Check out example codes for "dispatch after objective c". It will help you in understanding the concepts better.

Code Example 1

// Delay execution of my block for 1 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    //do something here
});

Code Example 2

// Delay execution of my block for 1 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    //do something here
});

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