GCD除了多线程的能力，我们常常还会利用栅栏、信号量等功能实现一些特定需求，本文将通过对libdispatch-1173.60.1源码的解读探究他的实现原理。

dispatch_once

通常我们常用GCD的dispatch_once创建单例，或者某些只执行一次的代码，例如：

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    <#code to be executed once#>
});

那么在底层的原理是怎样的呢？在dispatch_once函数的底层，是通过dispatch_once_f实现，先来看看源码

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);
}

这个方法中，首先会将传入的静态变量强转为dispatch_once_gate_t类型，然后通过os_atomic_load方法拿到任务标识v，而通过任务标识v,代码流程可能会走以下三个分支。

1. 如果v等于DLOCK_ONCE_DONE，说明这个任务已经执行过了，直接return
2. 通过_dispatch_once_gate_tryenter锁住当前任务，如果_dispatch_once_gate_tryenter(l)返回为true，说明这个任务block没有执行过，这通过_dispatch_once_callout执行任务block并开锁，同时在_dispatch_once_gate_tryenter(l)中将这个任务的标识符置为DLOCK_ONCE_DONE
3. 如果进入锁失败，说明当前任务正在执行被锁住，则通过_dispatch_once_wait(l)进入等待。

dispatch_semaphore

我们常常使用GCD的信号量去实现一些同步的功能。

通常我们是这么使用信号量的。

dispatch_semaphore_t semphore = dispatch_semaphore_create(1);
dispatch_semaphore_wait(semphore, DISPATCH_TIME_FOREVER);
NSLog(@"test");
dispatch_semaphore_signal(semphore);

这里有三个关键函数：dispatch_semaphore_create、dispatch_semaphore_wait、dispatch_semaphore_signal

dispatch_semaphore_create

dispatch_semaphore_t
dispatch_semaphore_create(long value)
{
    dispatch_semaphore_t dsema;

    // If the internal value is negative, then the absolute of the value is
    // equal to the number of waiting threads. Therefore it is bogus to
    // initialize the semaphore with a negative value.
    if (value < 0) {
        return DISPATCH_BAD_INPUT;
    }

    dsema = _dispatch_object_alloc(DISPATCH_VTABLE(semaphore),
            sizeof(struct dispatch_semaphore_s));
    dsema->do_next = DISPATCH_OBJECT_LISTLESS;
    dsema->do_targetq = _dispatch_get_default_queue(false);
    dsema->dsema_value = value;
    _dispatch_sema4_init(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
    dsema->dsema_orig = value;
    return dsema;
}

这是信号变量的创建过程，这里我们只需要主要，我们将传入的value传给了dsema_value成员。

dispatch_semaphore_wait

long
dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)
{
    long value = os_atomic_dec2o(dsema, dsema_value, acquire);
    if (likely(value >= 0)) {
        return 0;
    }
    return _dispatch_semaphore_wait_slow(dsema, timeout);
}

这里的代码有几个步骤，首先将传入的信号变量dsema的dsema_value进行进行-1操作，如果得到的值value大于或等于0，则直接返回0表示成功，否则进入_dispatch_semaphore_wait_slow进入长等待。

dispatch_semaphore_signal

long
dispatch_semaphore_signal(dispatch_semaphore_t dsema)
{
    long value = os_atomic_inc2o(dsema, dsema_value, release);
    if (likely(value > 0)) {
        return 0;
    }
    if (unlikely(value == LONG_MIN)) {
        DISPATCH_CLIENT_CRASH(value,
                "Unbalanced call to dispatch_semaphore_signal()");
    }
    return _dispatch_semaphore_signal_slow(dsema);
}

这里和dispatch_semaphore_wait相反，首先将dsema的dsema_value进行+1操作，如果value的值大于0，则返回0执行接下来的操作，如果value的值等于LONG_MIN，则抛出Unbalanced call to dispatch_semaphore_signal()的异常，如果value小于等于0，则进入长等待。

dispatch_group

举个例子，我们通常这么使用GCD的调度组

dispatch_group_t group = dispatch_group_create(); 
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务1");
}); 
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务2");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"任务完成");
});

