深刻理解GCD之dispatch_group

博客連接深刻理解GCD之dispatch_group

以前已經介紹了dispatch_semaphore的底層實現，dispatch_group的實現是基於前者的。在看源碼以前，咱們先看一下咱們是如何應用的。假設有這麼場景：有一個A耗時操做，B和C兩個網絡請求和一個耗時操做C當ABC都執行完成後，刷新頁面。咱們能夠用dispatch_group實現。關鍵以下：

- (void)viewDidLoad {
    [super viewDidLoad];
    
        __block NSInteger number = 0;
    
    dispatch_group_t group = dispatch_group_create();
    
    //A耗時操做
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        sleep(3);
        number += 2222;
    });
    
    //B網絡請求
    dispatch_group_enter(group);
    [self sendRequestWithCompletion:^(id response) {
        number += [response integerValue];
        dispatch_group_leave(group);
    }];
    
    //C網絡請求
    dispatch_group_enter(group);
    [self sendRequestWithCompletion:^(id response) {
        number += [response integerValue];
        dispatch_group_leave(group);
    }];
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"%zd", number);
    });
}

- (void)sendRequestWithCompletion:(void (^)(id response))completion {
    //模擬一個網絡請求
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        sleep(2);
        dispatch_async(dispatch_get_main_queue(), ^{
            if (completion) completion(@1111);
        });
    });
}
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接下來咱們根據上面的流程來看一下dispatch_group的相關API

dispatch_group_create

dispatch_group_t
dispatch_group_create(void)
{
	return (dispatch_group_t)dispatch_semaphore_create(LONG_MAX);
}
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dispatch_group_create其實就是建立了一個value爲LONG_MAX的dispatch_semaphore信號量

dispatch_group_async

void
dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_block_t db)
{
	dispatch_group_async_f(dg, dq, _dispatch_Block_copy(db),
			_dispatch_call_block_and_release);
}
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dispatch_group_async只是dispatch_group_async_f的封裝

dispatch_group_async_f

void
dispatch_group_async_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt,
		dispatch_function_t func)
{
	dispatch_continuation_t dc;

	_dispatch_retain(dg);
	dispatch_group_enter(dg);

	dc = fastpath(_dispatch_continuation_alloc_cacheonly());
	if (!dc) {
		dc = _dispatch_continuation_alloc_from_heap();
	}

	dc->do_vtable = (void *)(DISPATCH_OBJ_ASYNC_BIT | DISPATCH_OBJ_GROUP_BIT);
	dc->dc_func = func;
	dc->dc_ctxt = ctxt;
	dc->dc_group = dg;

	// No fastpath/slowpath hint because we simply don't know if (dq->dq_width != 1 && dq->do_targetq) { return _dispatch_async_f2(dq, dc); } _dispatch_queue_push(dq, dc); } 複製代碼

從上面的代碼咱們能夠看出dispatch_group_async_f和dispatch_async_f類似。dispatch_group_async_f多了dispatch_group_enter(dg);，另外在do_vtable的賦值中dispatch_group_async_f多了一個DISPATCH_OBJ_GROUP_BIT的標記符。既然添加了dispatch_group_enter一定會存在dispatch_group_leave。在以前《深刻理解GCD之dispatch_queue》介紹_dispatch_continuation_pop函數的源碼中有一段代碼以下：

_dispatch_client_callout(dc->dc_ctxt, dc->dc_func);
	if (dg) {
		//group須要進行調用dispatch_group_leave並釋放信號
		dispatch_group_leave(dg);
		_dispatch_release(dg);
	}
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因此dispatch_group_async_f函數中的dispatch_group_leave是在_dispatch_continuation_pop函數中調用的。

這裏歸納一下dispatch_group_async_f的工做流程：

1. 調用dispatch_group_enter；
2. 將block和queue等信息記錄到dispatch_continuation_t結構體中，並將它加入到group的鏈表中；
3. _dispatch_continuation_pop執行時會判斷任務是否爲group，是的話執行完任務再調用dispatch_group_leave以達到信號量的平衡。

dispatch_group_enter

void
dispatch_group_enter(dispatch_group_t dg)
{
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;

