Main queue and main thread in GCD

main_queue_thread_GCD.m

//The main queue must be associated with the main thread.
//The main thread is not exclusively associated with the main queue.
//For example, when disptach_sync is used to dispatch a task from queue A to queue B, 
//queue B will use the same thread queue A is using to execute the task. 
//So, if a task is dispatched to a non-main queue from the main queue with dispatch_sync,  
//the non-main queue will use the main thread to execute the task.

//Check if a task is running on the main thread,
[NSThread isMianThread];

//Check if a task is running on the main queue,
// Associate a key-value pair with the main queue
dispatch_queue_set_specific(
    dispatch_get_main_queue(), 
    mainQueueKey, 
    mainQueueValue, 
    nil
)

//use the key-value pair to do the check
if (dispatch_get_specific(mainQueueKey) == mainQueueValue) 
{
    NSLog(@"on the main queue");
}