一、栅栏函数

CPU的乱序执行能力让我们对多线程的安全保障的努力变得异常困难。因此要保证线程安全，阻止CPU换序是必需的。遗憾的是，现在并不存在可移植的阻止换序的方法。通常情况下是调用CPU提供的一条指令，这条指令常常被称为barrier。一条barrier指令会阻止CPU将该指令之前的指令交换到barrier之后，反之亦然。换句话说，barrier指令的作用类似于一个拦水坝，阻止换序穿透这个大坝。

栅栏函数最直接的作用：控制任务执行顺序，导致同步效果。
有两个函数：

dispatch_barrier_async：前面的任务执行完毕才会执行barrier中的逻辑，以及barrier后加入队列的任务。
dispatch_barrier_sync：作用相同，但是会堵塞线程，影响后面的任务执行。

⚠️：栅栏函数只能控制同一队列并发，相当于针对队列而言。

1.1 应用

1.1.1 dispatch_barrier_async 与 dispatch_barrier_sync 效果

有如下案例：

- (void)test {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("HotpotCat", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(concurrentQueue, ^{
        sleep(3);
        NSLog(@"1");
    });
    dispatch_async(concurrentQueue, ^{
        NSLog(@"2");
    });
    dispatch_barrier_async(concurrentQueue, ^{
        NSLog(@"3:%@",[NSThread currentThread]);
    });
    dispatch_async(concurrentQueue, ^{
        NSLog(@"4");
    });
    NSLog(@"5");
}

分析：barrier阻塞的是自己以及concurrentQueue队列中在它后面加入的任务。由于这里使用的是异步函数所以任务1，2，5顺序不定，3在4之前。
输出：

GCDDemo[49708:5622304] 5
GCDDemo[49708:5622437] 2
GCDDemo[49708:5622434] 1
GCDDemo[49708:5622434] 3:{number = 6, name = (null)}
GCDDemo[49708:5622434] 4

如果将dispatch_barrier_async改为dispatch_barrier_sync同步函数，则任务5会被阻塞。1、2（顺序不定）在3之前执行，4和5（顺序不定）在之后。

1.1.2 栅栏函数存在的问题

1.1.2.1 栅栏函数与全局队列

将concurrentQueue改为全局队列：

dispatch_queue_t concurrentQueue = dispatch_get_global_queue(0, 0);
dispatch_async(concurrentQueue, ^{
    NSLog(@"1");
});
dispatch_async(concurrentQueue, ^{
    NSLog(@"2");
});
dispatch_barrier_async(concurrentQueue, ^{
    NSLog(@"3:%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
    NSLog(@"4");
});
NSLog(@"5");

输出：

GCDDemo[49872:5632760] 5
GCDDemo[49872:5632979] 1
GCDDemo[49872:5633673] 2
GCDDemo[49872:5633675] 4
GCDDemo[49872:5633674] 3:{number = 10, name = (null)}

这个时候栅栏函数无论同步还是异步都无效了（有可能系统调度刚好符合预期）。
这也就意味着全局并发队列不允许使用栅栏函数，一定是自定义队列才能使用。

1.1.2.1 栅栏函数与不同队列

将任务2和4放入另外一个队列：

dispatch_queue_t concurrentQueue = dispatch_queue_create("Hotpot", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t concurrentQueue2 = dispatch_queue_create("Cat", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(concurrentQueue, ^{
    sleep(3);
    NSLog(@"1");
});
dispatch_async(concurrentQueue2, ^{
    NSLog(@"2");
});
dispatch_barrier_async(concurrentQueue, ^{
    NSLog(@"3:%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue2, ^{
    NSLog(@"4");
});
NSLog(@"5");

输出：

GCDDemo[49981:5639766] 5
GCDDemo[49981:5640003] 2
GCDDemo[49981:5639998] 4
GCDDemo[49981:5639997] 1
GCDDemo[49981:5639998] 3:{number = 5, name = (null)}

这个时候concurrentQueue2中的任务先执行了，它并不受栅栏函数的影响。那么说明 栅栏函数只对同一个队列中的任务起作用。

1.1.3 栅栏函数作为锁使用

有如下代码：

NSMutableArray *array = [NSMutableArray array];
dispatch_queue_t concurrentQueue = dispatch_queue_create("Hotpot", DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 1000; i++) {
    dispatch_async(concurrentQueue, ^{
        [array addObject:@(i)];
    });
}

多个线程同时操作array。
在addObject的时候有可能存在同一时间对同一块内存空间写入数据。
比如写第3个数据的时候，当前数组中数据是(1、2)这个时候有2个线程同时写入数据就存在了(1、2、3)和(1、2、4)`这个时候数据就发生了混乱造成了错误。

在运行的时候由于线程不安全（可变数组线程不安全），发生了写入错误直接报错：

image.png

将数组添加元素的操作放入dispatch_barrier_async中：

NSMutableArray *array = [NSMutableArray array];
dispatch_queue_t concurrentQueue = dispatch_queue_create("Hotpot", DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 1000; i++) {
    dispatch_async(concurrentQueue, ^{
        dispatch_barrier_async(concurrentQueue , ^{
            [array addObject:@(i)];
        });
    });
}

这样就没问题了，加入栅栏函数写入数据的时候相当于加了锁。

1.2 原理分析

根据1.1中的案例有3个问题：

1.为什么栅栏函数能起作用？
2.为什么全局队列无效？
3.为什么任务必须在同一队列才有效？

1.2.1 dispatch_barrier_sync

dispatch_barrier_sync源码如下：

void
dispatch_barrier_sync(dispatch_queue_t dq, dispatch_block_t work)
{
    uintptr_t dc_flags = DC_FLAG_BARRIER | DC_FLAG_BLOCK;
    if (unlikely(_dispatch_block_has_private_data(work))) {
        return _dispatch_sync_block_with_privdata(dq, work, dc_flags);
    }
    _dispatch_barrier_sync_f(dq, work, _dispatch_Block_invoke(work), dc_flags);
}

直接调用_dispatch_barrier_sync_f：

static void
_dispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt,
        dispatch_function_t func, uintptr_t dc_flags)
{
    _dispatch_barrier_sync_f_inline(dq, ctxt, func, dc_flags);
}

