Using GCD to wait for a condition - objective-c

Question

I'm trying to enforce a specific order for tasks to complete using Grand Central Dispatch but I'm having a bit of trouble understanding the correct way to do it. I tried using groups in the following way:
Initialization:
startup = dispatch_group_create();
Tasks that need to wait:
//Don't want to wait on the main thread, so dispatch async to a concurrent queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0L),^{
//Wait until we're finished starting up
dispatch_group_wait(startup,DISPATCH_TIME_FOREVER);
//Now we can do this stuff back on the main queue
dispatch_async(dispatch_get_main_queue(),^{
//Do work
});
});
Work that I need to wait for:
dispatch_group_async(startup,dispatch_get_main_queue(),^{ // work });
Due to the nature of my app, the tasks that need to wait can occur BEFORE the work that I need to wait for. What I really want is the ability to wait on a condition that way when the condition is done, it's done, and all future threads can do their thing. Does GCD have this?

Answer 1

I'm not sure of all the details of your implementation, so forgive me if I'm repeating what you already know.
Create a dispatch group
Use dispatch_group_async to a serial queue. Using the serial queue, you are assured that your tasks are processed in the order you wish. Use a concurrent queue if you the order doesn't matter, but your question said that they had to complete in a specific order.
After you are done dispatching all of your tasks, use dispatch_group_notify. This will execute a block on the queue you specify once all the tasks assigned to the group have finished processing.
dispatch_group_notify(startup, dispatch_get_main_queue(), ^{
// Don't forget to release the dispatch group!
dispatch_release(startup)
// perform work block;
});
For a particular parsing activity, I need to do some processing of data that can be done while the rest of the parsing activity is ongoing. I assign the processing to a group on a concurrent queue. Then when my parsing is done, I check to see if the group is done. If it is done, I clean things up. If not, utilize the dispatch_group_notify() and execute the work afterwards. Something along these lines:
double delayInSeconds = 2.0;
dispatch_time_t groupWaitTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
if (dispatch_group_wait(myDispatchGroup, groupWaitTime)==0){
NSLog(#"dispatch group completed in time");
dispatch_release(myDispatchGroup);
[self parsingCompleteWithActivity:activity];
}else{
NSLog(#"dispatch group did not complete in time");
dispatch_group_notify(myDispatchGroup, dispatch_get_main_queue(), ^{
dispatch_release(myDispatchGroup);
[self parsingCompleteWithActivity:activity];
});
}
Good luck!

Answer 2

Works if I use a semaphore and then signal after each call to wait.
Also works if I call dispatch_group_enter and dispatch_group_leave.