Installs a watch for the given
expression that calls the
notify function whenever the evaluation of
this 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.
the `this` argument to watch
callback to invoke when the expression changes
user data to pass to
destroy notify for
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 unwatch. You should call @ref if you want to keep the watch around.