  ///
+  /// The `root` render box is attached directly to the [renderView] and is
+  /// given constraints that require it to fill the window.
   RenderingFlutterBinding({ RenderBox root }) {
     assert(renderView != null);
     renderView.child = root;

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

@@ -25,7 +25,9 @@ typedef double _Constrainer(double value);
 class RenderBlock extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, BlockParentData>,
          RenderBoxContainerDefaultsMixin<RenderBox, BlockParentData> {
-
+  /// Creates a block render object.
+  ///
+  /// By default, the block positions children along the vertical axis.
   RenderBlock({
     List<RenderBox> children,
     Axis mainAxis: Axis.vertical

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

@@ -238,39 +238,43 @@ class BoxConstraints extends Constraints {
            (minHeight <= size.height) && (size.height <= maxHeight);
   }
 
-  BoxConstraints operator*(double other) {
+  /// Scales each constraint parameter by the given factor.
+  BoxConstraints operator*(double factor) {
     return new BoxConstraints(
-      minWidth: minWidth * other,
-      maxWidth: maxWidth * other,
-      minHeight: minHeight * other,
-      maxHeight: maxHeight * other
+      minWidth: minWidth * factor,
+      maxWidth: maxWidth * factor,
+      minHeight: minHeight * factor,
+      maxHeight: maxHeight * factor
     );
   }
 
-  BoxConstraints operator/(double other) {
+  /// Scales each constraint parameter by the inverse of the given factor.
+  BoxConstraints operator/(double factor) {
     return new BoxConstraints(
-      minWidth: minWidth / other,
-      maxWidth: maxWidth / other,
-      minHeight: minHeight / other,
-      maxHeight: maxHeight / other
+      minWidth: minWidth / factor,
+      maxWidth: maxWidth / factor,
+      minHeight: minHeight / factor,
+      maxHeight: maxHeight / factor
     );
   }
 
-  BoxConstraints operator~/(double other) {
+  /// Scales each constraint parameter by the inverse of the given factor, rounded to the nearest integer.
+  BoxConstraints operator~/(double factor) {
     return new BoxConstraints(
-      minWidth: (minWidth ~/ other).toDouble(),
-      maxWidth: (maxWidth ~/ other).toDouble(),
-      minHeight: (minHeight ~/ other).toDouble(),
-      maxHeight: (maxHeight ~/ other).toDouble()
+      minWidth: (minWidth ~/ factor).toDouble(),
+      maxWidth: (maxWidth ~/ factor).toDouble(),
+      minHeight: (minHeight ~/ factor).toDouble(),
+      maxHeight: (maxHeight ~/ factor).toDouble()
     );
   }
 
-  BoxConstraints operator%(double other) {
+  /// Computes the remainder of each constraint parameter by the given value.
+  BoxConstraints operator%(double value) {
     return new BoxConstraints(
-      minWidth: minWidth % other,
-      maxWidth: maxWidth % other,
-      minHeight: minHeight % other,
-      maxHeight: maxHeight % other
+      minWidth: minWidth % value,
+      maxWidth: maxWidth % value,
+      minHeight: minHeight % value,
+      maxHeight: maxHeight % value
     );
   }
 
@@ -374,6 +378,10 @@ class BoxConstraints extends Constraints {
     return isNormalized;
   }
 
+  /// Returns a box constraints that [isNormalized].
+  ///
+  /// The returned [maxWidth] is at least as large as the [minWidth]. Similarly,
+  /// the returned [maxHeight] is at least as large as the [minHeight].
   BoxConstraints normalize() {
     return new BoxConstraints(
       minWidth: minWidth,
@@ -425,6 +433,9 @@ class BoxConstraints extends Constraints {
 
 /// A hit test entry used by [RenderBox].
 class BoxHitTestEntry extends HitTestEntry {
+  /// Creates a box hit test entry.
+  ///
+  /// The [localPosition] argument must not be null.
   const BoxHitTestEntry(RenderBox target, this.localPosition) : super(target);
 
   @override
@@ -515,6 +526,9 @@ abstract class RenderBox extends RenderObject {
     return constraints.constrainHeight(0.0);
   }
 
