Gets a [class@Gio.
Cancellable] that will cancel when the promise has been discarded (and therefore result no longer necessary).
This is useful when manually implementing wrappers around various [callback@Gio.AsyncReadyCallback] based API.
If this was created with [ctor@Dex.Promise.new], then null is returned.
| this |
a [class@Dex.Promise] |
|
a [class@Gio.Cancellable] or null |