为什么要用 dispatch_sync

转自：http://stackoverflow.com/questions/4607125/using-dispatch-sync-in-grand-central-dispatch

You use it when you want to execute a block and wait for the results. One example of this is the pattern where you're using a dispatch queue instead of locks for synchronization. For example, assume you have a shared NSMutableArray a, with access mediated by dispatch queue q. A background thread might be appending to the array (async), while your foreground thread is pulling the first item off (synchronously): NSMutableArray *a = [[NSMutableArray alloc] init]; // All access to `a` is via this dispatch queue! dispatch_queue_t q = dispatch_queue_create("com.foo.samplequeue", NULL); dispatch_async(q, ^{ [a addObject:something]; }); // append to array, non-blocking __block Something *first = nil; // "__block" to make results from block available dispatch_sync(q, ^{ // note that these 3 statements... if ([a count] > 0) { // ...are all executed together... first = [a objectAtIndex:0]; // ...as part of a single block... [a removeObjectAtIndex:0]; // ...to ensure consistent results } }); share | improve this answer edited Nov 9 '12 at 18:46 RubberDuck 6,804 7 101 227 answered Jan 5 '11 at 18:18 David Gelhar 20.2k 2 35 54

I'll +1 this, since it's technically correct, although I there isn't much value in doing a single dispatch_async followed by a dispatch_sync on the same queue. However this same pattern is useful when you want to spawn multiple concurrent jobs on another queue and then wait for them all. –  kperryua Jan 5 '11 at 19:57 Thanks. This is beginning to make sense. What if I want to start multiple concurrent threads using dispatch_apply that access a single resource with mutual exclusion. How do I do this with GCD? Is the only way is to use a dispatch_async with serial queue within my dispatch_apply? Is there a way to use dispatch_sync? –  Rasputin Jones Jan 5 '11 at 20:56 @kperryua - sorry if the example wasn't clear - the idea is that the a separate thread would be doing multiple dispatch_async's to the queue –  David Gelhar Jan 6 '11 at 1:20 @David Gelhar - No problem. Just making mention for others who come looking. –  kperryua Jan 6 '11 at 6:01 5   I also like to think of it as being similar to using -performSelector:onThread:withObject:waitUntilDone: or performSelectorOnMainThread:withObject:waitUntilDone: and setting waitUntilDone to YES. –  Brad Larson♦ Jan 6 '11 at 18:42 show 3 more comments

up vote 31 down vote

First understand its brother dispatch_async //Do something dispatch_async(queue, ^{ //Do something else }); //Do More Stuff You use dispatch_async to create a new thread. When you do that, the current thread will not stop. That means //Do More Stuff may be executed before //Do something else finish What happens if you want the current thread to stop? You do not use dispatch at all. Just write the code normally //Do something //Do something else //Do More Stuff Now, say you want to do something on a DIFFERENT thread and yet wait as if and ensure that stuffs are done consecutively. There are many reason to do this. UI update, for example, is done on main thread. That's where you use dispatch_sync //Do something dispatch_sync(queue, ^{ //Do something else }); //Do More Stuff Here you got //Do something //Do something else and //Do More stuff done consecutively even though //Do something else is done on a different thread. Usually, when people use different thread, the whole purpose is so that something can get executed without waiting. Say you want to download large amount of data but you want to keep the UI smooth. Hence, dispatch_sync is rarely used. But it's there. I personally never used that. Why not ask for some sample code or project that does use dispatch_sync.