computeDryLayout()

computeDryLayout() answers the question "what size would you be under these constraints?" without actually performing layout. It is the engine behind getDryLayout(), which is called by IntrinsicWidth, IntrinsicHeight, and any other render object that needs to measure a child before committing to a layout pass.

Unlike performLayout(), computeDryLayout() must not have side effects. It cannot set size, position children, or call child.layout(). It may only call child.getDryLayout() to measure children, then return a Size.

Default Implementation#

The default implementation throws an UnsupportedError. Any render object that could appear inside an IntrinsicWidth or IntrinsicHeight must override this method. If your render object will never be used in that context, override it to assert instead of silently failing.

@override
Size computeDryLayout(BoxConstraints constraints) {
  assert(debugCannotComputeDryLayout(
    reason: 'MyRenderBox does not support dry layout.',
  ));
  return Size.zero;
}

No Children — Return a Fixed Size#

A leaf render object simply applies the constraints to its natural size with no children to measure.

@override
Size computeDryLayout(BoxConstraints constraints) {
  return constraints.constrain(const Size(200, 48));
}

No Children — Take Up All Available Space#

@override
Size computeDryLayout(BoxConstraints constraints) {
  return constraints.biggest;
}

One Child — Match Child Size#

Measure the child with getDryLayout() and return that size directly. This mirrors the performLayout() pattern but uses the dry-layout equivalents throughout.

@override
Size computeDryLayout(BoxConstraints constraints) {
  final Size childSize = child!.getDryLayout(constraints);
  return constraints.constrain(childSize);
}

One Child — Add Padding#

Apply padding to the constraints before measuring the child, then add it back to the returned size.

@override
Size computeDryLayout(BoxConstraints constraints) {
  const EdgeInsets padding = EdgeInsets.all(16);
  final BoxConstraints innerConstraints = constraints.deflate(padding);
  final Size childSize = child!.getDryLayout(innerConstraints);
  return constraints.constrain(padding.inflateSize(childSize));
}

Multiple Children — Largest Child Determines Size#

Measure every child and return the size of the largest one. Useful for stack-like layouts where all children occupy the same space.

@override
Size computeDryLayout(BoxConstraints constraints) {
  Size largest = Size.zero;
  RenderBox? child = firstChild;
  while (child != null) {
    final Size childSize = child.getDryLayout(constraints);
    if (childSize.width > largest.width || childSize.height > largest.height) {
      largest = childSize;
    }
    child = (child.parentData! as ContainerBoxParentData<RenderBox>).nextSibling;
  }
  return constraints.constrain(largest);
}

Multiple Children — Sum Along an Axis#

For row or column layouts, sum child sizes along the main axis and take the maximum along the cross axis.

@override
Size computeDryLayout(BoxConstraints constraints) {
  double totalWidth = 0;
  double maxHeight = 0;
  RenderBox? child = firstChild;
  while (child != null) {
    final Size childSize = child.getDryLayout(constraints.loosen());
    totalWidth += childSize.width;
    if (childSize.height > maxHeight) maxHeight = childSize.height;
    child = (child.parentData! as ContainerBoxParentData<RenderBox>).nextSibling;
  }
  return constraints.constrain(Size(totalWidth, maxHeight));
}