仍然是对_dispatch_barrier_sync_f_inline的调用：

static inline void
_dispatch_barrier_sync_f_inline(dispatch_queue_t dq, void *ctxt,
        dispatch_function_t func, uintptr_t dc_flags)
{
    dispatch_tid tid = _dispatch_tid_self();

    if (unlikely(dx_metatype(dq) != _DISPATCH_LANE_TYPE)) {
        DISPATCH_CLIENT_CRASH(0, "Queue type doesn't support dispatch_sync");
    }

    dispatch_lane_t dl = upcast(dq)._dl;

    if (unlikely(!_dispatch_queue_try_acquire_barrier_sync(dl, tid))) {
        //死锁走这里的逻辑，同步栅栏函数也走这里
        return _dispatch_sync_f_slow(dl, ctxt, func, DC_FLAG_BARRIER, dl,
                DC_FLAG_BARRIER | dc_flags);
    }

    if (unlikely(dl->do_targetq->do_targetq)) {
        return _dispatch_sync_recurse(dl, ctxt, func,
                DC_FLAG_BARRIER | dc_flags);
    }
    _dispatch_introspection_sync_begin(dl);
    _dispatch_lane_barrier_sync_invoke_and_complete(dl, ctxt, func
            DISPATCH_TRACE_ARG(_dispatch_trace_item_sync_push_pop(
                    dq, ctxt, func, dc_flags | DC_FLAG_BARRIER)));
}

栅栏函数这个时候走的也是_dispatch_sync_f_slow逻辑：

static void
_dispatch_sync_f_slow(dispatch_queue_class_t top_dqu, void *ctxt,
        dispatch_function_t func, uintptr_t top_dc_flags,
        dispatch_queue_class_t dqu, uintptr_t dc_flags)
{
    dispatch_queue_t top_dq = top_dqu._dq;
    dispatch_queue_t dq = dqu._dq;
    if (unlikely(!dq->do_targetq)) {
        return _dispatch_sync_function_invoke(dq, ctxt, func);
    }
    ......
    _dispatch_trace_item_push(top_dq, &dsc);
    //死锁报错
    __DISPATCH_WAIT_FOR_QUEUE__(&dsc, dq);

    if (dsc.dsc_func == NULL) {
        // dsc_func being cleared means that the block ran on another thread ie.
        // case (2) as listed in _dispatch_async_and_wait_f_slow.
        dispatch_queue_t stop_dq = dsc.dc_other;
        return _dispatch_sync_complete_recurse(top_dq, stop_dq, top_dc_flags);
    }

    _dispatch_introspection_sync_begin(top_dq);
    _dispatch_trace_item_pop(top_dq, &dsc);
    _dispatch_sync_invoke_and_complete_recurse(top_dq, ctxt, func,top_dc_flags
            DISPATCH_TRACE_ARG(&dsc));
}

断点调试走的是_dispatch_sync_complete_recurse：

static void
_dispatch_sync_complete_recurse(dispatch_queue_t dq, dispatch_queue_t stop_dq,
        uintptr_t dc_flags)
{
    bool barrier = (dc_flags & DC_FLAG_BARRIER);
    do {
        if (dq == stop_dq) return;
        if (barrier) {
            //唤醒执行
            //_dispatch_lane_wakeup
            dx_wakeup(dq, 0, DISPATCH_WAKEUP_BARRIER_COMPLETE);
        } else {
            //已经执行完成没有栅栏函数
            _dispatch_lane_non_barrier_complete(upcast(dq)._dl, 0);
        }
        dq = dq->do_targetq;
        barrier = (dq->dq_width == 1);
    } while (unlikely(dq->do_targetq));
}

这里进行了递归调用，循环条件是dq->do_targetq也就是 仅对当前队列有效。
唤醒执行栅栏前任务执行_dispatch_lane_wakeup逻辑。
当栅栏前的任务执行完毕走_dispatch_lane_non_barrier_complete逻辑。这也就是为什么栅栏起作用的原因。

dx_wakeup在全局队列是_dispatch_root_queue_wakeup，在自定义并行队列是_dispatch_lane_wakeup。

1.2.1.1 _dispatch_lane_wakeup

void
_dispatch_lane_wakeup(dispatch_lane_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags)
{
    dispatch_queue_wakeup_target_t target = DISPATCH_QUEUE_WAKEUP_NONE;

    if (unlikely(flags & DISPATCH_WAKEUP_BARRIER_COMPLETE)) {
        //barrier完成了就走这里的逻辑,barrier之前的任务执行完毕。
        return _dispatch_lane_barrier_complete(dqu, qos, flags);
    }
    if (_dispatch_queue_class_probe(dqu)) {
        target = DISPATCH_QUEUE_WAKEUP_TARGET;
    }
    //走这里
    return _dispatch_queue_wakeup(dqu, qos, flags, target);
}

在栅栏函数执行完毕后才走_dispatch_lane_barrier_complete与_dispatch_lane_non_barrier_complete中的逻辑就汇合了。
没有执行完毕的时候执行_dispatch_queue_wakeup。

_dispatch_queue_wakeup源码如下：

void
_dispatch_queue_wakeup(dispatch_queue_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags, dispatch_queue_wakeup_target_t target)
{
......

    if (likely((old_state ^ new_state) & enqueue)) {
......
        //_dispatch_queue_push_queue 断点断不住，走这里。
        return _dispatch_queue_push_queue(tq, dq, new_state);
    }
......
}

最终走的是_dispatch_queue_push_queue逻辑：

static inline void
_dispatch_queue_push_queue(dispatch_queue_t tq, dispatch_queue_class_t dq,
        uint64_t dq_state)
{
#if DISPATCH_USE_KEVENT_WORKLOOP
    if (likely(_dq_state_is_base_wlh(dq_state))) {
        _dispatch_trace_runtime_event(worker_request, dq._dq, 1);
        return _dispatch_event_loop_poke((dispatch_wlh_t)dq._dq, dq_state,
                DISPATCH_EVENT_LOOP_CONSUME_2);
    }
#endif // DISPATCH_USE_KEVENT_WORKLOOP
    _dispatch_trace_item_push(tq, dq);
    //_dispatch_lane_concurrent_push
    return dx_push(tq, dq, _dq_state_max_qos(dq_state));
}

