Fix documentation for UnconstrainedBox and code cleanup (#17249)

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

@@ -484,7 +484,6 @@ class RenderPositionedBox extends RenderAligningShiftedBox {
 ///  * [RenderSizedOverflowBox], a render object that is a specific size but
 ///    passes its original constraints through to its child, which it allows to
 ///    overflow.
-
 class RenderConstrainedOverflowBox extends RenderAligningShiftedBox {
   /// Creates a render object that lets its child overflow itself.
   RenderConstrainedOverflowBox({
@@ -584,10 +583,11 @@ class RenderConstrainedOverflowBox extends RenderAligningShiftedBox {
 /// render at its "natural" size.
 ///
 /// This allows a child to render at the size it would render if it were alone
-/// on an infinite canvas with no constraints. This box will then expand
-/// as much as it can within its own constraints and align the child based on
-/// [alignment]. If the box cannot expand enough to accommodate the entire
-/// child, the child will be clipped.
+/// on an infinite canvas with no constraints. This box will then attempt to
+/// adopt the same size, within the limits of its own constraints. If it ends
+/// up with a different size, it will align the child based on [alignment].
+/// If the box cannot expand enough to accommodate the entire child, the
+/// child will be clipped.
 ///
 /// In debug mode, if the child overflows the box, a warning will be printed on
 /// the console, and black and yellow striped areas will appear where the
@@ -595,9 +595,9 @@ class RenderConstrainedOverflowBox extends RenderAligningShiftedBox {
 ///
 /// See also:
 ///
-///  * [RenderConstrainedBox] renders a box which imposes constraints on its
-///    child.
-///  * [RenderConstrainedOverflowBox], renders a box that imposes different
+///  * [RenderConstrainedBox], which renders a box which imposes constraints
+///    on its child.
+///  * [RenderConstrainedOverflowBox], which renders a box that imposes different
 ///    constraints on its child than it gets from its parent, possibly allowing
 ///    the child to overflow the parent.
 ///  * [RenderSizedOverflowBox], a render object that is a specific size but
@@ -641,40 +641,31 @@ class RenderUnconstrainedBox extends RenderAligningShiftedBox with DebugOverflow
     if (child != null) {
       // Let the child lay itself out at it's "natural" size, but if
       // constrainedAxis is non-null, keep any constraints on that axis.
+      BoxConstraints childConstraints;
       if (constrainedAxis != null) {
         switch (constrainedAxis) {
           case Axis.horizontal:
-            child.layout(new BoxConstraints(
-              maxWidth: constraints.maxWidth, minWidth: constraints.minWidth),
-              parentUsesSize: true,
-            );
+            childConstraints = new BoxConstraints(maxWidth: constraints.maxWidth, minWidth: constraints.minWidth);
             break;
           case Axis.vertical:
-            child.layout(new BoxConstraints(
-              maxHeight: constraints.maxHeight, minHeight: constraints.minHeight),
-              parentUsesSize: true,
-            );
+            childConstraints = new BoxConstraints(maxHeight: constraints.maxHeight, minHeight: constraints.minHeight);
             break;
         }
       } else {
-        child.layout(const BoxConstraints(), parentUsesSize: true);
+        childConstraints = const BoxConstraints();
       }
+      child.layout(childConstraints, parentUsesSize: true);
       size = constraints.constrain(child.size);
       alignChild();
       final BoxParentData childParentData = child.parentData;
       _overflowContainerRect = Offset.zero & size;
       _overflowChildRect = childParentData.offset & child.size;
     } else {
-      size = constraints.constrain(Size.zero);
+      size = constraints.smallest;
       _overflowContainerRect = Rect.zero;
       _overflowChildRect = Rect.zero;
     }
-
-    final RelativeRect overflow = new RelativeRect.fromRect(_overflowContainerRect, _overflowChildRect);
-    _isOverflowing = overflow.left > 0.0 ||
-      overflow.right > 0.0 ||
-      overflow.top > 0.0 ||
-      overflow.bottom > 0.0;
+    _isOverflowing = new RelativeRect.fromRect(_overflowContainerRect, _overflowChildRect).hasInsets;
   }
 
   @override

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

@@ -77,6 +77,12 @@ class RelativeRect {
   /// May be negative if the bottom side of the rectangle is outside of the container.
   final double bottom;
 
+  /// Returns whether any of the values are greater than zero.
+  ///
+  /// This corresponds to one of the sides ([left], [top], [right], or [bottom]) having
+  /// some positive inset towards the center.
+  bool get hasInsets => left > 0.0 || top > 0.0 || right > 0.0 || bottom > 0.0;
+
   /// Returns a new rectangle object translated by the given offset.
   RelativeRect shift(Offset offset) {
     return new RelativeRect.fromLTRB(left + offset.dx, top + offset.dy, right - offset.dx, bottom - offset.dy);

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

@@ -1775,10 +1775,11 @@ class ConstrainedBox extends SingleChildRenderObjectWidget {
 /// at its "natural" size.
 ///
 /// This allows a child to render at the size it would render if it were alone
-/// on an infinite canvas with no constraints. This container will then expand
-/// as much as it can within its own constraints and align the child based on
-/// [alignment]. If the container cannot expand enough to accommodate the
-/// entire child, the child will be clipped.
+/// on an infinite canvas with no constraints. This container will then attempt
+/// to adopt the same size, within the limits of its own constraints. If it ends
+/// up with a different size, it will align the child based on [alignment].
+/// If the box cannot expand enough to accommodate the entire child, the
+/// child will be clipped.
 ///
 /// In debug mode, if the child overflows the container, a warning will be
 /// printed on the console, and black and yellow striped areas will appear where
@@ -1786,7 +1787,9 @@ class ConstrainedBox extends SingleChildRenderObjectWidget {
 ///
 /// See also:
 ///
-///  * [ConstrainedBox] for a box which imposes constraints on its child.
+///  * [ConstrainedBox], for a box which imposes constraints on its child.
+///  * [Align], which loosens the constraints given to the child rather than
+///    removing them entirely.
 ///  * [Container], a convenience widget that combines common painting,
 ///    positioning, and sizing widgets.
 ///  * [OverflowBox], a widget that imposes different constraints on its child