GCD學習(五) dispatch_barrier_async

先看段代碼

    dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-1");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-2");
    });
    dispatch_barrier_async(concurrentQueue, ^(){
        NSLog(@"dispatch-barrier"); 
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-3");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-4");
    });

dispatch_barrier_async 作用是在並行隊列中，等待前面兩個操作並行操作完成，這里是並行輸出

dispatch-1，dispatch-2

然后執行

dispatch_barrier_async中的操作，(現在就只會執行這一個操作)執行完成后，即輸出

"dispatch-barrier，
最后該並行隊列恢復原有執行狀態，繼續並行執行

dispatch-3,dispatch-4