1. override
void setupParentData(RenderObject child)

Override to setup parent data correctly for your children.

You can call this function to set up the parent data for child before the child is added to the parent's child list.

Source 

@override
void setupParentData(RenderObject child) {
  if (child.parentData is! BoxParentData)
    child.parentData = new BoxParentData();
}