+  /// Whether this render object has undergone layout and has a [size].
+  bool get hasSize => _size != null;
+
   /// The size of this render box computed during layout.
   ///
   /// This value is stale whenever this object is marked as needing layout.
@@ -546,7 +560,6 @@ abstract class RenderBox extends RenderObject {
     });
     return _size;
   }
-  bool get hasSize => _size != null;
   Size _size;
   set size(Size value) {
     assert(!(debugDoingThisResize && debugDoingThisLayout));
@@ -996,6 +1009,10 @@ abstract class RenderBox extends RenderObject {
       return true;
     });
   }
+
+  /// In debug mode, paints a border around this render box.
+  ///
+  /// Called for every [RenderBox] when [debugPaintSizeEnabled] is true.
   void debugPaintSize(PaintingContext context, Offset offset) {
     assert(() {
       Paint paint = new Paint()
@@ -1006,6 +1023,10 @@ abstract class RenderBox extends RenderObject {
       return true;
     });
   }
+
+  /// In debug mode, paints a line for each baseline.
+  ///
+  /// Called for every [RenderBox] when [debugPaintBaselinesEnabled] is true.
   void debugPaintBaselines(PaintingContext context, Offset offset) {
     assert(() {
       Paint paint = new Paint()
@@ -1033,6 +1054,10 @@ abstract class RenderBox extends RenderObject {
       return true;
     });
   }
+
+  /// In debug mode, paints a rectangle if this render box has received more pointer downs than pointer up events.
+  ///
+  /// Called for every [RenderBox] when [debugPaintPointersEnabled] is true.
   void debugPaintPointers(PaintingContext context, Offset offset) {
     assert(() {
       if (_debugActivePointers > 0) {
@@ -1127,8 +1152,13 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
     }
   }
 
+  /// Returns a list containing the children of this render object.
+  ///
+  /// This function is useful when you need random-access to the children of
+  /// this render object. If you're accessing the children in order, consider
+  /// walking the child list directly.
   List<ChildType> getChildrenAsList() {
-    List<ChildType> result = <ChildType>[];
+    final List<ChildType> result = <ChildType>[];
     RenderBox child = firstChild;
     while (child != null) {
       final ParentDataType childParentData = child.parentData;
@@ -1139,7 +1169,14 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
   }
 }
 
+/// An interpolation between two fractional offsets.
+///
+/// This class specializes the interpolation of Tween<FractionalOffset> to be
+/// appropriate for rectangles.
 class FractionalOffsetTween extends Tween<FractionalOffset> {
+  /// Creates a fractional offset tween.
+  ///
+  /// The [begin] and [end] arguments must not be null.
   FractionalOffsetTween({ FractionalOffset begin, FractionalOffset end }) : super(begin: begin, end: end);
 
   @override

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

@@ -208,10 +208,14 @@ class ChildViewConnection {
 
 /// (mojo-only) A view of a child application.
 class RenderChildView extends RenderBox {
+  /// Creates a child view render object.
+  ///
+  /// The [scale] argument must not be null.
   RenderChildView({
     ChildViewConnection child,
     double scale
   }) : _scale = scale {
+    assert(scale != null);
     this.child = child;
   }
 
@@ -243,6 +247,7 @@ class RenderChildView extends RenderBox {
   double get scale => _scale;
   double _scale;
   set scale (double value) {
+    assert(value != null);
     if (value == _scale)
       return;
     _scale = value;

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

@@ -205,6 +205,9 @@ abstract class MultiChildLayoutDelegate {
 class RenderCustomMultiChildLayoutBox extends RenderBox
   with ContainerRenderObjectMixin<RenderBox, MultiChildLayoutParentData>,
        RenderBoxContainerDefaultsMixin<RenderBox, MultiChildLayoutParentData> {
+  /// Creates a render object that customizes the layout of multiple children.
+  ///
+  /// The [delegate] argument must not be null.
   RenderCustomMultiChildLayoutBox({
     List<RenderBox> children,
     MultiChildLayoutDelegate delegate

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

@@ -66,6 +66,7 @@ bool debugPrintMarkNeedsLayoutStacks = false;
 /// Check the intrinsic sizes of each [RenderBox] during layout.
 bool debugCheckIntrinsicSizes = false;
 
+/// Returns a list of strings representing the given transform in a format useful for [RenderObject.debugFillDescription].
 List<String> debugDescribeTransform(Matrix4 transform) {
   List<String> matrix = transform.toString().split('\n').map((String s) => '  $s').toList();
   matrix.removeLast();

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

@@ -22,7 +22,12 @@ typedef void SelectionChangedHandler(TextSelection selection, RenderEditableLine
 /// Represents a global screen coordinate of the point in a selection, and the
 /// text direction at that point.
 class TextSelectionPoint {
-  TextSelectionPoint(this.point, this.direction);
+  /// Creates a description of a point in a text selection.
+  ///
+  /// The [point] argument must not be null.
+  TextSelectionPoint(this.point, this.direction) {
+    assert(point != null);
+  }
 
