LayoutCallback callback

Called during layout to determine the render object's children.

Typically the callback will mutate the child list appropriately, for example so the child list contains only visible children.

Source 

LayoutCallback get callback => _callback;
void callback=(LayoutCallback value)

Source 

set callback(LayoutCallback value) {
  if (value == _callback)
    return;
  _callback = value;
  markNeedsLayout();
}