dispatch_async底層

#ifdef __BLOCKS__
void
dispatch_async(dispatch_queue_t dq, dispatch_block_t work)
{
    dispatch_continuation_t dc = _dispatch_continuation_alloc();
    uintptr_t dc_flags = DC_FLAG_CONSUME;
    dispatch_qos_t qos;

    qos = _dispatch_continuation_init(dc, dq, work, 0, dc_flags);
    _dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}
#endif

dispatch_async的源碼不多,接下來我們要探索兩個方面:

• 子線程創(chuàng)建的時機(jī)點(diǎn)
• 任務(wù)block執(zhí)行的時機(jī)點(diǎn)

1.子線程創(chuàng)建流程

我們先進(jìn)入_dispatch_continuation_init的流程篮奄，看看是否有子線程創(chuàng)建把沼？

1.1_dispatch_continuation_init

image.png

接著來到_dispatch_continuation_init_f

• _dispatch_continuation_init_f

  
  
  image.png

• _dispatch_continuation_voucher_set

  
  
  image.png

• _dispatch_continuation_priority_set --> dc的dc_priority的設(shè)置

  
  
  image.png
  
  

  我們發(fā)現(xiàn)_dispatch_continuation_init里面并沒有子線程創(chuàng)建的代碼慈省，接下來我們?nèi)dispatch_continuation_async找找

1.2_dispatch_continuation_async

image.png

根據(jù)返回值踩晶，鎖定到了dx_push纽竣。全局搜索dx_push

image.png

繼續(xù)看dq_push

image.png

如果是并發(fā)隊列，那么.dq_push = _dispatch_lane_concurrent_push,接著來到
_dispatch_lane_concurrent_push

image.png

顯然是非柵欄函數(shù)悯衬，那么進(jìn)入_dispatch_continuation_redirect_push

image.png

那do_targetq是什么呢肾扰？得回到隊列的創(chuàng)建dispatch_queue_create去查看

image.png

image.png

image.png

那么，_dispatch_continuation_redirect_push里的dx_push時的隊列是_dispatch_get_root_queue()

image.png

image.png

image.png

image.png

image.png

image.png

到這里凝垛，我們應(yīng)該知道dispatch_async中子線程創(chuàng)建的調(diào)用流程：

1.dispatch_async --> _dispatch_continuation_async --> dx_push --> dq_push --> 并發(fā)隊列：_dispatch_lane_concurrent_push --> _dispatch_continuation_redirect_push
2._dispatch_continuation_redirect_push --> dx_push(此時是global_queue) -->_dispatch_root_queue_push --> _dispatch_root_queue_push_inline-->_dispatch_root_queue_poke-->_dispatch_root_queue_poke_slow -->線程池調(diào)度懊悯，創(chuàng)建線程pthread_create

2.任務(wù)Block的調(diào)用流程

那么接下來的問題就是 block何時調(diào)用?

再看哪里調(diào)用的block --> 類似這樣的代碼block(xxx)? 我們可以在dispatch_async的任務(wù)block中打斷點(diǎn)，然后bt查看調(diào)用棧

image.png

注意到在調(diào)用棧中有一個_dispatch_worker_thread2梦皮，現(xiàn)在重點(diǎn)就來到 --> 什么時候調(diào)起的_dispatch_worker_thread2炭分？

我們先全局搜索一下_dispatch_worker_thread2

image.png

發(fā)現(xiàn)全在一個方法里面 --> _dispatch_root_queues_init_once中，同理届氢，全局搜索

image.png

再全局搜索_dispatch_root_queues_init

image.png

_dispatch_root_queue_poke_slow是否很熟悉欠窒？ 就是我們上面在查找創(chuàng)建子線程時調(diào)用棧走過的方法，那么此時任務(wù)block的調(diào)用和子線程的創(chuàng)建產(chǎn)生了聯(lián)系退子，這個聯(lián)系就是_dispatch_root_queue_poke_slow

2.1 _dispatch_root_queues_init

DISPATCH_STATIC_GLOBAL(dispatch_once_t _dispatch_root_queues_pred);
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_root_queues_init(void)
{
    dispatch_once_f(&_dispatch_root_queues_pred, NULL,
            _dispatch_root_queues_init_once);
}

dispatch_once_f是否有些熟悉岖妄？莫非是單例?我們平時寫的單例是這樣的

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // input your code
    });

搜索一下dispatch_once源碼

image.png

接下來繼續(xù)探索dispatch_once_f

DISPATCH_NOINLINE
void
dispatch_once_f(dispatch_once_t *val, void *ctxt, dispatch_function_t func)
{
    dispatch_once_gate_t l = (dispatch_once_gate_t)val;

#if !DISPATCH_ONCE_INLINE_FASTPATH || DISPATCH_ONCE_USE_QUIESCENT_COUNTER
    uintptr_t v = os_atomic_load(&l->dgo_once, acquire);
    if (likely(v == DLOCK_ONCE_DONE)) {
        return;
    }
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
    if (likely(DISPATCH_ONCE_IS_GEN(v))) {
        return _dispatch_once_mark_done_if_quiesced(l, v);
    }
#endif
#endif
    if (_dispatch_once_gate_tryenter(l)) {
        return _dispatch_once_callout(l, ctxt, func);
    }
    return _dispatch_once_wait(l);
}

image.png

image.png

image.png

image.png

總之，這個_dispatch_once_gate_tryenter判斷條件寂祥，就能保證當(dāng)前只有一個線程進(jìn)去執(zhí)行代碼荐虐，那為什么只能執(zhí)行一次呢？還是看_dispatch_once_gate_broadcast 里的 _dispatch_once_mark_done

image.png

image.png

回到Block的調(diào)用時機(jī)
即什么時候調(diào)起的_dispatch_worker_thread2 丸凭？
在_dispatch_root_queues_init時福扬，單例執(zhí)行的任務(wù)block是_dispatch_root_queues_init_once

image.png

image.png

image.png

image.png

• _dispatch_worker_thread2底層

  
  
  image.png
  
  
  image.png
  
  
  image.png
  
  

  最終铛碑，我們來到了dx_invoke 和_dispatch_continuation_invoke_inline。

• _dispatch_continuation_invoke_inline

  
  
  image.png
  
  
  image.png

至此虽界，我們跟著底層源碼弄清楚了block()的調(diào)用流程??

1.通過在block任務(wù)中打斷點(diǎn)汽烦，LLDB bt指令查看調(diào)用棧信息，找到_dispatch_worker_thread2莉御；
2.搜索調(diào)用_dispatch_worker_thread2的地方撇吞，找到_dispatch_root_queues_init --> _dispatch_root_queues_init_once；
3.接著我們在_dispatch_root_queues_init_once中發(fā)現(xiàn)了子線程的創(chuàng)建礁叔，并綁定了block任務(wù)_dispatch_worker_thread2牍颈；
4.接著我們繼續(xù)查看_dispatch_worker_thread2的底層源碼，發(fā)現(xiàn)了調(diào)用block任務(wù)的時機(jī)點(diǎn)

dispatch_sync底層

image.png

image.png

接著看_dispatch_barrier_sync_f的源碼

image.png

image.png

再回頭看看_dispatch_queue_try_acquire_barrier_sync里的流程

image.png

image.png