dispatch_queue_t
dispatch_queue_create(const char *_Nullable label,
        dispatch_queue_attr_t _Nullable attr);

/*!
 * @typedef dispatch_queue_attr_t
 *
 * @abstract
 * Attribute for dispatch queues.
 */
DISPATCH_DECL(dispatch_queue_attr);

/*!
 * @const DISPATCH_QUEUE_SERIAL
 *
 * @discussion A dispatch queue that invokes blocks serially in FIFO order.
 */
#define DISPATCH_QUEUE_SERIAL NULL

/*!
 * @const DISPATCH_QUEUE_SERIAL_INACTIVE
 *
 * @discussion
 * A dispatch queue that invokes blocks serially in FIFO order, and that is
 * created initially inactive. See dispatch_queue_attr_make_initially_inactive().
 */
#define DISPATCH_QUEUE_SERIAL_INACTIVE \
        dispatch_queue_attr_make_initially_inactive(DISPATCH_QUEUE_SERIAL)

/*!
 * @const DISPATCH_QUEUE_CONCURRENT
 *
 * @discussion A dispatch queue that may invoke blocks concurrently and supports
 * barrier blocks submitted with the dispatch barrier API.
 */
#define DISPATCH_QUEUE_CONCURRENT \
        DISPATCH_GLOBAL_OBJECT(dispatch_queue_attr_t, \
        _dispatch_queue_attr_concurrent)
__OSX_AVAILABLE_STARTING(__MAC_10_7,__IPHONE_4_3)
DISPATCH_EXPORT
struct dispatch_queue_attr_s _dispatch_queue_attr_concurrent;

/*!
 * @const DISPATCH_QUEUE_CONCURRENT_INACTIVE
 *
 * @discussion
 * A dispatch queue that may invoke blocks concurrently and supports barrier
 * blocks submitted with the dispatch barrier API, and that is created initially
 * inactive. See dispatch_queue_attr_make_initially_inactive().
 */
#define DISPATCH_QUEUE_CONCURRENT_INACTIVE \
        dispatch_queue_attr_make_initially_inactive(DISPATCH_QUEUE_CONCURRENT)

dispatch_queue_t
dispatch_get_global_queue(long identifier, unsigned long flags);
 * well-known global concurrent queues:
 *  - QOS_CLASS_USER_INTERACTIVE
 *  - QOS_CLASS_USER_INITIATED
 *  - QOS_CLASS_DEFAULT
 *  - QOS_CLASS_UTILITY
 *  - QOS_CLASS_BACKGROUND
 *
 * The global concurrent queues may still be identified by their priority,
 * which map to the following QOS classes:
 *  - DISPATCH_QUEUE_PRIORITY_HIGH:         QOS_CLASS_USER_INITIATED
 *  - DISPATCH_QUEUE_PRIORITY_DEFAULT:      QOS_CLASS_DEFAULT
 *  - DISPATCH_QUEUE_PRIORITY_LOW:          QOS_CLASS_UTILITY
 *  - DISPATCH_QUEUE_PRIORITY_BACKGROUND:   QOS_CLASS_BACKGROUND
 *
 * @param flags
 * Reserved for future use. Passing any value other than zero may result in
 * a NULL return value.

舉個(gè)做飯的例子：
我們要做一頓飯，這個(gè)任務(wù)可以細(xì)分為下面幾個(gè)任務(wù)：

1. 蒸米飯
   淘米、漫長(zhǎng)的蒸米過(guò)程
2. 做菜
   擇菜、洗菜、切菜、切肉、腌肉、炒菜、悶熱20秒
3. 做完了叫人來(lái)吃飯

好了，假設(shè)有好多小伙伴，可以一起來(lái)做來(lái)提高效率。
米飯和菜可以同時(shí)來(lái)分開(kāi)做，
A擇菜，B切肉，C淘米
其中擇菜、洗菜、切菜是有先后順序的，
切肉、腌肉也有順序，淘米、蒸米也有順序
炒菜必須等菜和肉都到位才能進(jìn)行

dispatch_queue_t queue1 = dispatch_queue_create("米", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue2 = dispatch_queue_create("菜", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t queue3 = dispatch_queue_create("肉", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue4 = dispatch_queue_create("炒菜", DISPATCH_QUEUE_SERIAL);

    dispatch_async(queue1, ^{
        //淘米
    });
    dispatch_async(queue1, ^{
        //蒸米
    });
    
    dispatch_async(queue2, ^{
        //擇菜，有好多菜，可以分頭擇，并行
    });
     dispatch_barrier_async(queue2, ^{
         //洗菜，利用柵欄塊來(lái)完成順序執(zhí)行
     });
    dispatch_barrier_async(queue2, ^{
        //切菜，利用柵欄塊來(lái)完成順序執(zhí)行
    });
    
    dispatch_async(queue3, ^{
        //洗肉
    });
    dispatch_async(queue3, ^{
        //腌肉
    });
    dispatch_async(queue3, ^{
        //切肉
    });
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue2, ^{
       //擇菜，有好多菜，可以分頭擇，并行
    });
    dispatch_group_async(group, queue2, ^{
        //洗菜，利用柵欄塊來(lái)完成順序執(zhí)行
    });
    dispatch_group_async(group, queue2, ^{
        //切菜，利用柵欄塊來(lái)完成順序執(zhí)行
    });
    
    dispatch_group_async(group, queue3, ^{
        //洗肉
    });
    dispatch_group_async(group, queue3, ^{
        //腌肉
    });
    dispatch_group_async(group, queue3, ^{
        //切肉
    });
    
    dispatch_group_wait(group, 20*60);//同步等待完成
    dispatch_group_notify(group, queue4, ^{
        //材料準(zhǔn)備完畢，炒菜
        
    });
    

此外：
dispatch_barrier_async的順序執(zhí)行還是依賴queue的類型啊，必需要queue的類型為dispatch_queue_create創(chuàng)建的，而且attr參數(shù)值必需是DISPATCH_QUEUE_CONCURRENT類型，dispatch_get_global_queue獲得的并發(fā)隊(duì)列被歧視了。
延時(shí)執(zhí)行dispatch_after
執(zhí)行n次（通過(guò)數(shù)組的形式封裝任務(wù)，以實(shí)現(xiàn)group的效果）dispatch_apply(<#size_t iterations#>, <#dispatch_queue_t _Nonnull queue#>, <#^(size_t)block#>)
執(zhí)行1次 dispatch_once(<#dispatch_once_t * _Nonnull predicate#>, <#^(void)block#>)