dispatch_group_create(3)				   BSD Library Functions Manual 				  dispatch_group_create(3)

NAME

     dispatch_group_create, dispatch_group_async, dispatch_group_wait, dispatch_group_notify -- group blocks submitted to queues

SYNOPSIS

     #include <dispatch/dispatch.h>

     dispatch_group_t
     dispatch_group_create(void);

     void
     dispatch_group_enter(dispatch_group_t group);

     void
     dispatch_group_leave(dispatch_group_t group);

     long
     dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);

     void
     dispatch_group_notify(dispatch_group_t group, dispatch_queue_t queue, void (^block)(void));

     void
     dispatch_group_notify_f(dispatch_group_t group, dispatch_queue_t queue, void *context, void (*function)(void *));

     void
     dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, void (^block)(void));

     void
     dispatch_group_async_f(dispatch_group_t group, dispatch_queue_t queue, void *context, void (*function)(void *));

DESCRIPTION

     A dispatch group is an association of one or more blocks submitted to dispatch queues for asynchronous invocation.  Applications may use dis-
     patch groups to wait for the completion of blocks associated with the group.

     The dispatch_group_create() function returns a new and empty dispatch group.

     The dispatch_group_enter() and dispatch_group_leave() functions update the number of blocks running within a group.

     The dispatch_group_wait() function waits until all blocks associated with the group have completed, or until the specified timeout has
     elapsed.  If the group becomes empty within the specified amount of time, the function will return zero indicating success. Otherwise, a non-
     zero return code will be returned.  When DISPATCH_TIME_FOREVER is passed as the timeout, calls to this function will wait an unlimited amount
     of time until the group becomes empty and the return value is always zero.

     The dispatch_group_notify() function provides asynchronous notification of the completion of the blocks associated with the group by submit-
     ting the block to the specified queue once all blocks associated with the group have completed.  The system holds a reference to the dispatch
     group while an asynchronous notification is pending, therefore it is valid to release the group after setting a notification block.  The
     group will be empty at the time the notification block is submitted to the target queue. The group may either be released with
     dispatch_release() or reused for additional operations.

     The dispatch_group_async() convenience function behaves like so:

     void
     dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block)
     {
	     dispatch_retain(group);
	     dispatch_group_enter(group);
	     dispatch_async(queue, ^{
		     block();
		     dispatch_group_leave(group);
		     dispatch_release(group);
	     });
     }

RETURN VALUE

     The dispatch_group_create() function returns NULL on failure and non-NULL on success.

     The dispatch_group_wait() function returns zero upon success and non-zero after the timeout expires.  If the timeout is
     DISPATCH_TIME_FOREVER, then dispatch_group_wait() waits forever and always returns zero.

MEMORY MODEL

     Dispatch groups are retained and released via calls to dispatch_retain() and dispatch_release().

FUNDAMENTALS

     The dispatch_group_async() and dispatch_group_notify() functions are wrappers around dispatch_group_async_f() and dispatch_group_notify_f()
     respectively.

CAVEATS

     In order to ensure deterministic behavior, it is recommended to call dispatch_group_wait() only once all blocks have been submitted to the
     group. If it is later determined that new blocks should be run, it is recommended not to reuse an already-running group, but to create a new
     group.

     dispatch_group_wait() returns as soon as there are exactly zero enqueued or running blocks associated with a group (more precisely, as soon
     as every dispatch_group_enter() call has been balanced by a dispatch_group_leave() call). If one thread waits for a group while another
     thread submits new blocks to the group, then the count of associated blocks might momentarily reach zero before all blocks have been submit-
     ted. If this happens, dispatch_group_wait() will return too early: some blocks associated with the group have finished, but some have not yet
     been submitted or run.

     However, as a special case, a block associated with a group may submit new blocks associated with its own group. In this case, the behavior
     is deterministic: a waiting thread will not wake up until the newly submitted blocks have also finished.

     All of the foregoing also applies to dispath_group_notify() as well, with "block to be submitted" substituted for "waiting thread".

SEE ALSO

     dispatch(3), dispatch_async(3), dispatch_object(3), dispatch_queue_create(3), dispatch_semaphore_create(3), dispatch_time(3)

Darwin								    May 1, 2009 							    Darwin