Wait for Asynchronous Operations in Objective-C

Question

I'm googling like crazy and still confused about this.

I want to download an array of file urls to disk, and I want to update my view based on the bytes loaded of each file as they download. I already have something that will download a file, and report progress and completion via blocks.

How can I do this for each file in the array?

I'm ok doing them one at a time. I can calculate the total progress easily that way:

float progress = (numCompletedFiles + (currentDownloadedBytes / currentTotalBytes)) / totalFiles)

I mostly understand GCD and NSOperations, but how can you tell an operation or dispatch_async block to wait until a callback is called before being done? It seems possible by overriding NSOperation, but that seems like overkill. Is there another way? Is it possible with just GCD?

Answer 1

I'm not sure if I understand you correctly, but perhaps you need dispatch semaphores to achieve your goal. In one of my projects I use a dispatch semaphore to wait until another turn by another player is completed. This is partially the code I used.

for (int i = 0; i < _players.count; i++)
{

    // a semaphore is used to prevent execution until the asynchronous task is completed ...

    dispatch_semaphore_t sema = dispatch_semaphore_create(0);


    // player chooses a card - once card is chosen, animate choice by moving card to center of board ...

    [self.currentPlayer playCardWithPlayedCards:_currentTrick.cards trumpSuit:_trumpSuit completionHandler:^ (WSCard *card) {

        BOOL success = [self.currentTrick addCard:card];

        DLog(@"did add card to trick? %@", success ? @"YES" : @"NO");

        NSString *message = [NSString stringWithFormat:@"Card played by %@", _currentPlayer.name];
        [_messageView setMessage:message];

        [self turnCard:card];
        [self moveCardToCenter:card];


        // send a signal that indicates that this asynchronous task is completed ...

        dispatch_semaphore_signal(sema);

        DLog(@"<<< signal dispatched >>>");
    }];


    // execution is halted, until a signal is received from another thread ...

    DLog(@"<<< wait for signal >>>");

    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    dispatch_release(sema);


    DLog(@"<<< signal received >>>");

Answer 2

dispatch groups are the GCD facility designed to track completion of a set of independent or separately async'd blocks/tasks.

Either use dispatch_group_async() to submit the blocks in question, or dispatch_group_enter() the group before triggering the asynchronous task and dispatch_group_leave() the group when the task has completed.

You can then either get notified asynchronously via dispatch_group_notify() when all blocks/tasks in the group have completed, or if you must, you can synchronously wait for completion with dispatch_group_wait().