   /// Screen coordinates of the lower left or lower right corner of the selection.
   final Point point;
@@ -33,6 +38,7 @@ class TextSelectionPoint {
 
 /// A single line of editable text.
 class RenderEditableLine extends RenderBox {
+  /// Creates a render object for a single line of editable text.
   RenderEditableLine({
     TextSpan text,
     Color cursorColor,
@@ -56,7 +62,10 @@ class RenderEditableLine extends RenderBox {
       ..onLongPress = _handleLongPress;
   }
 
+  /// Called when the selection changes.
   SelectionChangedHandler onSelectionChanged;
+
+  /// Called when the inner or outer dimensions of this render object change.
   ViewportDimensionsChangeCallback onPaintOffsetUpdateNeeded;
 
   /// The text to display
@@ -72,6 +81,7 @@ class RenderEditableLine extends RenderBox {
     markNeedsLayout();
   }
 
+  /// The color to use when painting the cursor.
   Color get cursorColor => _cursorColor;
   Color _cursorColor;
   set cursorColor(Color value) {
@@ -81,6 +91,7 @@ class RenderEditableLine extends RenderBox {
     markNeedsPaint();
   }
 
+  /// Whether to paint the cursor.
   bool get showCursor => _showCursor;
   bool _showCursor;
   set showCursor(bool value) {
@@ -90,6 +101,7 @@ class RenderEditableLine extends RenderBox {
     markNeedsPaint();
   }
 
+  /// The color to use when painting the selection.
   Color get selectionColor => _selectionColor;
   Color _selectionColor;
   set selectionColor(Color value) {
@@ -101,6 +113,7 @@ class RenderEditableLine extends RenderBox {
 
   List<ui.TextBox> _selectionRects;
 
+  /// The region of text that is selected, if any.
   TextSelection get selection => _selection;
   TextSelection _selection;
   set selection(TextSelection value) {
@@ -111,6 +124,11 @@ class RenderEditableLine extends RenderBox {
     markNeedsPaint();
   }
 
+  /// The offset at which the text should be painted.
+  ///
+  /// If the text content is larger than the editable line itself, the editable
+  /// line clips the text. This property controls which part of the text is
+  /// visible by shifting the text by the given offset before clipping.
   Offset get paintOffset => _paintOffset;
   Offset _paintOffset;
   set paintOffset(Offset value) {
@@ -120,8 +138,18 @@ class RenderEditableLine extends RenderBox {
     markNeedsPaint();
   }
 
+  /// Returns the global coordinates of the endpoints of the given selection.
+  ///
+  /// If the selection is collapsed (and therefore occupies a single point), the
+  /// returned list is of length one. Otherwise, the selection is not collapsed
+  /// and the returned list is of length two. In this case, however, the two
+  /// points might actually be co-located (e.g., because of a bidirectional
+  /// selection that contains some text but whose ends meet in the middle).
   List<TextSelectionPoint> getEndpointsForSelection(TextSelection selection) {
-    _textPainter.layout(); // TODO(mpcomplete): is this hacky?
+    // TODO(mpcomplete): We should be more disciplined about when we dirty the
+    // layout state of the text painter so that we can know that the layout is
+    // clean at this point. 
+    _textPainter.layout();
 
     Offset offset = _paintOffset + new Offset(0.0, -_kCaretHeightOffset);
 
@@ -141,9 +169,10 @@ class RenderEditableLine extends RenderBox {
     }
   }
 
-  TextPosition getPositionForPoint(Point global) {
-    global += -paintOffset;
-    return _textPainter.getPositionForOffset(globalToLocal(global).toOffset());
+  /// Returns the position in the text for the given global coordinate.
+  TextPosition getPositionForPoint(Point globalPosition) {
+    globalPosition += -paintOffset;
+    return _textPainter.getPositionForOffset(globalToLocal(globalPosition).toOffset());
   }
 
   Size _contentSize;

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

@@ -90,7 +90,10 @@ typedef double _ChildSizingFunction(RenderBox child, BoxConstraints constraints)
 /// positioned along the cross axis according to the [crossAxisAlignment] property.
 class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, FlexParentData>,
                                         RenderBoxContainerDefaultsMixin<RenderBox, FlexParentData> {
-
+  /// Creates a flex render object.
+  ///
+  /// By default, the flex layout is horizontal and children are aligned to the
+  /// start of the main axis and the center of the cross axis.
   RenderFlex({
     List<RenderBox> children,
     FlexDirection direction: FlexDirection.horizontal,

