How to wait past dispatch_async before proceeding? - objective-c

Question

I have a series of dispatch_async that I am performing and I would like to only update the UI when they are all done. Problem is the method within dispatch_async calls something in a separate thread so it returns before the data is fully loaded and dispatch_group_notify is called before everything is loaded.
So I introduce a infinite loop to make it wait until a flag is set.
Is this the best way? See code below.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();
for (...) {
dispatch_group_async(group, queue, ^{
__block BOOL dataLoaded = NO;
[thirdPartyCodeCallWithCompletion:^{
dataLoaded = YES;
}];
// prevent infinite loop
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)),
queue, ^{
dataLoaded = YES;
});
// infinite loop to wait until data is loaded
while (1) {
if (dataLoaded) break;
}
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
//update UI
});
}

Answer 1

You're already aware of dispatch groups. Why not just use dispatch_group_wait(), which includes support for a timeout? You can use dispatch_group_enter() and dispatch_group_leave() rather than dispatch_group_async() to make the group not done until the internal block for the third-party call with completion is finished.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();
for (...) {
dispatch_group_enter(group);
dispatch_async(queue, ^{
[thirdPartyCodeCallWithCompletion:^{
dispatch_group_leave(group);
}];
}
}
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, NSECS_PER_SEC));
dispatch_async(dispatch_get_main_queue(), ^{
//update UI
});
The use of dispatch_group_wait() does make this code synchronous, which is bad if run on the main thread. Depending on what exactly is supposed to happen if it times out, you could use dispatch_group_notify() as you were and use dispatch_after() to just updates the UI rather than trying to pretend the block completed.
Update: I tweaked my code to make sure that "update UI" happens on the main queue, just in case this code isn't already on the main thread.
By the way, I only used dispatch_async() for the block which calls thirdPartyCodeCallWithCompletion: because your original used dispatch_group_async() and I wasn't sure that the hypothetical method was asynchronous. Most APIs which take a completion block are asynchronous, though. If that one is, then you can just invoke it directly.

Answer 2

Another method is to use semaphore and the dispatch_semaphore_wait:
// Create your semaphore, 0 is specifying the initial pool size
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
#autoreleasepool {
// Your code goes here
}
// Release the resource and signal the semaphore
dispatch_semaphore_signal(semaphore);
});
// Wait for the above block execution, AKA Waits for (decrements) a semaphore.
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
// After this line you can now safely assert anything you want regarding the async operation since it is done.