内部是对_dispatch_lane_concurrent_push的调用：

void
_dispatch_lane_concurrent_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
    if (dq->dq_items_tail == NULL &&
            !_dispatch_object_is_waiter(dou) &&
            !_dispatch_object_is_barrier(dou) &&
            _dispatch_queue_try_acquire_async(dq)) {
        return _dispatch_continuation_redirect_push(dq, dou, qos);
    }

    _dispatch_lane_push(dq, dou, qos);
}

这里直接调用_dispatch_lane_push：

void
_dispatch_lane_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
......
    if (flags) {
        //栅栏函数走这里。
        //#define dx_wakeup(x, y, z) dx_vtable(x)->dq_wakeup(x, y, z)
        //dx_wakeup 对应 dq_wakeup 自定义全局队列对应 _dispatch_lane_wakeup，全局队列对应 _dispatch_root_queue_wakeup
        return dx_wakeup(dq, qos, flags);
    }
}

又调用回了_dispatch_lane_wakeup，相当于一直扫描。

1.2.1.2 _dispatch_lane_non_barrier_complete

static void
_dispatch_lane_non_barrier_complete(dispatch_lane_t dq,
        dispatch_wakeup_flags_t flags)
{
    ......
    _dispatch_lane_non_barrier_complete_finish(dq, flags, old_state, new_state);
}

其中是对_dispatch_lane_non_barrier_complete_finish的调用。

DISPATCH_ALWAYS_INLINE
static void
_dispatch_lane_non_barrier_complete_finish(dispatch_lane_t dq,
        dispatch_wakeup_flags_t flags, uint64_t old_state, uint64_t new_state)
{
    if (_dq_state_received_override(old_state)) {
        // Ensure that the root queue sees that this thread was overridden.
        _dispatch_set_basepri_override_qos(_dq_state_max_qos(old_state));
    }

    if ((old_state ^ new_state) & DISPATCH_QUEUE_IN_BARRIER) {
        if (_dq_state_is_dirty(old_state)) {
        //走_dispatch_lane_barrier_complete逻辑
        return _dispatch_lane_barrier_complete(dq, 0, flags);
    }

    if ((old_state ^ new_state) & DISPATCH_QUEUE_ENQUEUED) {
        if (!(flags & DISPATCH_WAKEUP_CONSUME_2)) {
            _dispatch_retain_2(dq);
        }
        dispatch_assert(!_dq_state_is_base_wlh(new_state));
        _dispatch_trace_item_push(dq->do_targetq, dq);
        return dx_push(dq->do_targetq, dq, _dq_state_max_qos(new_state));
    }

    if (flags & DISPATCH_WAKEUP_CONSUME_2) {
        _dispatch_release_2_tailcall(dq);
    }
}

走的是_dispatch_lane_barrier_complete逻辑：

DISPATCH_NOINLINE
static void
_dispatch_lane_barrier_complete(dispatch_lane_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags)
{
    dispatch_queue_wakeup_target_t target = DISPATCH_QUEUE_WAKEUP_NONE;
    dispatch_lane_t dq = dqu._dl;

    if (dq->dq_items_tail && !DISPATCH_QUEUE_IS_SUSPENDED(dq)) {
        struct dispatch_object_s *dc = _dispatch_queue_get_head(dq);
        //串行队列
        if (likely(dq->dq_width == 1 || _dispatch_object_is_barrier(dc))) {
            if (_dispatch_object_is_waiter(dc)) {
                //栅栏中的任务逻辑
                return _dispatch_lane_drain_barrier_waiter(dq, dc, flags, 0);
            }
        } else if (dq->dq_width > 1 && !_dispatch_object_is_barrier(dc)) {
            return _dispatch_lane_drain_non_barriers(dq, dc, flags);
        }

        if (!(flags & DISPATCH_WAKEUP_CONSUME_2)) {
            _dispatch_retain_2(dq);
            flags |= DISPATCH_WAKEUP_CONSUME_2;
        }
        target = DISPATCH_QUEUE_WAKEUP_TARGET;
    }

    uint64_t owned = DISPATCH_QUEUE_IN_BARRIER +
            dq->dq_width * DISPATCH_QUEUE_WIDTH_INTERVAL;
    //执行栅栏后续的代码
    return _dispatch_lane_class_barrier_complete(dq, qos, flags, target, owned);
}

_dispatch_lane_drain_barrier_waiter执行栅栏函数中的任务。
_dispatch_lane_class_barrier_complete执行栅栏函数后续的代码。

调用_dispatch_lane_drain_barrier_waiter执行栅栏函数中的任务：

static void
_dispatch_lane_drain_barrier_waiter(dispatch_lane_t dq,
        struct dispatch_object_s *dc, dispatch_wakeup_flags_t flags,
        uint64_t enqueued_bits)
{
    ......
    return _dispatch_barrier_waiter_redirect_or_wake(dq, dc, flags,
            old_state, new_state);
}

直接调用_dispatch_barrier_waiter_redirect_or_wake：

static void
_dispatch_barrier_waiter_redirect_or_wake(dispatch_queue_class_t dqu,
        dispatch_object_t dc, dispatch_wakeup_flags_t flags,
        uint64_t old_state, uint64_t new_state)
{
    ......
    return _dispatch_waiter_wake(dsc, wlh, old_state, new_state);
}

调用_dispatch_waiter_wake：

static void
_dispatch_waiter_wake(dispatch_sync_context_t dsc, dispatch_wlh_t wlh,
        uint64_t old_state, uint64_t new_state)
{
    dispatch_wlh_t waiter_wlh = dsc->dc_data;

    if ((_dq_state_is_base_wlh(old_state) && !dsc->dsc_from_async) ||
            _dq_state_is_base_wlh(new_state) ||
            waiter_wlh != DISPATCH_WLH_ANON) {
        _dispatch_event_loop_wake_owner(dsc, wlh, old_state, new_state);
    }
    if (unlikely(waiter_wlh == DISPATCH_WLH_ANON)) {
        //走这里
        _dispatch_waiter_wake_wlh_anon(dsc);
    }
}

