gobject:object, common-lisp:standard-object, common-lisp:t

None

The gio:cancellable object allows operations to be cancelled. The g:cancellable object is a thread-safe operation cancellation stack used throughout GIO to allow for cancellation of synchronous and asynchronous operations.

The "cancelled" signal

lambda (cancellable)    :run-last      

cancellable

The g:cancellable object.

Emitted when the operation has been cancelled. Can be used by implementations of cancellable operations. If the operation is cancelled from another thread, the signal will be emitted in the thread that cancelled the operation, not the thread that is running the operation.

Note that disconnecting from this signal, or any signal, in a multi-threaded program is prone to race conditions. For instance it is possible that a signal handler may be invoked even after a call to the g:signal-handler-disconnect function for that handler has already returned.

There is also a problem when cancellation happen right before connecting to the signal. If this happens the signal will unexpectedly not be emitted, and checking before connecting to the signal leaves a race condition where this is still happening.

In order to make it safe and easy to connect handlers there are two helper functions: the g:cancellable-connect and g:cancellable-disconnect functions which protect against problems like this.

An example of how to us this:

/* Make sure we don't do any unnecessary work if already cancelled */
if (g_cancellable_set_error_if_cancelled (cancellable))
  return;
/* Set up all the data needed to be able to
 * handle cancellation of the operation */
my_data = my_data_new (...);


id = 0;
if (cancellable)
  id = g_cancellable_connect (cancellable,
                  G_CALLBACK (cancelled_handler)
                  data, NULL);


/* cancellable operation here... */


g_cancellable_disconnect (cancellable, id);


/* cancelled_handler is never called after this, it
 * is now safe to free the data */
my_data_free (my_data);      

Note that the cancelled signal is emitted in the thread that the user cancelled from, which may be the main thread. So, the cancellable signal should not do something that can block.