Objective-c – Understanding dispatch_async

asynchronousdispatch-asyncobjective c

I have question around this code 

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSData* data = [NSData dataWithContentsOfURL: 
      kLatestKivaLoansURL];
    [self performSelectorOnMainThread:@selector(fetchedData:) 
      withObject:data waitUntilDone:YES];
});

The first parameter of this code is 

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

Are we asking this code to perform serial tasks on global queue whose definition itself is that it returns global concurrent queue of a given priority level?

What is advantage of using dispatch_get_global_queue over the main queue?

I am confused. Could you please help me to understand this better.

Best Answer

The main reason you use the default queue over the main queue is to run tasks in the background.

For instance, if I am downloading a file from the internet and I want to update the user on the progress of the download, I will run the download in the priority default queue and update the UI in the main queue asynchronously.

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    //Background Thread
    dispatch_async(dispatch_get_main_queue(), ^(void){
        //Run UI Updates
    });
});
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