gi-gio-2.0.18: Gio bindings

CopyrightWill Thompson Iñaki García Etxebarria and Jonas Platte
MaintainerIñaki García Etxebarria (
Safe HaskellNone




Provides a base class for implementing asynchronous function results.

Asynchronous operations are broken up into two separate operations which are chained together by a AsyncReadyCallback. To begin an asynchronous operation, provide a AsyncReadyCallback to the asynchronous function. This callback will be triggered when the operation has completed, and must be run in a later iteration of the [thread-default main context][g-main-context-push-thread-default] from where the operation was initiated. It will be passed a AsyncResult instance filled with the details of the operation's success or failure, the object the asynchronous function was started for and any error codes returned. The asynchronous callback function is then expected to call the corresponding "_finish()" function, passing the object the function was called for, the AsyncResult instance, and (optionally) an error to grab any error conditions that may have occurred.

The "_finish()" function for an operation takes the generic result (of type AsyncResult) and returns the specific result that the operation in question yields (e.g. a FileEnumerator for a "enumerate children" operation). If the result or error status of the operation is not needed, there is no need to call the "_finish()" function; GIO will take care of cleaning up the result and error information after the AsyncReadyCallback returns. You can pass Nothing for the AsyncReadyCallback if you don't need to take any action at all after the operation completes. Applications may also take a reference to the AsyncResult and call "_finish()" later; however, the "_finish()" function may be called at most once.

Example of a typical asynchronous operation flow:

C code

void _theoretical_frobnitz_async (Theoretical         *t,
                                  GCancellable        *c,
                                  GAsyncReadyCallback  cb,
                                  gpointer             u);

gboolean _theoretical_frobnitz_finish (Theoretical   *t,
                                       GAsyncResult  *res,
                                       GError       **e);

static void
frobnitz_result_func (GObject      *source_object,
		 GAsyncResult *res,
		 gpointer      user_data)
  gboolean success = FALSE;

  success = _theoretical_frobnitz_finish (source_object, res, NULL);

  if (success)
    g_printf ("Hurray!\n");
    g_printf ("Uh oh!\n");



int main (int argc, void *argv[])

   _theoretical_frobnitz_async (theoretical_data,


The callback for an asynchronous operation is called only once, and is always called, even in the case of a cancelled operation. On cancellation the result is a IOErrorEnumCancelled error.

# {io-priority}

Many I/O-related asynchronous operations have a priority parameter, which is used in certain cases to determine the order in which operations are executed. They are not used to determine system-wide I/O scheduling. Priorities are integers, with lower numbers indicating higher priority. It is recommended to choose priorities between PRIORITY_LOW and PRIORITY_HIGH, with PRIORITY_DEFAULT as a default.


Exported types

newtype AsyncResult Source #

Memory-managed wrapper type.

class GObject o => IsAsyncResult o Source #

Type class for types which can be safely cast to AsyncResult, for instance with toAsyncResult.

toAsyncResult :: (MonadIO m, IsAsyncResult o) => o -> m AsyncResult Source #

Cast to AsyncResult, for types for which this is known to be safe. For general casts, use castTo.



asyncResultGetSourceObject Source #


:: (HasCallStack, MonadIO m, IsAsyncResult a) 
=> a

res: a AsyncResult

-> m (Maybe Object)

Returns: a new reference to the source object for the res, or Nothing if there is none.

Gets the source object from a AsyncResult.


asyncResultGetUserData Source #


:: (HasCallStack, MonadIO m, IsAsyncResult a) 
=> a

res: a AsyncResult.

-> m (Ptr ())

Returns: the user data for res.

Gets the user data from a AsyncResult.


asyncResultIsTagged Source #


:: (HasCallStack, MonadIO m, IsAsyncResult a) 
=> a

res: a AsyncResult

-> Ptr ()

sourceTag: an application-defined tag

-> m Bool

Returns: True if res has the indicated sourceTag, False if not.

Checks if res has the given sourceTag (generally a function pointer indicating the function res was created by).

Since: 2.34


asyncResultLegacyPropagateError Source #


:: (HasCallStack, MonadIO m, IsAsyncResult a) 
=> a

res: a AsyncResult

-> m ()

(Can throw GError)

If res is a SimpleAsyncResult, this is equivalent to simpleAsyncResultPropagateError. Otherwise it returns False.

This can be used for legacy error handling in async *_finish() wrapper functions that traditionally handled SimpleAsyncResult error returns themselves rather than calling into the virtual method. This should not be used in new code; AsyncResult errors that are set by virtual methods should also be extracted by virtual methods, to enable subclasses to chain up correctly.

Since: 2.34