调用_dispatch_waiter_wake_wlh_anon:

static void
_dispatch_waiter_wake_wlh_anon(dispatch_sync_context_t dsc)
{
    if (dsc->dsc_override_qos > dsc->dsc_override_qos_floor) {
        _dispatch_wqthread_override_start(dsc->dsc_waiter,
                dsc->dsc_override_qos);
    }
    //执行
    _dispatch_thread_event_signal(&dsc->dsc_event);
}

其中是对线程发送信号。

对于_dispatch_root_queue_wakeup而言：

void
_dispatch_root_queue_wakeup(dispatch_queue_global_t dq,
        DISPATCH_UNUSED dispatch_qos_t qos, dispatch_wakeup_flags_t flags)
{
    if (!(flags & DISPATCH_WAKEUP_BLOCK_WAIT)) {
        DISPATCH_INTERNAL_CRASH(dq->dq_priority,
                "Don't try to wake up or override a root queue");
    }
    if (flags & DISPATCH_WAKEUP_CONSUME_2) {
        return _dispatch_release_2_tailcall(dq);
    }
}

内部没有对barrier的处理，所以全局队列栅栏函数无效。

因为全局队列不仅有你的任务，还有其它系统的任务。如果加barrier不仅影响你自己的任务还会影响系统的任务。对于全局队列而言栅栏函数就是个普通的异步函数。

整个流程如下：

dispatch_barrier_sync逻辑

1.2.2 dispatch_barrier_async

dispatch_barrier_async源码如下：

void
dispatch_barrier_async(dispatch_queue_t dq, dispatch_block_t work)
{
    dispatch_continuation_t dc = _dispatch_continuation_alloc();
    uintptr_t dc_flags = DC_FLAG_CONSUME | DC_FLAG_BARRIER;
    dispatch_qos_t qos;

    qos = _dispatch_continuation_init(dc, dq, work, 0, dc_flags);
    _dispatch_continuation_async(dq, dc, qos, dc_flags);
}

调用的是_dispatch_continuation_async：

static inline void
_dispatch_continuation_async(dispatch_queue_class_t dqu,
        dispatch_continuation_t dc, dispatch_qos_t qos, uintptr_t dc_flags)
{
#if DISPATCH_INTROSPECTION
    if (!(dc_flags & DC_FLAG_NO_INTROSPECTION)) {
        _dispatch_trace_item_push(dqu, dc);
    }
#else
    (void)dc_flags;
#endif
    return dx_push(dqu._dq, dc, qos);
}

调用了dx_push，对应的自定义队列是_dispatch_lane_concurrent_push。全局队列是_dispatch_root_queue_push。

_dispatch_lane_concurrent_push:

void
_dispatch_lane_concurrent_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
    if (dq->dq_items_tail == NULL &&
            !_dispatch_object_is_waiter(dou) &&
            !_dispatch_object_is_barrier(dou) &&
            _dispatch_queue_try_acquire_async(dq)) {
        return _dispatch_continuation_redirect_push(dq, dou, qos);
    }

    _dispatch_lane_push(dq, dou, qos);
}

断点跟踪走的是_dispatch_lane_push：

DISPATCH_NOINLINE
void
_dispatch_lane_push(dispatch_lane_t dq, dispatch_object_t dou,
        dispatch_qos_t qos)
{
    dispatch_wakeup_flags_t flags = 0;
    struct dispatch_object_s *prev;

    if (unlikely(_dispatch_object_is_waiter(dou))) {
        return _dispatch_lane_push_waiter(dq, dou._dsc, qos);
    }

    dispatch_assert(!_dispatch_object_is_global(dq));
    qos = _dispatch_queue_push_qos(dq, qos);

    prev = os_mpsc_push_update_tail(os_mpsc(dq, dq_items), dou._do, do_next);
    if (unlikely(os_mpsc_push_was_empty(prev))) {
        _dispatch_retain_2_unsafe(dq);
        flags = DISPATCH_WAKEUP_CONSUME_2 | DISPATCH_WAKEUP_MAKE_DIRTY;
    } else if (unlikely(_dispatch_queue_need_override(dq, qos))) {
        _dispatch_retain_2_unsafe(dq);
        flags = DISPATCH_WAKEUP_CONSUME_2;
    }
    os_mpsc_push_update_prev(os_mpsc(dq, dq_items), prev, dou._do, do_next);
    if (flags) {
        //栅栏函数走这里。
        //#define dx_wakeup(x, y, z) dx_vtable(x)->dq_wakeup(x, y, z)
        //dx_wakeup 对应 dq_wakeup 自定义全局队列对应 _dispatch_lane_wakeup，全局队列对应 _dispatch_root_queue_wakeup
        return dx_wakeup(dq, qos, flags);
    }
}

栅栏函数走_dispatch_lane_wakeup逻辑：

void
_dispatch_lane_wakeup(dispatch_lane_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags)
{
    dispatch_queue_wakeup_target_t target = DISPATCH_QUEUE_WAKEUP_NONE;

    if (unlikely(flags & DISPATCH_WAKEUP_BARRIER_COMPLETE)) {
        return _dispatch_lane_barrier_complete(dqu, qos, flags);
    }
    if (_dispatch_queue_class_probe(dqu)) {
        target = DISPATCH_QUEUE_WAKEUP_TARGET;
    }
    //走这里
    return _dispatch_queue_wakeup(dqu, qos, flags, target);
}

继续断点走_dispatch_queue_wakeup逻辑：

void
_dispatch_queue_wakeup(dispatch_queue_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags, dispatch_queue_wakeup_target_t target)
{
    ......

    if (unlikely(flags & DISPATCH_WAKEUP_BARRIER_COMPLETE)) {
    ......
        //loop  _dispatch_lane_wakeup  //_dq_state_merge_qos
        return _dispatch_lane_class_barrier_complete(upcast(dq)._dl, qos,
                flags, target, DISPATCH_QUEUE_SERIAL_DRAIN_OWNED);
    }

    if (target) {
    ......
#if HAVE_PTHREAD_WORKQUEUE_QOS
    } else if (qos) {
    ......
    if (likely((old_state ^ new_state) & enqueue)) {
        ...... //_dispatch_queue_push_queue断点断不住，断它内部断点
        return _dispatch_queue_push_queue(tq, dq, new_state);
    }
#if HAVE_PTHREAD_WORKQUEUE_QOS
    if (unlikely((old_state ^ new_state) & DISPATCH_QUEUE_MAX_QOS_MASK)) {
        if (_dq_state_should_override(new_state)) {
            return _dispatch_queue_wakeup_with_override(dq, new_state,
                    flags);
        }
    }
#endif // HAVE_PTHREAD_WORKQUEUE_QOS
done:
    if (likely(flags & DISPATCH_WAKEUP_CONSUME_2)) {
        return _dispatch_release_2_tailcall(dq);
    }
}

