6个结果搞懂 GCD中 QUEUE 和 async/sync

同一个blockObject

SSViewController *firstVC = (SSViewController *)self.window.rootViewController;
    [firstVC gcdTestMethod:^{
        for (int i=1; i<5; i++)
        {
            NSString *isMain = [[NSThread currentThread] isMainThread]?@"YES":@"NO";
            NSLog(@"    1 ====% 2d  %@ Main:%@",i,[NSThread currentThread],isMain);
        }
    }];
    [firstVC gcdTestMethod:^{
        for (int i=1; i<5; i++)
        {
            NSString *isMain = [[NSThread currentThread] isMainThread]?@"YES":@"NO";
            NSLog(@"    2 ====% 2d  %@ Main:%@",i,[NSThread currentThread],isMain);
        }
    }];

这里给6个不一样实现和不一样的结果:

======================== 1 ============================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
    if (!_concurrentQ){
        _concurrentQ =
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
    }
    dispatch_async(_concurrentQ, oneGcdObject);
}

======================== 2 ============================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
    if (!_concurrentQ){
        _concurrentQ =
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
    }
    dispatch_sync(_concurrentQ, oneGcdObject);
}

======================== 3 ============================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
    if (!_concurrentQ){
        _concurrentQ =
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
    }
    dispatch_async(_concurrentQ, oneGcdObject);
}

========================= 4 ===========================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
    if (!_concurrentQ){
        _concurrentQ =
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
    }
    dispatch_sync(_concurrentQ, oneGcdObject);
}

========================= 5 ===========================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
//    if (!_concurrentQ){
        _concurrentQ =
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
//    }
    dispatch_sync(_concurrentQ, oneGcdObject);
}

========================= 6 ===========================

- (void)gcdTestMethod:(gcdBlockObject)oneGcdObject
{
//    if (!_concurrentQ){
        _concurrentQ =
//        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_CONCURRENT);
        dispatch_queue_create(oneQueueName, DISPATCH_QUEUE_SERIAL);
//    }
    dispatch_async(_concurrentQ, oneGcdObject);
}

====================================================

1和2对比： DISPATCH_QUEUE_CONCURRENT 的queue 在使用sync时优先使用主线程(当前线程)
3和4对比： DISPATCH_QUEUE_SERIAL 的queue 不管使用sync仍是aync都将遵循FIFO，但使用sync时优先使用主线程(当前线程)
5和6对比： DISPATCH_QUEUE_CONCURRENT 看似建立不一样的queue 但因为都是使用sync，实际上都交给了主线程(当前线程)，但不是同一个queue；而DISPATCH_QUEUE_SERIAL 看似没有遵循FIFO，实际是彻底在两个不一样的线程中执行的，这是因为的确建立了两个不一样的queue。

Reference about dispatch_sync in "Grand Central Dispatch (GCD) Reference": 
"As an optimization, this function invokes the block on the current thread when possible."