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 做用是在並行隊列中，等待前面兩個操做並行操做完成，這裏是並行輸出

dispatch-1，dispatch-2

而後執行

dispatch_barrier_async中的操做，(如今就只會執行這一個操做)執行完成後，即輸出

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

dispatch-3,dispatch-4