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

@@ -33,6 +33,16 @@ List<double> _generateRegularOffsets(int count, double size) {
   return result;
 }
 
+/// Specifies the geometry of tiles in a grid.
+///
+/// A grid specificiation divides a fixed width and height into a certain number
+/// of rows and columns, each with a specific size.
+///
+/// See also:
+///
+///  * [CustomGrid]
+///  * [GridDelegate]
+///  * [RenderGrid]
 class GridSpecification {
   /// Creates a grid specification from an explicit list of offsets.
   GridSpecification.fromOffsets({
@@ -118,7 +128,17 @@ class GridSpecification {
 }
 
 /// Where to place a child within a grid.
+///
+/// See also:
+///
+///  * [CustomGrid]
+///  * [GridDelegate]
+///  * [RenderGrid]
 class GridChildPlacement {
+  /// Creates a placement for a child in a grid.
+  ///
+  /// The [column] and [row] arguments must not be null. By default, the child
+  /// spans a single column and row.
   GridChildPlacement({
     this.column,
     this.row,
@@ -186,6 +206,9 @@ abstract class GridDelegate {
 
 /// A [GridDelegate] the places its children in order throughout the grid.
 abstract class GridDelegateWithInOrderChildPlacement extends GridDelegate {
+  /// Initializes [columnSpacing], [rowSpacing], and [padding] for subclasses.
+  ///
+  /// By default, the [columnSpacing], [rowSpacing], and [padding] are zero.
   GridDelegateWithInOrderChildPlacement({
     this.columnSpacing: 0.0,
     this.rowSpacing: 0.0,
@@ -202,7 +225,7 @@ abstract class GridDelegateWithInOrderChildPlacement extends GridDelegate {
   /// The vertical distance between rows.
   final double rowSpacing;
 
-  // Insets for the entire grid.
+  /// Insets for the entire grid.
   final EdgeInsets padding;
 
   @override
@@ -225,6 +248,9 @@ abstract class GridDelegateWithInOrderChildPlacement extends GridDelegate {
 
 /// A [GridDelegate] that divides the grid's width evenly amount a fixed number of columns.
 class FixedColumnCountGridDelegate extends GridDelegateWithInOrderChildPlacement {
+  /// Creates a grid delegate that uses a fixed column count.
+  ///
+  /// The [columnCount] argument must not be null.
   FixedColumnCountGridDelegate({
     this.columnCount,
     double columnSpacing: 0.0,
@@ -286,6 +312,9 @@ class FixedColumnCountGridDelegate extends GridDelegateWithInOrderChildPlacement
 ///  - The tile width is at most [maxTileWidth].
 ///
 class MaxTileWidthGridDelegate extends GridDelegateWithInOrderChildPlacement {
+  /// Creates a grid delegate that uses a max tile width.
+  ///
+  /// The [maxTileWidth] argument must not be null.
   MaxTileWidthGridDelegate({
     this.maxTileWidth,
     this.tileAspectRatio: 1.0,
@@ -363,6 +392,9 @@ class GridParentData extends ContainerBoxParentDataMixin<RenderBox> {
 /// Additionally, grid layout materializes all of its children, which makes it
 /// most useful for grids containing a moderate number of tiles.
 class RenderGrid extends RenderVirtualViewport<GridParentData> {
+  /// Creates a grid render object.
+  ///
+  /// The [delegate] argument must not be null.
   RenderGrid({
     List<RenderBox> children,
     GridDelegate delegate,
@@ -382,6 +414,17 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
   }
 
   /// The delegate that controls the layout of the children.
+  ///
+  /// If the new delegate is the same as the previous one, this does nothing.
+  ///
+  /// If the new delegate is the same class as the previous one, then the new
+  /// delegate has its [GridDelegate.shouldRelayout] called; if the result is
+  /// `true`, then the delegate will be called.
+  ///
+  /// If the new delegate is a different class than the previous one, then the
+  /// delegate will be called.
+  ///
+  /// The delegate must not be null.
   GridDelegate get delegate => _delegate;
   GridDelegate _delegate;
   set delegate (GridDelegate newDelegate) {
@@ -457,6 +500,10 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
     return defaultComputeDistanceToHighestActualBaseline(baseline);
   }
 
+  /// The specification of this grid.
+  ///
+  /// The grid specification cannot be set directly. Instead, set a [delegate]
+  /// to control the specification.
   GridSpecification get specification => _specification;
   GridSpecification _specification;
   int _specificationChildCount;