	(void)dispatch_semaphore_wait(dsema, DISPATCH_TIME_FOREVER);
}
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dispatch_group_enter將dispatch_group_t轉換成dispatch_semaphore_t，並調用dispatch_semaphore_wait，原子性減1後，進入等待狀態直到有信號喚醒。因此說dispatch_group_enter就是對dispatch_semaphore_wait的封裝。

dispatch_group_leave

void
dispatch_group_leave(dispatch_group_t dg)
{
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;
	dispatch_atomic_release_barrier();
	long value = dispatch_atomic_inc2o(dsema, dsema_value);//dsema_value原子性加1
	if (slowpath(value == LONG_MIN)) {//內存溢出，因爲dispatch_group_leave在dispatch_group_enter以前調用
		DISPATCH_CLIENT_CRASH("Unbalanced call to dispatch_group_leave()");
	}
	if (slowpath(value == dsema->dsema_orig)) {//表示全部任務已經完成，喚醒group
		(void)_dispatch_group_wake(dsema);
	}
}
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從上面的源代碼中咱們看到dispatch_group_leave將dispatch_group_t轉換成dispatch_semaphore_t後將dsema_value的值原子性加1。若是value爲LONG_MIN程序crash；若是value等於dsema_orig表示全部任務已完成，調用_dispatch_group_wake喚醒group（_dispatch_group_wake的用於和notify有關，咱們會在後面介紹）。由於在enter的時候進行了原子性減1操做。因此在leave的時候須要原子性加1。

這裏先說明一下enter和leave之間的關係：

1. dispatch_group_leave與dispatch_group_enter配對使用。當調用了dispatch_group_enter而沒有調用dispatch_group_leave時，因爲value不等於dsema_orig不會走到喚醒邏輯，dispatch_group_notify中的任務沒法執行或者dispatch_group_wait收不到信號而卡住線程。

2. dispatch_group_enter必須在dispatch_group_leave以前出現。當dispatch_group_leave比dispatch_group_enter多調用了一次或者說在dispatch_group_enter以前被調用的時候，dispatch_group_leave進行原子性加1操做，至關於value爲LONGMAX+1，發生數據長度溢出，變成LONG_MIN，因爲value == LONG_MIN成立，程序發生crash。

dispatch_group_notify

void
dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
		dispatch_block_t db)
{
	dispatch_group_notify_f(dg, dq, _dispatch_Block_copy(db),
			_dispatch_call_block_and_release);
}
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dispatch_group_notify是dispatch_group_notify_f的封裝，具體實如今後者。

dispatch_group_notify_f

void
dispatch_group_notify_f(dispatch_group_t dg, dispatch_queue_t dq, void *ctxt,
		void (*func)(void *))
{
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;
	struct dispatch_sema_notify_s *dsn, *prev;

	//封裝dispatch_continuation_t結構體
	// FIXME -- this should be updated to use the continuation cache
	while (!(dsn = calloc(1, sizeof(*dsn)))) {
		sleep(1);
	}

	dsn->dsn_queue = dq;
	dsn->dsn_ctxt = ctxt;
	dsn->dsn_func = func;
	_dispatch_retain(dq);
	dispatch_atomic_store_barrier();
	//將結構體放到鏈表尾部，若是鏈表爲空同時設置鏈表頭部節點並喚醒group
	prev = dispatch_atomic_xchg2o(dsema, dsema_notify_tail, dsn);
	if (fastpath(prev)) {
		prev->dsn_next = dsn;
	} else {
		_dispatch_retain(dg);
		(void)dispatch_atomic_xchg2o(dsema, dsema_notify_head, dsn);
		if (dsema->dsema_value == dsema->dsema_orig) {//任務已經完成，喚醒group
			_dispatch_group_wake(dsema);
		}
	}
}
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因此dispatch_group_notify函數只是用鏈表把全部回調通知保存起來，等待調用。

_dispatch_group_wake

static long
_dispatch_group_wake(dispatch_semaphore_t dsema)
{
	struct dispatch_sema_notify_s *next, *head, *tail = NULL;
	long rval;
	//將dsema的dsema_notify_head賦值爲NULL，同時將以前的內容賦給head
	head = dispatch_atomic_xchg2o(dsema, dsema_notify_head, NULL);
	if (head) {
		// snapshot before anything is notified/woken <rdar://problem/8554546>
		//將dsema的dsema_notify_tail賦值爲NULL，同時將以前的內容賦給tail
		tail = dispatch_atomic_xchg2o(dsema, dsema_notify_tail, NULL);
	}
	//將dsema的dsema_group_waiters設置爲0，並返回原來的值
	rval = dispatch_atomic_xchg2o(dsema, dsema_group_waiters, 0);
	if (rval) {
		//循環調用semaphore_signal喚醒當初等待group的信號量，使得dispatch_group_wait函數返回。
		// wake group waiters
#if USE_MACH_SEM
		_dispatch_semaphore_create_port(&dsema->dsema_waiter_port);
		do {
			kern_return_t kr = semaphore_signal(dsema->dsema_waiter_port);
			DISPATCH_SEMAPHORE_VERIFY_KR(kr);
		} while (--rval);
#elif USE_POSIX_SEM
		do {
			int ret = sem_post(&dsema->dsema_sem);
			DISPATCH_SEMAPHORE_VERIFY_RET(ret);
		} while (--rval);
#endif
	}
	if (head) {
		//獲取鏈表，依次調用dispatch_async_f異步執行在notify函數中的任務即Block。
		// async group notify blocks
		do {
			dispatch_async_f(head->dsn_queue, head->dsn_ctxt, head->dsn_func);
			_dispatch_release(head->dsn_queue);
			next = fastpath(head->dsn_next);
			if (!next && head != tail) {
				while (!(next = fastpath(head->dsn_next))) {
					_dispatch_hardware_pause();
				}
			}
			free(head);
		} while ((head = next));
		_dispatch_release(dsema);
	}
	return 0;
}
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_dispatch_group_wake主要的做用有兩個：

