the this argument to watch
callback to invoke when the expression changes
user data to pass to the notify callback
destroy notify for user_data
The newly installed watch. Note that the only reference held to the watch will be released when the watch is unwatched which can happen automatically, and not just via [method@Gtk.ExpressionWatch.unwatch]. You should call [method@Gtk.ExpressionWatch.ref] if you want to keep the watch around.
Installs a watch for the given expression that calls the notify function whenever the evaluation of self may have changed.
GTK cannot guarantee that the evaluation did indeed change when the @notify gets invoked, but it guarantees the opposite: When it did in fact change, the notify will be invoked.