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

@@ -16,6 +16,7 @@ export 'package:flutter/painting.dart' show
 /// The render image attempts to find a size for itself that fits in the given
 /// constraints and preserves the image's intrinisc aspect ratio.
 class RenderImage extends RenderBox {
+  /// Creates a render box that displays an image.
   RenderImage({
     ui.Image image,
     double width,

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

@@ -73,8 +73,13 @@ abstract class Layer {
   @override
   String toString() => '$runtimeType';
 
+  /// The object responsible for creating this layer.
+  ///
+  /// Defaults to the value of [RenderObject.debugCreator] for the render object
+  /// that created this layer. Used in debug messages.
   dynamic debugCreator;
 
+  /// Returns a string representation of this layer and its descendants.
   String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {
     String result = '$prefixLineOne$this\n';
     final String childrenDescription = debugDescribeChildren(prefixOtherLines);
@@ -88,11 +93,13 @@ abstract class Layer {
     return result;
   }
 
+  /// Add additional information to the given description for use by [toStringDeep].
   void debugFillDescription(List<String> description) {
     if (debugCreator != null)
       description.add('creator: $debugCreator');
   }
 
+  /// Returns a description of this layer's children for use by [toStringDeep].
   String debugDescribeChildren(String prefix) => '';
 }
 
@@ -109,8 +116,15 @@ class PictureLayer extends Layer {
   }
 }
 
+/// (mojo-only) A layer that represents content from another process.
 class ChildSceneLayer extends Layer {
-  ChildSceneLayer({ this.offset, this.devicePixelRatio, this.physicalWidth, this.physicalHeight, this.sceneToken });
+  ChildSceneLayer({
+    this.offset,
+    this.devicePixelRatio,
+    this.physicalWidth,
+    this.physicalHeight,
+    this.sceneToken
+  });
 
   Offset offset;
   double devicePixelRatio;
@@ -142,6 +156,7 @@ class ChildSceneLayer extends Layer {
 /// A layer that indicates to the compositor that it should display
 /// certain performance statistics within it.
 class PerformanceOverlayLayer extends Layer {
+  /// Creates a layer that displays a performance overlay.
   PerformanceOverlayLayer({
     this.overlayRect,
     this.optionsMask,
@@ -151,9 +166,13 @@ class PerformanceOverlayLayer extends Layer {
   /// The rectangle in this layer's coodinate system that the overlay should occupy.
   Rect overlayRect;
 
-  /// A mask specifying the statistics to display.
+  /// The mask is created by shifting 1 by the index of the specific
+  /// [PerformanceOverlayOption] to enable.
   final int optionsMask;
 
+  /// The rasterizer threshold is an integer specifying the number of frame
+  /// intervals that the rasterizer must miss before it decides that the frame
+  /// is suitable for capturing an SkPicture trace for further analysis.
   final int rasterizerThreshold;
 
   @override
@@ -164,7 +183,6 @@ class PerformanceOverlayLayer extends Layer {
   }
 }
 
-
 /// A composited layer that has a list of children
 class ContainerLayer extends Layer {
   /// The first composited layer in this layer's child list
@@ -284,7 +302,19 @@ class ContainerLayer extends Layer {
   }
 }
 
+/// A layer that is displayed at an offset from its parent layer.
+///
+/// Offset layers are key to efficient repainting because they are created by
+/// repaint boundaries in the [RenderObject] tree (see
+/// [RenderObject.isRepaintBoundary]). When a render object that is a repaint
+/// boundary is asked to paint at given offset in a [PaintingContext], the
+/// render object first checks whether it needs to repaint itself. If not, it
+/// reuses its existing [OffsetLayer] (and its entire subtree) by mutating its
+/// [offset] property, cutting off the paint walk.
 class OffsetLayer extends ContainerLayer {
+  /// Creates an offset layer.
+  ///
+  /// By default, [offset] is zero.
   OffsetLayer({ this.offset: Offset.zero });
 
   /// Offset from parent in the parent's coordinate system.
@@ -305,12 +335,14 @@ class OffsetLayer extends ContainerLayer {
 
 /// A composite layer that clips its children using a rectangle
 class ClipRectLayer extends ContainerLayer {
+  /// Creates a layer with a rectangular clip.
+  ///
+  /// The [clipRect] property must be non-null before the compositing phase of
+  /// the pipeline.
   ClipRectLayer({ this.clipRect });
 
   /// The rectangle to clip in the parent's coordinate system
   Rect clipRect;
-  // TODO(abarth): Why is the rectangle in the parent's coordinate system
-  // instead of in the coordinate system of this layer?
 
