positionChild method

void positionChild (Object childId, Offset offset)

Specify the child's origin relative to this origin.

Call this from your performLayout function to position each child. If you do not call this for a child, its position will remain unchanged. Children initially have their position set to (0,0), i.e. the top left of the RenderCustomMultiChildLayoutBox.

Implementation 

void positionChild(Object childId, Offset offset) {
  final RenderBox child = _idToChild[childId];
  assert(() {
    if (child == null) {
      throw FlutterError(
        'The $this custom multichild layout delegate tried to position out a non-existent child:\n'
        'There is no child with the id "$childId".'
      );
    }
    if (offset == null) {
      throw FlutterError(
        'The $this custom multichild layout delegate provided a null position for the child with id "$childId".'
      );
    }
    return true;
  }());
  final MultiChildLayoutParentData childParentData = child.parentData;
  childParentData.offset = offset;
}