GAsyncResult

GAsyncResult — Asynchronous Function Results

Types and Values

Object Hierarchy

    GInterface
    ╰── GAsyncResult

Prerequisites

GAsyncResult requires GObject.

Known Implementations

GAsyncResult is implemented by GSimpleAsyncResult and GTask.

Includes

#include <gio/gio.h>

Description

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 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 GFileEnumerator 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:

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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.

Functions

GAsyncReadyCallback ()

void
(*GAsyncReadyCallback) (GObject *source_object,
                        GAsyncResult *res,
                        gpointer user_data);

Type definition for a function that will be called back when an asynchronous operation within GIO has been completed.

Parameters

source_object

the object the asynchronous operation was started with.

 

res

a GAsyncResult.

 

user_data

user data passed to the callback.

 

g_async_result_get_user_data ()

gpointer
g_async_result_get_user_data (GAsyncResult *res);

Gets the user data from a GAsyncResult.

Parameters

res

a GAsyncResult.

 

Returns

the user data for res .

[transfer full]


g_async_result_get_source_object ()

GObject *
g_async_result_get_source_object (GAsyncResult *res);

Gets the source object from a GAsyncResult.

Parameters

res

a GAsyncResult

 

Returns

a new reference to the source object for the res , or NULL if there is none.

[transfer full]


g_async_result_is_tagged ()

gboolean
g_async_result_is_tagged (GAsyncResult *res,
                          gpointer source_tag);

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

Parameters

res

a GAsyncResult

 

source_tag

an application-defined tag

 

Returns

TRUE if res has the indicated source_tag , FALSE if not.

Since: 2.34


g_async_result_legacy_propagate_error ()

gboolean
g_async_result_legacy_propagate_error (GAsyncResult *res,
                                       GError **error);

If res is a GSimpleAsyncResult, this is equivalent to g_simple_async_result_propagate_error(). Otherwise it returns FALSE.

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

Parameters

res

a GAsyncResult

 

error

a location to propagate the error to.

[out]

Returns

TRUE if error is has been filled in with an error from res , FALSE if not.

Since: 2.34

Types and Values

GAsyncResult

typedef struct _GAsyncResult GAsyncResult;

Holds results information for an asynchronous operation, usually passed directly to a asynchronous _finish() operation.


struct GAsyncResultIface

struct GAsyncResultIface {
  GTypeInterface g_iface;

  /* Virtual Table */

  gpointer  (* get_user_data)     (GAsyncResult *res);
  GObject * (* get_source_object) (GAsyncResult *res);

  gboolean  (* is_tagged)         (GAsyncResult *res,
				   gpointer      source_tag);
};

Interface definition for GAsyncResult.

Members

get_user_data ()

Gets the user data passed to the callback.

 

get_source_object ()

Gets the source object that issued the asynchronous operation.

 

is_tagged ()

Checks if a result is tagged with a particular source.

 

See Also

GTask