   @override
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
@@ -328,12 +360,14 @@ class ClipRectLayer extends ContainerLayer {
 
 /// A composite layer that clips its children using a rounded rectangle
 class ClipRRectLayer extends ContainerLayer {
+  /// Creates a layer with a rounded-rectangular clip.
+  ///
+  /// The [clipRRect] property must be non-null before the compositing phase of
+  /// the pipeline.
   ClipRRectLayer({ this.clipRRect });
 
   /// The rounded-rect to clip in the parent's coordinate system
   RRect clipRRect;
-  // TODO(abarth): Why is the rounded-rect in the parent's coordinate system
-  // instead of in the coordinate system of this layer?
 
   @override
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
@@ -351,12 +385,14 @@ class ClipRRectLayer extends ContainerLayer {
 
 /// A composite layer that clips its children using a path
 class ClipPathLayer extends ContainerLayer {
+  /// Creates a layer with a path-based clip.
+  ///
+  /// The [clipPath] property must be non-null before the compositing phase of
+  /// the pipeline.
   ClipPathLayer({ this.clipPath });
 
   /// The path to clip in the parent's coordinate system
   Path clipPath;
-  // TODO(abarth): Why is the path in the parent's coordinate system instead of
-  // in the coordinate system of this layer?
 
   @override
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
@@ -374,7 +410,14 @@ class ClipPathLayer extends ContainerLayer {
 
 /// A composited layer that applies a transformation matrix to its children
 class TransformLayer extends OffsetLayer {
-  TransformLayer({ Offset offset: Offset.zero, this.transform }): super(offset: offset);
+  /// Creates a transform layer.
+  ///
+  /// The [transform] property must be non-null before the compositing phase of
+  /// the pipeline.
+  TransformLayer({
+    Offset offset: Offset.zero,
+    this.transform
+  }): super(offset: offset);
 
   /// The matrix to apply
   Matrix4 transform;
@@ -398,6 +441,10 @@ class TransformLayer extends OffsetLayer {
 
 /// A composited layer that makes its children partially transparent
 class OpacityLayer extends ContainerLayer {
+  /// Creates an opacity layer.
+  ///
+  /// The [alpha] property must be non-null before the compositing phase of
+  /// the pipeline.
   OpacityLayer({ this.alpha });
 
   /// The amount to multiply into the alpha channel
@@ -422,6 +469,10 @@ class OpacityLayer extends ContainerLayer {
 
 /// A composited layer that applies a shader to hits children.
 class ShaderMaskLayer extends ContainerLayer {
+  /// Creates a shader mask layer.
+  ///
+  /// The [shader], [maskRect], and [transferMode] properties must be non-null
+  /// before the compositing phase of the pipeline.
   ShaderMaskLayer({ this.shader, this.maskRect, this.transferMode });
 
   /// The shader to apply to the children.
@@ -451,6 +502,10 @@ class ShaderMaskLayer extends ContainerLayer {
 
 /// A composited layer that applies a filter to the existing contents of the scene.
 class BackdropFilterLayer extends ContainerLayer {
+  /// Creates a backdrop filter layer.
+  ///
+  /// The [filter] property must be non-null before the compositing phase of the
+  /// pipeline.
   BackdropFilterLayer({ this.filter });
 
   /// The filter to apply to the existing contents of the scene.

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

@@ -11,7 +11,20 @@ import 'viewport.dart';
 /// Parent data for use with [RenderList].
 class ListParentData extends ContainerBoxParentDataMixin<RenderBox> { }
 
+/// A linear layout of children intended for use as a virtual viewport.
+///
+/// Children are layout out in order along the main axis. If [itemExtent] is
+/// non-null, each child is required to have exactly [itemExtent] extent in the
+/// main axis. If [itemExtent] is null, each child is required to have the same
+/// extent in the main axis as the list itself.
+///
+/// In the cross axis, the render list expands to fill the available space and
+/// each child is required to have the same extent in the cross axis as the list
+/// itself.
 class RenderList extends RenderVirtualViewport<ListParentData> {
+  /// Creates a render list.
+  ///
+  /// By default, the list is oriented vertically and anchored at the start.
   RenderList({
     List<RenderBox> children,
     double itemExtent,
@@ -35,16 +48,20 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
     addAll(children);
   }
 
