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 submitting
     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 associ-
     ated blocks might momentarily reach zero before all blocks have been sub-
     mitted. 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 submit-
     ted 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