这里断点走了_dispatch_queue_push_queue逻辑（_dispatch_queue_push_queue本身断不住，断它内部断点）：

static inline void
_dispatch_queue_push_queue(dispatch_queue_t tq, dispatch_queue_class_t dq,
        uint64_t dq_state)
{
#if DISPATCH_USE_KEVENT_WORKLOOP
    if (likely(_dq_state_is_base_wlh(dq_state))) {
        _dispatch_trace_runtime_event(worker_request, dq._dq, 1);
        return _dispatch_event_loop_poke((dispatch_wlh_t)dq._dq, dq_state,
                DISPATCH_EVENT_LOOP_CONSUME_2);
    }
#endif // DISPATCH_USE_KEVENT_WORKLOOP
    _dispatch_trace_item_push(tq, dq);
    //_dispatch_lane_concurrent_push
    return dx_push(tq, dq, _dq_state_max_qos(dq_state));
}

内部走的是_dispatch_lane_concurrent_push逻辑，这里又继续走了_dispatch_lane_push的逻辑了，在这里就造成了循环等待。当队列中任务执行完毕后_dispatch_lane_wakeup中就走_dispatch_lane_barrier_complete逻辑了。

可以通过barrier前面的任务加延迟去验证。直接断点_dispatch_lane_barrier_complete，当前面的任务执行完毕后就进入_dispatch_lane_barrier_complete断点了。

_dispatch_lane_barrier_complete源码如下：

static void
_dispatch_lane_barrier_complete(dispatch_lane_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags)
{
......
    return _dispatch_lane_class_barrier_complete(dq, qos, flags, target, owned);
}

走了_dispatch_lane_class_barrier_complete逻辑：

static void
_dispatch_lane_class_barrier_complete(dispatch_lane_t dq, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags, dispatch_queue_wakeup_target_t target,
        uint64_t owned)
{
......
again:
    os_atomic_rmw_loop2o(dq, dq_state, old_state, new_state, release, {
    ......
        } else if (unlikely(_dq_state_is_dirty(old_state))) {
    ......
                flags |= DISPATCH_WAKEUP_BARRIER_COMPLETE;
                //自定义并行队列 _dispatch_lane_wakeup
                return dx_wakeup(dq, qos, flags);
            });
        } else {
            new_state &= ~DISPATCH_QUEUE_MAX_QOS_MASK;
        }
    });
......
}

调用走的是_dispatch_lane_wakeup：

void
_dispatch_lane_wakeup(dispatch_lane_class_t dqu, dispatch_qos_t qos,
        dispatch_wakeup_flags_t flags)
{
    dispatch_queue_wakeup_target_t target = DISPATCH_QUEUE_WAKEUP_NONE;

    if (unlikely(flags & DISPATCH_WAKEUP_BARRIER_COMPLETE)) {
        //barrier完成了就走这里的逻辑,barrier之前的任务执行完毕。
        return _dispatch_lane_barrier_complete(dqu, qos, flags);
    }
    if (_dispatch_queue_class_probe(dqu)) {
        target = DISPATCH_QUEUE_WAKEUP_TARGET;
    }
    //走这里
    return _dispatch_queue_wakeup(dqu, qos, flags, target);
}

这个时候就又走到了_dispatch_lane_barrier_complete。

DISPATCH_WAKEUP_BARRIER_COMPLETE状态是在_dispatch_lane_resume中进行变更的:

image.png

_dispatch_root_queue_push内部并没有对barrier的处理，与全局队列逻辑一致。所以barrier函数传递全局队列无效。

整个过程如下：

dispatch_barrier_async逻辑

二、信号量（`dispatch_semaphore_t`）

2.1 应用

    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_semaphore_t sem = dispatch_semaphore_create(1);
    dispatch_queue_t queue1 = dispatch_queue_create("HotpotCat", NULL);
 
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"1 start");
        NSLog(@"1 end");
        dispatch_semaphore_signal(sem);
    });
    
    dispatch_async(queue1, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"2 start");
        NSLog(@"2 end");
        dispatch_semaphore_signal(sem);
    });
    
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"3 start");
        NSLog(@"3 end");
        dispatch_semaphore_signal(sem);
    });
    
    dispatch_async(queue1, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"4 start");
        NSLog(@"4 end");
        dispatch_semaphore_signal(sem);
    });

对于上面的例子输出：

1 start
1 end
2 start
2 end
3 start
3 end
4 start
4 end

这个时候信号量初始化的是1，全局队列与自定义串行队列中的任务按顺序依次执行。
当将信号量改为2后输出：

1 start
2 start
2 end
1 end
3 start
4 start
3 end
4 end

这个时候1、2先执行无序，3、4后执行无序。这样就控制了GCD任务的最大并发数。

修改代码如下：

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_semaphore_t sem = dispatch_semaphore_create(0);

dispatch_async(queue, ^{
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    NSLog(@"1 start");
    NSLog(@"1 end");
});

dispatch_async(queue, ^{
    sleep(2);
    NSLog(@"2 start");
    NSLog(@"2 end");
    dispatch_semaphore_signal(sem);
});

信号量初始值修改为0，在任务1中wait，在任务2中signal，这个时候输出如下：

2 start
2 end
1 start
1 end

任务2比任务1先执行了。由于信号量初始化为0，wait函数后面任务就执行不了一直等待；等到signal执行后发送信号wait就可以执行了。这样就达到了控制流程。任务2中的信号控制了任务1的执行。

2.2 源码分析

2.2.1 dispatch_semaphore_create

/*
 * @param dsema
 * The semaphore. The result of passing NULL in this parameter is undefined.
*/

dispatch_semaphore_t
dispatch_semaphore_create(intptr_t 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) { //>=0 才有用，否则直接返回
        return DISPATCH_BAD_INPUT;// 0
    }

