Asynchronously checks if this exists.
When the operation is finished, callback will be invoked in the <link linkend="g-main-context-push-thread-default">
thread-default main loop</link> of the thread you are calling this method from. You can then call
exists.end to get the result of the operation.
| this |
A Subject. |
| cancellable |
A Cancellable or null. |
| callback |
A TaskReadyCallback to call when the request is satisfied |
| user_data |
The data to pass to |