或者

dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务1");
    dispatch_group_leave(group);
});
dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
    NSLog(@"任务2");
    dispatch_group_leave(group);
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"任务完成");
});

不管哪种方式，进组和出组必须成对出现，如果进组后没有出组，则dispatch_group_notify的任务不会执行，如果没有进组便出组，则会导致崩溃。

关于调度组原理的探究，可以先看dispatch_group_enter的源码

void
dispatch_group_enter(dispatch_group_t dg)
{
    // The value is decremented on a 32bits wide atomic so that the carry
    // for the 0 -> -1 transition is not propagated to the upper 32bits.
    uint32_t old_bits = os_atomic_sub_orig2o(dg, dg_bits,
            DISPATCH_GROUP_VALUE_INTERVAL, acquire);
    uint32_t old_value = old_bits & DISPATCH_GROUP_VALUE_MASK;
    if (unlikely(old_value == 0)) {
        _dispatch_retain(dg); // <rdar://problem/22318411>
    }
    if (unlikely(old_value == DISPATCH_GROUP_VALUE_MAX)) {
        DISPATCH_CLIENT_CRASH(old_bits,
                "Too many nested calls to dispatch_group_enter()");
    }
}

其实不用怎么看代码，注释已经帮我解释了，这里将调度组dg的old_bits进行-1操作

再来看dispatch_group_leave

void
dispatch_group_leave(dispatch_group_t dg)
{
    // The value is incremented on a 64bits wide atomic so that the carry for
    // the -1 -> 0 transition increments the generation atomically.
    uint64_t new_state, old_state = os_atomic_add_orig2o(dg, dg_state,
            DISPATCH_GROUP_VALUE_INTERVAL, release);
    uint32_t old_value = (uint32_t)(old_state & DISPATCH_GROUP_VALUE_MASK);

    if (unlikely(old_value == DISPATCH_GROUP_VALUE_1)) {
        old_state += DISPATCH_GROUP_VALUE_INTERVAL;
        do {
            new_state = old_state;
            if ((old_state & DISPATCH_GROUP_VALUE_MASK) == 0) {
                new_state &= ~DISPATCH_GROUP_HAS_WAITERS;
                new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
            } else {
                // If the group was entered again since the atomic_add above,
                // we can't clear the waiters bit anymore as we don't know for
                // which generation the waiters are for
                new_state &= ~DISPATCH_GROUP_HAS_NOTIFS;
            }
            if (old_state == new_state) break;
        } while (unlikely(!os_atomic_cmpxchgv2o(dg, dg_state,
                old_state, new_state, &old_state, relaxed)));
        return _dispatch_group_wake(dg, old_state, true);
    }

    if (unlikely(old_value == 0)) {
        DISPATCH_CLIENT_CRASH((uintptr_t)old_value,
                "Unbalanced call to dispatch_group_leave()");
    }
}

也可以通过注释看出，这里是与enter相反的操作，这里将group的标记进行了+1操作，如果old_value等于-1，也就是调度组的enter和leave是成对的出现，那么就调用_dispatch_group_wake进行下一步操作。

static void
_dispatch_group_wake(dispatch_group_t dg, uint64_t dg_state, bool needs_release)
{
    uint16_t refs = needs_release ? 1 : 0; // <rdar://problem/22318411>

    if (dg_state & DISPATCH_GROUP_HAS_NOTIFS) {
        dispatch_continuation_t dc, next_dc, tail;

