dispatch_group_create

Section: C Library Functions (3)
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NAME

dispatch_group_create dispatch_group_async dispatch_group_wait dispatch_group_notify - group blocks submitted to queues

SYNOPSIS

Fd #include <dispatch/dispatch.h> Ft dispatch_group_t Fo dispatch_group_create Fa void Fc Ft void Fo dispatch_group_enter Fa dispatch_group_t group Fc Ft void Fo dispatch_group_leave Fa dispatch_group_t group Fc Ft long Fo dispatch_group_wait Fa dispatch_group_t group dispatch_time_t timeout Fc Ft void Fo dispatch_group_notify Fa dispatch_group_t group dispatch_queue_t queue void (^block)(void) Fc Ft void Fo dispatch_group_notify_f Fa dispatch_group_t group dispatch_queue_t queue void *context void (*function)(void *) Fc Ft void Fo dispatch_group_async Fa dispatch_group_t group dispatch_queue_t queue void (^block)(void) Fc Ft void Fo dispatch_group_async_f Fa dispatch_group_t group dispatch_queue_t queue void *context void (*function)(void *) Fc

DESCRIPTION

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

The Fn dispatch_group_create function returns a new and empty dispatch group.

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

The Fn dispatch_group_wait function waits until all blocks associated with the Fa group have completed, or until the specified Fa timeout has elapsed. If the Fa 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 Fa 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 Fn dispatch_group_notify function provides asynchronous notification of the completion of the blocks associated with the Fa group by submitting the Fa block to the specified Fa queue once all blocks associated with the Fa 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 Fa 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 Fn dispatch_release or reused for additional operations.

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

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

MEMORY MODEL

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

FUNDAMENTALS

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

CAVEATS

In order to ensure deterministic behavior, it is recommended to call Fn 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.

Fn 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 Fn dispatch_group_enter call has been balanced by a Fn 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 submitted. If this happens, Fn 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 Fn dispath_group_notify as well, with "block to be submitted" substituted for "waiting thread".