Question

I need to send multiple tasks to Grand Central Dispatch to run. Some tasks will finish first, while some will finish last.

How do I know all my tasks in Grand Central Dispatch finished?

Should I use a counter to record the number of tasks finished? Any smarter method?

Answer 1

You can use dispatch groups to be notified when all tasks completed. This is an example from http://cocoasamurai.blogspot.com/2009/09/guide-to-blocks-grand-central-dispatch.html

dispatch_queue_t queue = dispatch_get_global_queue(0,0);
dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group,queue,^{
    NSLog(@"Block 1");
});

dispatch_group_async(group,queue,^{
    NSLog(@"Block 2");
});

dispatch_group_notify(group,queue,^{
    NSLog(@"Final block is executed last after 1 and 2");
});