        // Snapshot before anything is notified/woken <rdar://problem/8554546>
        dc = os_mpsc_capture_snapshot(os_mpsc(dg, dg_notify), &tail);
        do {
            dispatch_queue_t dsn_queue = (dispatch_queue_t)dc->dc_data;
            next_dc = os_mpsc_pop_snapshot_head(dc, tail, do_next);
            _dispatch_continuation_async(dsn_queue, dc,
                    _dispatch_qos_from_pp(dc->dc_priority), dc->dc_flags);
            _dispatch_release(dsn_queue);
        } while ((dc = next_dc));

        refs++;
    }

    if (dg_state & DISPATCH_GROUP_HAS_WAITERS) {
        _dispatch_wake_by_address(&dg->dg_gen);
    }

    if (refs) _dispatch_release_n(dg, refs);
}

_dispatch_group_wake内部通过一个do-while循环，最终调用的是_dispatch_continuation_async去执行目标任务，而_dispatch_continuation_async实际就是异步执行任务的方法。

也就是说dispatch_group_leave是可以唤醒调度组执行最终任务的。

除了dispatch_group_leave，另一个可以执行目标任务的方法是dispatch_group_notify，再来看它的实现。

static inline void
_dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
        dispatch_continuation_t dsn)
{
    uint64_t old_state, new_state;
    dispatch_continuation_t prev;

    dsn->dc_data = dq;
    _dispatch_retain(dq);

    prev = os_mpsc_push_update_tail(os_mpsc(dg, dg_notify), dsn, do_next);
    if (os_mpsc_push_was_empty(prev)) _dispatch_retain(dg);
    os_mpsc_push_update_prev(os_mpsc(dg, dg_notify), prev, dsn, do_next);
    if (os_mpsc_push_was_empty(prev)) {
        os_atomic_rmw_loop2o(dg, dg_state, old_state, new_state, release, {
            new_state = old_state | DISPATCH_GROUP_HAS_NOTIFS;
            if ((uint32_t)old_state == 0) {
                os_atomic_rmw_loop_give_up({
                    return _dispatch_group_wake(dg, new_state, false);
                });
            }
        });
    }
}

可以看到，这里也是判断state是否为0，也就当前调度组dg进出租是否成对，再通过_dispatch_group_wake去执行目标任务。

而调度组相关的有另一个方法dispatch_group_async

void
dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq,
        dispatch_block_t db)
{
    dispatch_continuation_t dc = _dispatch_continuation_alloc();
    uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_GROUP_ASYNC;
    dispatch_qos_t qos;

    qos = _dispatch_continuation_init(dc, dq, db, 0, dc_flags);
    _dispatch_continuation_group_async(dg, dq, dc, qos);
}

接着看关键方法_dispatch_continuation_group_async

static inline void
_dispatch_continuation_group_async(dispatch_group_t dg, dispatch_queue_t dq,
        dispatch_continuation_t dc, dispatch_qos_t qos)
{
    dispatch_group_enter(dg);
    dc->dc_data = dg;
    _dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}

显而易见，这里也有进组的操作，和我们显式写进组出组实际是一样的。

那么他是何时出组的呢？我们可以通过断点调试查看他出组的位置。

搜索_dispatch_client_callout，可以发现在_dispatch_continuation_with_group_invoke方法中有调用

static inline void
_dispatch_continuation_with_group_invoke(dispatch_continuation_t dc)
{
    struct dispatch_object_s *dou = dc->dc_data;
    unsigned long type = dx_type(dou);
    if (type == DISPATCH_GROUP_TYPE) {
        _dispatch_client_callout(dc->dc_ctxt, dc->dc_func);
        _dispatch_trace_item_complete(dc);
        dispatch_group_leave((dispatch_group_t)dou);
    } else {
        DISPATCH_INTERNAL_CRASH(dx_type(dou), "Unexpected object type");
    }
}

这里可以发现dispatch_group_leave这里调用了，所以dispatch_group_async和dispatch_group_enter、dispatch_group_leave的用法本质上其实是一样的。

至此我们可以得出结论，调度的进组和出组以成对的的，当dispatch_group_leave或者dispatch_group_notify调用时便会唤醒调度组执行目标任务。