CODEWITHSUNDEEP
iosgrand-central-dispatch
suneel
suneel

Reputation: 103

how to know dispatch_async is running ios

I am looking for a small scenario that how can we trace the "dispatch_async" is running or not?.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 
                                         (unsigned long)NULL), ^(void) {
   //back ground process
});

In my case, my app will be in foreground I started the back ground thread and when I bring app from background to foreground I need to check whether it is still running or not. I should not call the same process if it is still running. any idea?

Upvotes: 0

Views: 194

Answers (2)

gnasher729
gnasher729

Reputation: 52602

The question is wrong - dispatch_async is running while you call it and stops running when the call returns, which is practically immediately. What you really want to know is whether the dispatched block is running or not. The simplest way is something along the lines of 

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 
                                     (unsigned long)NULL), ^(void) {
   [self blockIsRunning:YES];
   // do stuff
   [self blockIsRunning:NO];
});

or if you want to know whether the block has run once, you would do something like

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 
                                     (unsigned long)NULL), ^(void) {
   [self blockStarted];
   // do stuff
   [self blockFinished];
});

Alternatively, use NSOperationQueue and a subclass of NSOperation so instead of an anonymous block you have a proper object that you can ask whether it is ready, cancelled, executing, or finished.

Upvotes: 2

TheCodingArt
TheCodingArt

Reputation: 3429

The easiest way to do this (without keeping a reference to every dispatch or a flag for entering/leaving asynchronous tasks) is by using dispatch_group notifications. See the example link and code below:

    - (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
    // 1
    __block NSError *error;
    dispatch_group_t downloadGroup = dispatch_group_create(); 

    for (NSInteger i = 0; i < 3; i++) {
        NSURL *url;
        switch (i) {
            case 0:
                url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
                break;
            case 1:
                url = [NSURL URLWithString:kSuccessKidURLString];
                break;
            case 2:
                url = [NSURL URLWithString:kLotsOfFacesURLString];
                break;
            default:
                break;
        }

        dispatch_group_enter(downloadGroup); // 2
        Photo *photo = [[Photo alloc] initwithURL:url
                              withCompletionBlock:^(UIImage *image, NSError *_error) {
                                  if (_error) {
                                      error = _error;
                                  }
                                  dispatch_group_leave(downloadGroup); // 3
                              }];

        [[PhotoManager sharedManager] addPhoto:photo];
    }

    dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
        if (completionBlock) {
            completionBlock(error);
        }
    });
}

Note how:

dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
    if (completionBlock) {
        completionBlock(error);
    }
});

will not be called until after

dispatch_group_leave(downloadGroup); // 3

is called.

You should setup your threading to where you can work with callbacks like this to determine states. You should try to avoid using boolean flags at all costs, as this is exactly what dispatch groups are for. It's also hard to keep track of numerous asynchronous calls using boolean states.

link: dispatch groups

Upvotes: 2

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