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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 must be run in a later iteration of
the thread-default main context
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 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. GAsyncReadyCallback callbacks from GTask are guaranteed to be invoked in a later iteration of the thread-default main context where the GTask was created. All other users of GAsyncReadyCallback must likewise call it asynchronously in a later iteration of the main context.
Parameters
source_object |
the object the asynchronous operation was started with. |
[nullable] |
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.
g_async_result_get_source_object ()
GObject *
g_async_result_get_source_object (GAsyncResult *res);
Gets the source object from a GAsyncResult.
Returns
a new reference to the source
object for the res
, or NULL if there is none.
[transfer full][nullable]
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).
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.
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.