GCD可以設置線程最大並發量

-(void)dispatchSignal{

     //crate的value表示，最多幾個資源可訪問

     dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);   

     dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

     

     //任務1

     dispatch_async(quene, ^{

         dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

         NSLog (@ "run task 1" );

         sleep(1);

         NSLog (@ "complete task 1" );

         dispatch_semaphore_signal(semaphore);       

     });<br>

     //任務2

     dispatch_async(quene, ^{

         dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

         NSLog (@ "run task 2" );

         sleep(1);

         NSLog (@ "complete task 2" );

         dispatch_semaphore_signal(semaphore);       

     });<br>

     //任務3

     dispatch_async(quene, ^{

         dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

         NSLog (@ "run task 3" );

         sleep(1);

         NSLog (@ "complete task 3" );

         dispatch_semaphore_signal(semaphore);       

     });   

}