Registers a finalization notifier which will be called when the reference count of this goes down to 0.
Multiple finalization notifiers on a single closure are invoked in unspecified order. If a single call to unref results in the closure being both invalidated and finalized, then the invalidate notifiers will be run before the finalize notifiers.
| this |
a Closure |
| notify_data |
data to pass to |
| notify_func |
the callback function to register |