Improve Stack documentation.

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

@@ -18,7 +18,6 @@ import 'object.dart';
 /// If you create the RelativeRect with null values, the methods on
 /// RelativeRect will not work usefully (or at all).
 class RelativeRect {
-
   /// Creates a RelativeRect with the given values.
   const RelativeRect.fromLTRB(this.left, this.top, this.right, this.bottom);
 
@@ -136,16 +135,16 @@ class RelativeRect {
 
 /// Parent data for use with [RenderStack]
 class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
-  /// The offset of the child's top edge from the top of the stack.
+  /// The distance by which the child's top edge is inset from the top of the stack.
   double top;
 
-  /// The offset of the child's right edge from the right of the stack.
+  /// The distance by which the child's right edge is inset from the right of the stack.
   double right;
 
-  /// The offset of the child's bottom edge from the bottom of the stack.
+  /// The distance by which the child's bottom edge is inset from the bottom of the stack.
   double bottom;
 
-  /// The offset of the child's left edge from the left of the stack.
+  /// The distance by which the child's left edge is inset from the left of the stack.
   double left;
 
   /// The child's width.
@@ -167,9 +166,9 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
     left = value.left;
   }
 
-  /// Whether this child is considered positioned
+  /// Whether this child is considered positioned.
   ///
-  /// A child is positioned if any of the top, right, bottom, or left offsets
+  /// A child is positioned if any of the top, right, bottom, or left properties
   /// are non-null. Positioned children do not factor into determining the size
   /// of the stack but are instead placed relative to the non-positioned
   /// children in the stack.
@@ -411,7 +410,7 @@ abstract class RenderStackBase extends RenderBox
 /// In a stack layout, the children are positioned on top of each other in the
 /// order in which they appear in the child list. First, the non-positioned
 /// children (those with null values for top, right, bottom, and left) are
-/// initially layed out and placed in the upper-left corner of the stack. The
+/// laid out and initially placed in the upper-left corner of the stack. The
 /// stack is then sized to enclose all of the non-positioned children. If there
 /// are no non-positioned children, the stack becomes as large as possible.
 ///
@@ -419,23 +418,25 @@ abstract class RenderStackBase extends RenderBox
 /// parameter. The left of each non-positioned child becomes the
 /// difference between the child's width and the stack's width scaled by
 /// alignment.x. The top of each non-positioned child is computed
-/// similarly and scaled by alignement.y. So if the alignment x and y properties
+/// similarly and scaled by alignment.y. So if the alignment x and y properties
 /// are 0.0 (the default) then the non-positioned children remain in the
 /// upper-left corner. If the alignment x and y properties are 0.5 then the
 /// non-positioned children are centered within the stack.
 ///
 /// Next, the positioned children are laid out. If a child has top and bottom
 /// values that are both non-null, the child is given a fixed height determined
-/// by deflating the width of the stack by the sum of the top and bottom values.
+/// by subtracting the sum of the top and bottom values from the height of the stack.
-/// Similarly, if the child has rigth and left values that are both non-null,
+/// Similarly, if the child has right and left values that are both non-null,
-/// the child is given a fixed width. Otherwise, the child is given unbounded
+/// the child is given a fixed width derived from the stack's width.
-/// space in the non-fixed dimensions.
+/// Otherwise, the child is given unbounded constraints in the non-fixed dimensions.
 ///
-/// Once the child is laid out, the stack positions the child according to the
+/// Once the child is laid out, the stack positions the child
-/// top, right, bottom, and left offsets. For example, if the top value is 10.0,
+/// according to the top, right, bottom, and left properties of their
-/// the top edge of the child will be placed 10.0 pixels from the top edge of
+/// [StackParentData]. For example, if the bottom value is 10.0, the
-/// the stack. If the child extends beyond the bounds of the stack, the stack
+/// bottom edge of the child will be inset 10.0 pixels from the bottom
-/// will clip the child's painting to the bounds of the stack.
+/// edge of the stack. If the child extends beyond the bounds of the
+/// stack, the stack will clip the child's painting to the bounds of
+/// the stack.
 class RenderStack extends RenderStackBase {
   RenderStack({
     List<RenderBox> children,

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

@@ -1161,6 +1161,12 @@ class IndexedStack extends StackRenderObjectWidgetBase {
 /// to its enclosing [Stack] must contain only [StatelessWidget]s or
 /// [StatefulWidget]s (not other kinds of widgets, like [RenderObjectWidget]s).
 class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
+  /// Creates a Positioned object with the given values.
+  ///
+  /// Only two out of the three horizontal values ([left], [right],
+  /// [width]), and only two out of the three vertical values ([top],
+  /// [bottom], [height]), can be set. In each case, at least one of
+  /// the three must be null.
   Positioned({
     Key key,
     Widget child,
@@ -1175,6 +1181,11 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
     assert(top == null || bottom == null || height == null);
   }
 
+  /// Creates a Positioned object with the values from the given [Rect].
+  ///
+  /// This sets the [left], [top], [width], and [height] properties
+  /// from the given [Rect]. The [right] and [bottom] properties are
+  /// set to null.
   Positioned.fromRect({
     Key key,
     Widget child,
@@ -1187,27 +1198,39 @@ class Positioned extends ParentDataWidget<StackRenderObjectWidgetBase> {
        bottom = null,
        super(key: key, child: child);
 
-  /// The offset of the child's left edge from the left of the stack.
+  /// The distance that the child's left edge is inset from the left of the stack.
+  ///
+  /// Only two out of the three horizontal values ([left], [right], [width]) can be
+  /// set. The third must be null.
   final double left;
 
-  /// The offset of the child's top edge from the top of the stack.
+  /// The distance that the child's top edge is inset from the top of the stack.
+  ///
+  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
+  /// set. The third must be null.
   final double top;
 
-  /// The offset of the child's right edge from the right of the stack.
+  /// The distance that the child's right edge is inset from the right of the stack.
+  ///
+  /// Only two out of the three horizontal values ([left], [right], [width]) can be
+  /// set. The third must be null.
   final double right;
 
-  /// The offset of the child's bottom edge from the bottom of the stack.
+  /// The distance that the child's bottom edge is inset from the bottom of the stack.
+  ///
+  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
+  /// set. The third must be null.
   final double bottom;
 
   /// The child's width.
   ///
-  /// Only two out of the three horizontal values (left, right, width) can be
+  /// Only two out of the three horizontal values ([left], [right], [width]) can be
   /// set. The third must be null.
   final double width;
 
   /// The child's height.
   ///
-  /// Only two out of the three vertical values (top, bottom, height) can be
+  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
   /// set. The third must be null.
   final double height;