Question

I need to perform three tasks that are independent one from each other, so I’d like to execute them concurrently. But I need them all to have finished to notify another object. AFAIK, *dispatch_apply* creates concurrent threads, but it iterates a collection or an array of objects and performs the same task a number of loops, and I want to perform a different task for each thread. Is it possible to do what I want by using GCD? If not, what should be the best way?

Thanks!

Answer 1

Use a dispatch_group. The Concurrency Programming Guide gives one example, and there is more API that might also help you.

• Create a dispatch group using dispatch_group_create.

• Put each "task" in the group by enqueuing it using dispatch_group_async.

  (Or, manually tell GCD when each task starts and stops, using dispatch_group_enter and dispatch_group_leave.)

• To run a block when all tasks in the group have completed, enqueue it using dispatch_group_notify.

  (Or, in the unlikely case that the design of your app allows you to wait synchronously, use dispatch_group_wait instead.)

• When you’re done with the group, dispatch_release it.