Why dispatch_sync( ) call on main queue is blocking the main queue?

Question

I know this is not a strong question but I have to clear my mind on this concept.

I have defined myBlock as follows.

void(^myBlock)(v         


        

Answer 1

When you run an async task, it will create a new thread and your code in the block will be executed in that new thread. In that method you call dispatch_sync on main thread because you want to run in the main queue. Please try to understand it with this example.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

    if ([NSThread isMainThread])
    {
        NSLog(@"Running on main Thread in dispatch_async");
    }
    else
    {
        NSLog(@"Running on another Thread in dispatch_async");
    }

    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

        if ([NSThread isMainThread])
        {
            NSLog(@"Running on main Thread in dispatch_sync");
        }
        else
        {
            NSLog(@"Running on another Thread in dispatch_sync");
        }
    });

    dispatch_sync(dispatch_get_main_queue(), ^(void) {

        if ([NSThread isMainThread])
        {
            NSLog(@"Running on main Thread in dispatch_sync");
        }
        else
        {
            NSLog(@"Running on another Thread in dispatch_sync");
        }
    });

});

Output is:

Running on another Thread in dispatch_async
Running on another Thread in dispatch_sync
Running on main Thread in dispatch_sync