問題

如何用GCD實現圖中要求：任務1、2、3并行；任務0等1、2、3都結束后執(zhí)行；任務4、5、6并行并且等任務0結束后才執(zhí)行 ？

方法一：group

dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, globalQueue, ^{
        NSLog(@"任務 1");
    });
dispatch_group_async(group, globalQueue, ^{
        NSLog(@"任務 2");
    });
dispatch_group_async(group, globalQueue, ^{
        NSLog(@"任務 3");
    });

dispatch_group_t group2 = dispatch_group_create();
//手動管理Group計數，必須配套dispatch_group_leave，計數減為零則觸發(fā)Notifiy;否則依賴該Group的block會直接執(zhí)行
dispatch_group_enter(group2); 
dispatch_group_notify(group, globalQueue, ^{
        dispatch_async(globalQueue, ^{
            NSLog(@"任務 0");
            dispatch_group_leave(group2);
        });
    });

dispatch_group_notify(group2, globalQueue, ^{
        dispatch_async(globalQueue, ^{
            NSLog(@"任務 4");
        });
        dispatch_async(globalQueue, ^{
            NSLog(@"任務 5");
        });
        dispatch_async(globalQueue, ^{
            NSLog(@"任務 6");
        });
    });

方法二：barrier

    dispatch_queue_t cocurrentQueue = dispatch_queue_create("concurrent queue", DISPATCH_QUEUE_CONCURRENT);
   dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 1");
    });
    dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 2");
    });
    dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 3");
    });
    dispatch_barrier_async(cocurrentQueue, ^{
        NSLog(@"任務 0");
    });
    dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 4");
    });
    dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 5");
    });
   dispatch_async(cocurrentQueue, ^{
        NSLog(@"任務 6");
    });

注意：barrier常配合并行隊列，但只是對自己創(chuàng)建的并行隊列有效，對于系統(tǒng)提供的全局并行隊列無效，可能蘋果是出于安全考慮而做了特殊處理