Question

I got this code

-(void)changeText
{

  dispatch_queue_t gqueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

  dispatch_async(gqueue, ^(void){
    //simulate a network traffic delay
    [NSThread sleepForTimeInterval:5];
    NSLog(@"start executing");
    self.mylabel.text = @"Yeah! Text Changed";
    NSLog(@"stop exec");
  });
}

Problem is, it take too much time to change label text than normally do. If I use main queue, it will do instantly but UI will be blocked for 5 seconds.

What is the proper way to use GCD so that I can download stuff in another thread, my UI will not be blocked, and as soon as my work done, my UI will change instantly?

Answer 1

You cannot modify UIKit objects (such as UILabel) on a background thread. The above should be:

-(void)changeText
{

  dispatch_queue_t gqueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

  dispatch_async(gqueue, ^(void){
    //simulate a network traffic delay
    [NSThread sleepForTimeInterval:5];
    NSLog(@"start executing");
    dispatch_async(dispatch_get_main_queue(), ^{
      self.mylabel.text = @"Yeah! Text Changed"; });
    NSLog(@"stop exec");
  });
}

You can also use dispatch_sync rather than display_async to wait for the main thread to process the change, but be careful of deadlock.