AsyncResult

class AsyncResult(*args, **kwargs)

Implementations: SimpleAsyncResult, Task

GAsyncResult provides a base class for implementing asynchronous function results.

Asynchronous operations are broken up into two separate operations which are chained together by a GAsyncReadyCallback. To begin an asynchronous operation, provide a GAsyncReadyCallback 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 (see push_thread_default) from where the operation was initiated. It will be passed a GAsyncResult 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 GAsyncResult 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 GAsyncResult) 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 GAsyncReadyCallback returns. You can pass NULL for the GAsyncReadyCallback if you don’t need to take any action at all after the operation completes. Applications may also take a reference to the GAsyncResult and call _finish() later; however, the _finish() function may be called at most once.

Example of a typical asynchronous operation flow:

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");
  else
    g_printf ("Uh oh!\n");

  ...

}

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

   _theoretical_frobnitz_async (theoretical_data,
                                NULL,
                                frobnitz_result_func,
                                NULL);

   ...
}

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 G_IO_ERROR_CANCELLED error.

I/O 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 G_PRIORITY_LOW and G_PRIORITY_HIGH, with G_PRIORITY_DEFAULT as a default.

Methods

class AsyncResult

get_source_object() → Object | None

Gets the source object from a AsyncResult.

get_user_data() → Any | None

Gets the user data from a AsyncResult.

is_tagged(source_tag: Any = None) → bool

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

Added in version 2.34.

Parameters:

source_tag – an application-defined tag

legacy_propagate_error() → bool

If res is a SimpleAsyncResult, this is equivalent to propagate_error. 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.

Added in version 2.34.

Virtual Methods

class AsyncResult

do_get_source_object() → Object | None

Gets the source object from a AsyncResult.

do_get_user_data() → Any | None

Gets the user data from a AsyncResult.

do_is_tagged(source_tag: Any = None) → bool

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

Added in version 2.34.

Parameters:

source_tag – an application-defined tag