CODEWITHSUNDEEP
objective-cmultithreadingcocoa-touchios9grand-central-dispatch
Alex Stone
Alex Stone

Reputation: 47354

Check if a dispatch queue cannot keep up with tasks and is backlogged

I'm using a serial dispatch queue to alter images coming from the camera for a timelapse video app. So far I'm using GCD to offload image processing to a second thread.

I expect the app to be running for a long time and don't want to somehow overwhelm the device with processing requests.

Is there a way to check if the dispatch queue cannot keep up with the number of operations added to it (creating a backlog)?

dispatch_async(backgroundQueue, ^{

    __block UIImage* backupImage = self.thermalImage;
    backupImage = [self imageByDrawingCircleOnImage:backupImage];

    dispatch_async(dispatch_get_main_queue(), ^{
        [self.thermalImageView setImage:backupImage];
    });
});

Upvotes: 3

Views: 159

Answers (1)

Rich Schonthal
Rich Schonthal

Reputation: 452

Do you have a specific number of tasks you want to execute in this queue before deciding that it's backlogged? If so, use a semaphore to limit the number of tasks in the queue. 

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
    dispatch_queue_t queue = dispatch_queue_create("com.foo.queue", DISPATCH_QUEUE_CONCURRENT);

    for(int i = 1; i < 7; i++) {
        dispatch_async(queue, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            NSLog(@"hello from #%d", i);
            [NSThread sleepForTimeInterval:2];
            dispatch_semaphore_signal(semaphore);
        });
    }

You can specify a timeout and then check the return value of dispatch_semaphore_wait to see if you timed out and then create a new queue or just wait until the queue frees up etc.

Upvotes: 5

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