SingleChildLayoutDelegate delegate

A delegate that controls this object's layout.

Source 

SingleChildLayoutDelegate get delegate => _delegate;
void delegate=(SingleChildLayoutDelegate newDelegate)

Source 

set delegate (SingleChildLayoutDelegate newDelegate) {
  assert(newDelegate != null);
  if (_delegate == newDelegate)
    return;
  if (newDelegate.runtimeType != _delegate.runtimeType || newDelegate.shouldRelayout(_delegate))
    markNeedsLayout();
  _delegate = newDelegate;
}