AutoLayoutDelegate delegate

The delegate that generates constraints for the layout.

If the new delegate is the same as the previous one, this does nothing.

If the new delegate is the same class as the previous one, then the new delegate has its AutoLayoutDelegate.shouldUpdateConstraints called; if the result is true, then the delegate will be called.

If the new delegate is a different class than the previous one, then the delegate will be called.

If the delgate is null, the layout is unconstrained.

Source 

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

Source 

set delegate(AutoLayoutDelegate newDelegate) {
  if (_delegate == newDelegate)
    return;
  AutoLayoutDelegate oldDelegate = _delegate;
  _delegate = newDelegate;
  if (newDelegate == null) {
    assert(oldDelegate != null);
    _needToUpdateConstraints = true;
    markNeedsLayout();
  } else if (oldDelegate == null ||
      newDelegate.runtimeType != oldDelegate.runtimeType ||
      newDelegate.shouldUpdateConstraints(oldDelegate)) {
    _needToUpdateConstraints = true;
    markNeedsLayout();
  }
}