1. 調用semaphore_signal喚醒當初等待group的信號量，使得dispatch_group_wait函數返回。

2. 獲取鏈表，依次調用dispatch_async_f異步執行在notify函數中的任務即Block。

到這裏咱們已經差很少知道了dispatch_group工做過程，咱們用一張圖表示：

dispatch_group_wait

long
dispatch_group_wait(dispatch_group_t dg, dispatch_time_t timeout)
{
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg;

	if (dsema->dsema_value == dsema->dsema_orig) {//沒有須要執行的任務
		return 0;
	}
	if (timeout == 0) {//返回超時
#if USE_MACH_SEM
		return KERN_OPERATION_TIMED_OUT;
#elif USE_POSIX_SEM
		errno = ETIMEDOUT;
		return (-1);
#endif
	}
	return _dispatch_group_wait_slow(dsema, timeout);
}
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dispatch_group_wait用於等待group中的任務完成。

_dispatch_group_wait_slow

static long
_dispatch_group_wait_slow(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
	long orig;

again:
	// check before we cause another signal to be sent by incrementing
	// dsema->dsema_group_waiters
	if (dsema->dsema_value == dsema->dsema_orig) {
		return _dispatch_group_wake(dsema);
	}
	// Mach semaphores appear to sometimes spuriously wake up. Therefore,
	// we keep a parallel count of the number of times a Mach semaphore is
	// signaled (6880961).
	(void)dispatch_atomic_inc2o(dsema, dsema_group_waiters);
	// check the values again in case we need to wake any threads
	if (dsema->dsema_value == dsema->dsema_orig) {
		return _dispatch_group_wake(dsema);
	}

#if USE_MACH_SEM
	mach_timespec_t _timeout;
	kern_return_t kr;

	_dispatch_semaphore_create_port(&dsema->dsema_waiter_port);

	// From xnu/osfmk/kern/sync_sema.c:
	// wait_semaphore->count = -1; /* we don't keep an actual count */ // // The code above does not match the documentation, and that fact is // not surprising. The documented semantics are clumsy to use in any // practical way. The above hack effectively tricks the rest of the // Mach semaphore logic to behave like the libdispatch algorithm. switch (timeout) { default: do { uint64_t nsec = _dispatch_timeout(timeout); _timeout.tv_sec = (typeof(_timeout.tv_sec))(nsec / NSEC_PER_SEC); _timeout.tv_nsec = (typeof(_timeout.tv_nsec))(nsec % NSEC_PER_SEC); kr = slowpath(semaphore_timedwait(dsema->dsema_waiter_port, _timeout)); } while (kr == KERN_ABORTED); if (kr != KERN_OPERATION_TIMED_OUT) { DISPATCH_SEMAPHORE_VERIFY_KR(kr); break; } // Fall through and try to undo the earlier change to // dsema->dsema_group_waiters case DISPATCH_TIME_NOW: while ((orig = dsema->dsema_group_waiters)) { if (dispatch_atomic_cmpxchg2o(dsema, dsema_group_waiters, orig, orig - 1)) { return KERN_OPERATION_TIMED_OUT; } } // Another thread called semaphore_signal(). // Fall through and drain the wakeup. case DISPATCH_TIME_FOREVER: do { kr = semaphore_wait(dsema->dsema_waiter_port); } while (kr == KERN_ABORTED); DISPATCH_SEMAPHORE_VERIFY_KR(kr); break; } #elif USE_POSIX_SEM //這部分代碼省略 #endif goto again; } 複製代碼

從上面的代碼咱們發現_dispatch_group_wait_slow和_dispatch_semaphore_wait_slow的邏輯很接近。都利用mach內核的semaphore進行信號的發送。區別在於_dispatch_semaphore_wait_slow在等待結束後是return，而_dispatch_group_wait_slow在等待結束是調用_dispatch_group_wake去喚醒這個group。

總結

1. dispatch_group是一個初始值爲LONG_MAX的信號量，group中的任務完成是判斷其value是否恢復成初始值。

2. dispatch_group_enter和dispatch_group_leave必須成對使用而且支持嵌套。

3. 若是dispatch_group_enter比dispatch_group_leave多，因爲value不等於dsema_orig不會走到喚醒邏輯，dispatch_group_notify中的任務沒法執行或者dispatch_group_wait收不到信號而卡住線程。若是是dispatch_group_leave多，則會引發崩潰。