+  /// The main-axis extent of each item in the list.
+  ///
+  /// If [itemExtent] is null, the items are required to match the main-axis
+  /// extent of the list itself.
   double get itemExtent => _itemExtent;
   double _itemExtent;
   set itemExtent (double newValue) {
-    assert(newValue != null);
     if (_itemExtent == newValue)
       return;
     _itemExtent = newValue;
     markNeedsLayout();
   }
 
+  /// The amount of space by which to inset the children inside the list.
   EdgeInsets get padding => _padding;
   EdgeInsets _padding;
   set padding (EdgeInsets newValue) {

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

@@ -35,6 +35,10 @@ class ParentData {
   String toString() => '<none>';
 }
 
+/// Signature for painting into a [PaintingContext].
+///
+/// The `offset` argument is the offset from the origin of the coordinate system
+/// of the [PaintingContext.canvas] to the coordinate system of the callee.
 typedef void PaintingContextCallback(PaintingContext context, Offset offset);
 
 /// A place to paint.
@@ -208,6 +212,7 @@ class PaintingContext {
     ));
   }
 
+  /// (mojo-only) Draws content from another process.
   void pushChildScene(Offset offset, double devicePixelRatio, int physicalWidth, int physicalHeight, mojom.SceneToken sceneToken) {
     _stopRecordingIfNeeded();
     _appendLayer(new ChildSceneLayer(
@@ -458,9 +463,11 @@ abstract class Constraints {
   });
 }
 
+/// Signature for a function that is called for each [RenderObject].
 typedef void RenderObjectVisitor(RenderObject child);
+
+/// Signature for a function that is called during layout.
 typedef void LayoutCallback(Constraints constraints);
-typedef double ExtentCallback(Constraints constraints);
 
 class _SemanticsGeometry {
   _SemanticsGeometry() : transform = new Matrix4.identity();
@@ -975,6 +982,9 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// Override in subclasses with children and call the visitor for each child
   void visitChildren(RenderObjectVisitor visitor) { }
 
+  /// The object responsible for creating this render object.
+  ///
+  /// Used in debug messages.
   dynamic debugCreator;
   void _debugReportException(String method, dynamic exception, StackTrace stack) {
     FlutterError.reportError(new FlutterErrorDetailsForRendering(
@@ -1601,7 +1611,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// called on).
   ///
   /// This might be called if, e.g., the device pixel ratio changed.
-  void replaceRootLayer(ContainerLayer rootLayer) {
+  void replaceRootLayer(OffsetLayer rootLayer) {
     assert(attached);
     assert(parent is! RenderObject);
     assert(!owner._debugDoingPaint);

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

@@ -9,6 +9,7 @@ import 'object.dart';
 /// without making the child available for hit testing, and without taking any
 /// room in the parent.
 class RenderOffStage extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  /// Creates an off-stage render object.
   RenderOffStage({ RenderBox child }) {
     this.child = child;
   }

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

@@ -57,6 +57,9 @@ class RenderPerformanceOverlay extends RenderBox {
     markNeedsPaint();
   }
 
+  /// The rasterizer threshold is an integer specifying the number of frame
+  /// intervals that the rasterizer must miss before it decides that the frame
+  /// is suitable for capturing an SkPicture trace for further analysis.
   int get rasterizerThreshold => _rasterizerThreshold;
   int _rasterizerThreshold;
   set rasterizerThreshold (int threshold) {
@@ -82,7 +85,7 @@ class RenderPerformanceOverlay extends RenderBox {
     return constraints.constrainWidth(0.0);
   }
 
-  double get intrinsicHeight {
+  double get _intrinsicHeight {
     const double kDefaultGraphHeight = 80.0;
     double result = 0.0;
     if ((optionsMask | (1 << PerformanceOverlayOption.displayRasterizerStatistics.index) > 0) ||
@@ -96,17 +99,17 @@ class RenderPerformanceOverlay extends RenderBox {
 
   @override
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(intrinsicHeight);
+    return constraints.constrainHeight(_intrinsicHeight);
   }
 
   @override
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(intrinsicHeight);
+    return constraints.constrainHeight(_intrinsicHeight);
   }
 
   @override
   void performResize() {
-    size = constraints.constrain(new Size(double.INFINITY, intrinsicHeight));
+    size = constraints.constrain(new Size(double.INFINITY, _intrinsicHeight));
   }
 
