BoxyChild

To lay out child widgets we first grab a BoxyChild instance, call layout, and optionally position it.

@override
Size layout() {
  // Find our children, in this case we assume there is exactly one
  final BoxyChild child = children.single;

  // Call layout on the child to obtain their size, we just pass the
  // constraints given to the CustomBoxy by its parent
  final Size size = child.layout(constraints);

  // Optionally position the child
  child.position(Offset.zero);

  // Return a size
  return size;
}

BoxyChild is just a fancy wrapper around RenderBox, it has a bunch of methods and properties that are useful for layout:

• layout, a method that lays out the child given some constraints
• layoutFit, a method that lays out and transforms the child according to a Rect and BoxFit, just like a FittedBox
• layoutRect, a method that lays out and positions the child so that it fits a Rect with an optional alignment property that behaves like an Align
• position, a method that sets the offset of the child
• setTransform, a more advanced version of position that takes a Matrix4 transform
• size, the size of the child after it's laid out
• context, the Element (aka BuildContext) of the child
• id, the id of the child which is provided by either BoxyId or an incrementing integer