iOS GCD Sync with Async Block

Question

I have an async function with a block :

[self performAsyncTaskCompletion:(void(^) () )
 {
   //Do Something
 }
];

I need to call this funct

Answer 1

You can use dispatch_async with semaphores:

Example:

- (void)performAsyncTaskCompletion:(void (^)())block {
    if (block) {
        block();
    }
}

- (void)runChainOfOperations {
    static dispatch_once_t onceToken;
    static dispatch_semaphore_t semaphore;
    static dispatch_queue_t queue;

    dispatch_once(&onceToken, ^{
        semaphore = dispatch_semaphore_create(1);
        queue = dispatch_queue_create("com.example.MyApp", NULL);
    });

    NSArray *array = @[@1, @2, @3, @4, @5];

    static long counter = 0;
    for (int i = 0; i < array.count; i++) {
        dispatch_async(queue, ^{
            dispatch_semaphore_wait(semaphore,  DISPATCH_TIME_FOREVER);

            [self performAsyncTaskCompletion:^{
                sleep(10);

                dispatch_async(dispatch_get_main_queue(), ^{
                     NSLog(@"%ld", counter++);
                });

                dispatch_semaphore_signal(semaphore);
             }];  
        });
    }
}

Console output:

2015-04-12 21:28:06.047 HKTest[9497:1136830] 0
2015-04-12 21:28:16.023 HKTest[9497:1136830] 1
2015-04-12 21:28:26.025 HKTest[9497:1136830] 2
2015-04-12 21:28:36.029 HKTest[9497:1136830] 3
2015-04-12 21:28:46.031 HKTest[9497:1136830] 4