delegate property
The delegate that controls the layout of the children.
Implementation
MultiChildLayoutDelegate get delegate => _delegate;Implementation
set delegate(MultiChildLayoutDelegate value) {
  assert(value != null);
  if (_delegate == value)
    return;
  if (value.runtimeType != _delegate.runtimeType || value.shouldRelayout(_delegate))
    markNeedsLayout();
  _delegate = value;
}