    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;
}

当value < 0的时候无效，只有>= 0才有效，才会执行后续流程。

2.2.2 dispatch_semaphore_wait

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

--后value大于等于0直接返回0。执行dispatch_semaphore_wait后续的代码。
否则执行_dispatch_semaphore_wait_slow（相当于do-while循环）。

_dispatch_semaphore_wait_slow
当信号量为0的时候调用wait后（< 0）就走_dispatch_semaphore_wait_slow逻辑了：

DISPATCH_NOINLINE
static intptr_t
_dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema,
        dispatch_time_t timeout)
{
    long orig;

    _dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
    //超时直接break
    switch (timeout) {
    default:
        if (!_dispatch_sema4_timedwait(&dsema->dsema_sema, timeout)) {
            break;
        }
        // Fall through and try to undo what the fast path did to
        // dsema->dsema_value
    //NOW的情况下进行超时处理
    case DISPATCH_TIME_NOW:
        orig = dsema->dsema_value;
        while (orig < 0) {
            if (os_atomic_cmpxchgv2o(dsema, dsema_value, orig, orig + 1,
                    &orig, relaxed)) {
                return _DSEMA4_TIMEOUT();
            }
        }
        // Another thread called semaphore_signal().
        // Fall through and drain the wakeup.
    //FOREVER则进入wait逻辑。
    case DISPATCH_TIME_FOREVER:
        _dispatch_sema4_wait(&dsema->dsema_sema);
        break;
    }
    return 0;
}

当值为timeout的时候直接break。
当值为DISPATCH_TIME_NOW的时候循环调用_DSEMA4_TIMEOUT()。

#define _DSEMA4_TIMEOUT() KERN_OPERATION_TIMED_OUT

当值为DISPATCH_TIME_FOREVER的时候调用_dispatch_sema4_wait。

_dispatch_sema4_wait

//    void
//    _dispatch_sema4_wait(_dispatch_sema4_t *sema)
//    {
//        int ret = 0;
//        do {
//            ret = sem_wait(sema);
//        } while (ret == -1 && errno == EINTR);
//        DISPATCH_SEMAPHORE_VERIFY_RET(ret);
//    }

void
_dispatch_sema4_wait(_dispatch_sema4_t *sema)
{
    kern_return_t kr;
    do {
        kr = semaphore_wait(*sema);
    } while (kr == KERN_ABORTED);
    DISPATCH_SEMAPHORE_VERIFY_KR(kr);
}

semaphore_wait并没有搜到实现，这是pthread内核封装的实现。
_dispatch_sema4_wait本质上是一个do-while循环，相当于在这里直接卡住执行不到后面的逻辑了。相当于：

dispatch_async(queue, ^{
//  dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    do {
        //循环
    } while (signal <= 0);
    NSLog(@"1 start");
    NSLog(@"1 end");
});

结论：当value >= 0的时候执行后续的代码，否则do-while循环卡住后续逻辑。

2.2.3 dispatch_semaphore_signal

/*!
 * @function dispatch_semaphore_signal
 *
 * @abstract
 * Signal (increment) a semaphore.
 *
 * @discussion
 * Increment the counting semaphore. If the previous value was less than zero,
 * this function wakes a waiting thread before returning.
 *
 * @param dsema The counting semaphore.
 * The result of passing NULL in this parameter is undefined.
 *
 * @result
 * This function returns non-zero if a thread is woken. Otherwise, zero is
 * returned.
 */
intptr_t
dispatch_semaphore_signal(dispatch_semaphore_t dsema)
{
    //++操作
    long value = os_atomic_inc2o(dsema, dsema_value, release);
    if (likely(value > 0)) {
        return 0;
    }
    //++ 后还 < 0，则表示做wait操作(--)过多。报错。
    if (unlikely(value == LONG_MIN)) {
        DISPATCH_CLIENT_CRASH(value,
                "Unbalanced call to dispatch_semaphore_signal()");
    }
    //发送信号量逻辑，恢复wait等待的操作。
    return _dispatch_semaphore_signal_slow(dsema);
}

os_atomic_inc2o执行++后值大于0直接返回能够执行。
只有<= 0的时候才执行后续流程，调用_dispatch_semaphore_signal_slow进行异常处理。
注释说明了当值< 0的时候在return之前唤醒一个等待线程。

_dispatch_semaphore_signal_slow

intptr_t
_dispatch_semaphore_signal_slow(dispatch_semaphore_t dsema)
{
    _dispatch_sema4_create(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
    _dispatch_sema4_signal(&dsema->dsema_sema, 1);
    return 1;
}

直接调用_dispatch_sema4_signal。

_dispatch_sema4_signal

#define DISPATCH_SEMAPHORE_VERIFY_KR(x) do { \
        DISPATCH_VERIFY_MIG(x); \
        if (unlikely((x) == KERN_INVALID_NAME)) { \
            DISPATCH_CLIENT_CRASH((x), \
                "Use-after-free of dispatch_semaphore_t or dispatch_group_t"); \
        } else if (unlikely(x)) { \
            DISPATCH_INTERNAL_CRASH((x), "mach semaphore API failure"); \
        } \
    } while (0)

//经过调试走的是这个逻辑
void
_dispatch_sema4_signal(_dispatch_sema4_t *sema, long count)
{
    do {
        kern_return_t kr = semaphore_signal(*sema);//+1
        DISPATCH_SEMAPHORE_VERIFY_KR(kr);// == -1 报错
    } while (--count);//do-while(0) 只执行一次
}

相当于内部做了+1操作。这也是当信号量初始值为0的时候dispatch_semaphore_signal执行完毕后dispatch_semaphore_wait能够执行的原因。

小结：

dispatch_semaphore_wait进行--操作，减完是负值进入do-while循环，阻塞后续流程。
dispatch_semaphore_signal进行++操作，加完值不大于0进入后续报错流程。
semaphore_signal 与 semaphore_wait才是信号量能控制最大并发数的根本原因，否则dispatch_semaphore_signal与dispatch_semaphore_signal都是判断后直接返回，相当于什么都没做。

semaphore_signal & semaphore_wait

三、调度组

最直接的作用: 控制任务执行顺序。
相关API:

dispatch_group_create 创建组
dispatch_group_async 进组任务（与dispatch_group_enter和dispatch_group_leave搭配使用效果相同）
- dispatch_group_enter 进组
- dispatch_group_leave 出组
dispatch_group_notify 进组任务执行完毕通知
dispatch_group_wait 进组任务执行等待时间

