Retrieves the [iface@Gtk.ConstraintTarget] used as the source for the constraint.
If the source is set to NULL at creation, the constraint will use the widget using the [class@Gtk.ConstraintLayout] as the source.
the source of the constraint
See Implementation
Retrieves the [iface@Gtk.ConstraintTarget] used as the source for the constraint.
If the source is set to NULL at creation, the constraint will use the widget using the [class@Gtk.ConstraintLayout] as the source.