How do I wait for an asynchronously dispatched block to finish?

Question

I am testing some code that does asynchronous processing using Grand Central Dispatch. The testing code looks like this:

[object runSomeLongOperationAndDo:^{
    STAssert…
}];

The tests have to wait for the operation to finish. My current solution looks like this:

__block BOOL finished = NO;
[object runSomeLongOperationAndDo:^{
    STAssert…
    finished = YES;
}];
while (!finished);

Which looks a bit crude, do you know a better way? I could expose the queue and then block by calling dispatch_sync:

[object runSomeLongOperationAndDo:^{
    STAssert…
}];
dispatch_sync(object.queue, ^{});

…but that’s maybe exposing too much on the object.

Answer 1

Trying to use a dispatch_semaphore. It should look something like this:

dispatch_semaphore_t sema = dispatch_semaphore_create(0);

[object runSomeLongOperationAndDo:^{
    STAssert…

    dispatch_semaphore_signal(sema);
}];

if (![NSThread isMainThread]) {
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
} else {
    while (dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW)) { 
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:0]]; 
    }
}

This should behave correctly even if runSomeLongOperationAndDo: decides that the operation isn't actually long enough to merit threading and runs synchronously instead.