3.1 应用

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_queue_t queue1 = dispatch_queue_create("test", DISPATCH_QUEUE_CONCURRENT);

dispatch_group_async(group, queue, ^{
    sleep(3);
    NSLog(@"1");
});

dispatch_group_async(group, queue1, ^{
    sleep(2);
    NSLog(@"2");
});

dispatch_group_async(group, queue1, ^{
    sleep(1);
    NSLog(@"3");
});

dispatch_group_async(group, queue, ^{
    NSLog(@"4");
});

dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    NSLog(@"5");
});

有如上案例，任务5永远在任务1、2、3、4之后执行。

当然也可以使用enter与leave配合dispatch_async使用：

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_queue_t queue1 = dispatch_queue_create("test", DISPATCH_QUEUE_CONCURRENT);

//先 enter 再 leave
dispatch_group_enter(group);
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"1");
    dispatch_group_leave(group);
});

dispatch_group_enter(group);
dispatch_async(queue1, ^{
    sleep(2);
    NSLog(@"2");
    dispatch_group_leave(group);
});

dispatch_group_enter(group);
dispatch_async(queue1, ^{
    sleep(1);
    NSLog(@"3");
    dispatch_group_leave(group);
});

dispatch_group_enter(group);
dispatch_async(queue, ^{
    NSLog(@"4");
    dispatch_group_leave(group);
});

dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    NSLog(@"5");
});

效果相同，需要注意的是dispatch_group_enter要比dispatch_group_leave先调用，并且必须成对出现，否则会崩溃。当然两种形式也可以混着用。

3.2 源码分析

根据上面的分析有3个问题：

1.dispatch_group_enter为什么要比dispatch_group_leave先调用，否则崩溃?
2.能够实现同步的原理是什么？
3.dispatch_group_async为什么等价于dispatch_group_enter + dispatch_group_leave?

之前的版本调度组是封装了信号量，目前新版本的是调度组自己写了一套逻辑。

3.2.1 dispatch_group_create

dispatch_group_t
dispatch_group_create(void)
{
    return _dispatch_group_create_with_count(0);
}

//creat & enter 写在一起的写法，信号量标记位1
dispatch_group_t
_dispatch_group_create_and_enter(void)
{
    return _dispatch_group_create_with_count(1);
}

是对_dispatch_group_create_with_count的调用：

static inline dispatch_group_t
_dispatch_group_create_with_count(uint32_t n)
{
    dispatch_group_t dg = _dispatch_object_alloc(DISPATCH_VTABLE(group),
            sizeof(struct dispatch_group_s));
    dg->do_next = DISPATCH_OBJECT_LISTLESS;
    dg->do_targetq = _dispatch_get_default_queue(false);
    if (n) {
        os_atomic_store2o(dg, dg_bits,
                (uint32_t)-n * DISPATCH_GROUP_VALUE_INTERVAL, relaxed);
        os_atomic_store2o(dg, do_ref_cnt, 1, relaxed); // 
    }
    return dg;
}

调用_dispatch_object_alloc创建group，与信号量写法相似

3.2.2 dispatch_group_enter

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.
    //0-- -> -1，与信号量不同的是没有wait
    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); // 
    }
    if (unlikely(old_value == DISPATCH_GROUP_VALUE_MAX)) {
        DISPATCH_CLIENT_CRASH(old_bits,
                "Too many nested calls to dispatch_group_enter()");
    }
}

0--变为-1，与信号量不同的是没有wait操作。

3.2.3 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.
    //-1++ -> 0
    uint64_t new_state, old_state = os_atomic_add_orig2o(dg, dg_state,
            DISPATCH_GROUP_VALUE_INTERVAL, release);
    //#define DISPATCH_GROUP_VALUE_MASK       0x00000000fffffffcULL
    // old_state & DISPATCH_GROUP_VALUE_MASK 是一个很大的值
    uint32_t old_value = (uint32_t)(old_state & DISPATCH_GROUP_VALUE_MASK);
    //-1 & DISPATCH_GROUP_VALUE_MASK == DISPATCH_GROUP_VALUE_1，old_value = -1
    if (unlikely(old_value == DISPATCH_GROUP_VALUE_1)) {//old_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)));
        //调用 _dispatch_group_wake，唤醒 dispatch_group_notify
        return _dispatch_group_wake(dg, old_state, true);
    }
    //old_value 为0的情况下直接报错，也就是先leave的情况下直接报错
    if (unlikely(old_value == 0)) {
        DISPATCH_CLIENT_CRASH((uintptr_t)old_value,
                "Unbalanced call to dispatch_group_leave()");
    }
}

-1++变为0，当old_value == -1的时候调用_dispatch_group_wake唤醒dispatch_group_notify。
既然old_value == -1的时候才唤醒，那么多次enter只有最后一次leave的时候才能唤醒。
当old_value == 0的时候直接报错，这也就是为什么先调用leave直接发生了crash。

3.2.4 dispatch_group_notify

void
dispatch_group_notify(dispatch_group_t dg, dispatch_queue_t dq,
        dispatch_block_t db)
{
    dispatch_continuation_t dsn = _dispatch_continuation_alloc();
    _dispatch_continuation_init(dsn, dq, db, 0, DC_FLAG_CONSUME);
    _dispatch_group_notify(dg, dq, dsn);
}

调用_dispatch_group_notify：

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) {//循环判断 old_state == 0 的时候 wake
                os_atomic_rmw_loop_give_up({
                    return _dispatch_group_wake(dg, new_state, false);
                });
            }
        });
    }
}

当old_state == 0的时候调用_dispatch_group_wake，也就是调用block的callout。与leave调用了同一个方法。

为什么两个地方都调用了？
因为在leave的时候dispatch_group_notify可能已经执行过了，任务已经保存在了group中，leave的时候本身尝试调用一次。
当然leave中也可能是一个延时任务，当调用leave的时候notify可能还没有执行，就导致notify任务还不存在。所以需要在notify中也调用。