   @override

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

@@ -32,6 +32,11 @@ export 'package:flutter/gestures.dart' show
 /// for render objects that wish to mimic most, but not all, of the properties
 /// of their child.
 class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  /// Creates a proxy render box.
+  ///
+  /// Proxy render boxes are rarely created directly because they simply proxy
+  /// the render box protocol to [child]. Instead, consider using one of the
+  /// subclasses.
   RenderProxyBox([RenderBox child = null]) {
     this.child = child;
   }

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

@@ -18,6 +18,9 @@ const double _kQuarterTurnsInRadians = math.PI / 2.0;
 /// this object applies its rotation prior to layout, which means the entire
 /// rotated box consumes only as much space as required by the rotated child.
 class RenderRotatedBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  /// Creates a rotated render box.
+  ///
+  /// The [quarterTurns] argument must not be null.
   RenderRotatedBox({
     int quarterTurns,
     RenderBox child

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

@@ -11,6 +11,7 @@ import 'object.dart';
 /// Abstract class for one-child-layout render boxes that provide control over
 /// the child's position.
 abstract class RenderShiftedBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  /// Initializes the [child] property for sublasses.
   RenderShiftedBox(RenderBox child) {
     this.child = child;
   }

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

@@ -49,6 +49,7 @@ class RelativeRect {
     );
   }
 
+  /// A rect that covers the entire container.
   static final RelativeRect fill = new RelativeRect.fromLTRB(0.0, 0.0, 0.0, 0.0);
 
   /// Distance from the left side of the container to the left side of this rectangle.

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

@@ -10,6 +10,7 @@ import 'object.dart';
 
 /// Parent data used by [RenderTable] for its children.
 class TableCellParentData extends BoxParentData {
+  /// Where this cell should be placed vertically.
   TableCellVerticalAlignment verticalAlignment;
 
   /// The column that the child was in the last time it was laid out.
@@ -74,7 +75,7 @@ abstract class TableColumnWidth {
 /// A flex value can be provided. If specified (and non-null), the
 /// column will participate in the distribution of remaining space
 /// once all the non-flexible columns have been sized.
-class IntrinsicColumnWidth extends TableColumnWidth { 
+class IntrinsicColumnWidth extends TableColumnWidth {
   const IntrinsicColumnWidth({ double flex }) : _flex = flex;
 
   @override
@@ -500,7 +501,7 @@ class RenderTable extends RenderBox {
     _border = value;
     markNeedsPaint();
   }
- 
+
   List<Decoration> get rowDecorations => new List<Decoration>.unmodifiable(_rowDecorations ?? const <Decoration>[]);
   List<Decoration> _rowDecorations;
   List<BoxPainter> _rowDecorationPainters;
@@ -830,7 +831,7 @@ class RenderTable extends RenderBox {
       // least as big as the target width.
       double targetWidth;
       if (maxWidthConstraint.isFinite) {
-        targetWidth = maxWidthConstraint; 
+        targetWidth = maxWidthConstraint;
       } else {
         targetWidth = minWidthConstraint;
       }
@@ -1098,7 +1099,7 @@ class RenderTable extends RenderBox {
     for (int index = 0; index < _children.length; index += 1) {
       RenderBox child = _children[index];
       if (child != null) {
-        final BoxParentData childParentData = child.parentData; 
+        final BoxParentData childParentData = child.parentData;
         context.paintChild(child, childParentData.offset + offset);
       }
     }

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

@@ -15,6 +15,9 @@ import 'binding.dart';
 
 /// The layout constraints for the root render object.
 class ViewConfiguration {
+  /// Creates a view configuration.
+  ///
+  /// By default, the view has zero [size] and a [devicePixelRatio] of 1.0.
   const ViewConfiguration({
     this.size: Size.zero,
     this.devicePixelRatio: 1.0,

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

@@ -24,6 +24,9 @@ enum ViewportAnchor {
 
 /// The interior and exterior dimensions of a viewport.
 class ViewportDimensions {
+  /// Creates dimensions for a viewport.
+  ///
+  /// By default, the content and container sizes are zero.
   const ViewportDimensions({
     this.contentSize: Size.zero,
     this.containerSize: Size.zero

packages/flutter/lib/src/widgets/child_view.dart

@@ -27,7 +27,9 @@ class _ChildViewWidget extends LeafRenderObjectWidget {
   _ChildViewWidget({
     ChildViewConnection child,
     this.scale
-  }) : child = child, super(key: new GlobalObjectKey(child));
+  }) : child = child, super(key: new GlobalObjectKey(child)) {
+    assert(scale != null);
+  }
 
   final ChildViewConnection child;
   final double scale;