Waiting on Groups of Queued Tasks

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Waiting on Groups of Queued Tasks

Dispatch groups are a way to block a thread until one or more tasks finish executing. You can use this behavior in places where you cannot make progress until all of the specified tasks are complete. For example, after dispatching several tasks to compute some data, you might use a group to wait on those tasks and then process the results when they are done. Another way to use dispatch groups is as an alternative to thread joins. Instead of starting several child threads and then joining with each of them, you could add the corresponding tasks to a dispatch group and wait on the entire group. 

Listing 3-6 shows the basic process for setting up a group, dispatching tasks to it, and waiting on the results. Instead of dispatching tasks to a queue using the dispatch_async function, you use the dispatch_group_async function instead. This function associates the task with the group and queues it for execution. To wait on a group of tasks to finish, you then use the dispatch_group_wait function, passing in the appropriate group.

Listing 3-6  Waiting on asynchronous tasks

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_t group = dispatch_group_create();

// Add a task to the group

dispatch_group_async(group, queue, ^{

   // Some asynchronous work

});

// Do some other work while the tasks execute.

// When you cannot make any more forward progress,

// wait on the group to block the current thread.

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

// Release the group when it is no longer needed.

dispatch_release(group);