_dispatch_group_wake

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

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

        // Snapshot before anything is notified/woken 
        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);
            //异步回调，执行block callout
            _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);
}

调用_dispatch_continuation_async相当于调用的是dispatch_async执行notify的任务。
任务先保存在在group中，唤醒notify的时候才将任务加入队列。

3.2.5 dispatch_group_async

dispatch_group_async(dispatch_group_t dg, dispatch_queue_t dq,
        dispatch_block_t db)
{
    dispatch_continuation_t dc = _dispatch_continuation_alloc();
       //标记 DC_FLAG_GROUP_ASYNC
    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);
}

调用_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)
{
    //调用enter
    dispatch_group_enter(dg);
    dc->dc_data = dg;
    //dispatch_async
    _dispatch_continuation_async(dq, dc, qos, dc->dc_flags);
}

内部先调用dispatch_group_enter，在这里就等待wakeup的调用了
再调用_dispatch_continuation_async，相当于dispatch_async。

那么leave在什么时候调用呢？
肯定要在callout执行完毕后调用。_dispatch_continuation_async的调用以全局队列为例调用_dispatch_root_queue_push，最终会调用到_dispatch_continuation_invoke_inline：

image.png

在这里就进行了逻辑区分，有group的情况下（dispatch_group_async的时候dc_flags进行了标记）调用的是_dispatch_continuation_with_group_invoke：

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) {
        //callout
        _dispatch_client_callout(dc->dc_ctxt, dc->dc_func);
        _dispatch_trace_item_complete(dc);
        //leave
        dispatch_group_leave((dispatch_group_t)dou);
    } else {
        DISPATCH_INTERNAL_CRASH(dx_type(dou), "Unexpected object type");
    }
}

在callout后调用了dispatch_group_leave。

dispatch_group_async 底层是对 dispatch_group_enter + dispatch_group_leave 的封装

在dispatch_group_async中先进行dispatch_group_enter，然后执行dispatch_async。
在回调中先_dispatch_client_callout然后dispatch_group_leave。

四、Dispatch Source

在任一线程上调用它的的一个函数 dispatch_source_merge_data 后，会执行 dispatch source 事先定义好的句柄(可以把句柄简单理解为一个 block ) 这个过程叫 用户事件(Custom event)。是 dispatch source 支持处理的一种事件。

句柄是一种指向指针的指针，它指向的就是一个类或者结构，它和系统有很密切的关系。比如：实例句柄(HINSTANCE)，位图句柄(HBITMAP)，设备表述句柄(HDC)，图标句柄(HICON)等。这当中还有一个通用的句柄，就是HANDLE。

Dispatch Source有两点：

CPU 负荷非常小，尽量不占用资源。
联结的优势。
dispatch source不受runloop的影响，底层封装的是pthread。

4.1 应用

dispatch_source_t
dispatch_source_create(dispatch_source_type_t type,
    uintptr_t handle,
    uintptr_t mask,
    dispatch_queue_t _Nullable queue);

type：dispatch 源可处理的事件。比如：DISPATCH_SOURCE_TYPE_TIMER、DISPATCH_SOURCE_TYPE_DATA_ADD。
- DISPATCH_SOURCE_TYPE_DATA_ADD: 将所有触发结果相加，最后统一执行响应。间隔的时间越长，则每次触发都会响应；如果间隔的时间很短，则会将触发后的结果相加后统一触发。也就是利用CPU空闲时间进行回调。
handle：可以理解为句柄、索引或id，如果要监听进程，需要传入进程的ID。
mask：可以理解为描述，具体要监听什么。
queue：处理handle的队列。

有如下一个进度条的案例：

self.completed = 0;
self.queue = dispatch_queue_create("HotpotCat", NULL);
self.source = dispatch_source_create(DISPATCH_SOURCE_TYPE_DATA_ADD, 0, 0, dispatch_get_main_queue());
//设置句柄
dispatch_source_set_event_handler(self.source, ^{
    NSLog(@"%@",[NSThread currentThread]);
    NSUInteger value = dispatch_source_get_data(self.source);
    self.completed += value;
    double progress = self.completed / 100.0;
    NSLog(@"progress: %.2f",progress);
    self.progressView.progress = progress;
});
self.isRunning = YES;
//创建后默认是挂起状态
dispatch_resume(self.source);

创建了一个ADD类型的source，在handle获取进度增量并更新进度条。由于创建后source处于挂起状态，需要先恢复。

可以在按钮的点击事件中进行任务的挂起和恢复：

if (self.isRunning) {
    dispatch_suspend(self.source);
    dispatch_suspend(self.queue);
    NSLog(@"pause");
    self.isRunning = NO;
    [sender setTitle:@"pause" forState:UIControlStateNormal];
} else {
    dispatch_resume(self.source);
    dispatch_resume(self.queue);
    NSLog(@"running");
    self.isRunning = YES;
    [sender setTitle:@"running" forState:UIControlStateNormal];
}

任务的执行是一个简单的循环：

for (NSInteger i = 0; i < 100; i++) {
    dispatch_async(self.queue, ^{
        NSLog(@"merge");
        //加不加 sleep 影响 handler 的执行次数。
        sleep(1);
        dispatch_source_merge_data(self.source, 1);//+1
    });
}

在循环中调用dispatch_source_merge_data触发回调。当queue挂起后后续任务就不再执行了。
在不加sleep的情况下handler的回调是小于100次的，任务会被合并。

4.2 源码解析

4.2.1 dispatch_source_create

dispatch_source_t
dispatch_source_create(dispatch_source_type_t dst, uintptr_t handle,
        uintptr_t mask, dispatch_queue_t dq)
{
    dispatch_source_refs_t dr;
    dispatch_source_t ds;
    //add对应 _dispatch_source_data_create timer对应 _dispatch_source_timer_create
    dr = dux_create(dst, handle, mask)._dr;
    if (unlikely(!dr)) {
        return DISPATCH_BAD_INPUT;
    }
    //创建队列
    ds = _dispatch_queue_alloc(source,
            dux_type(dr)->dst_strict ? DSF_STRICT : DQF_MUTABLE, 1,
            DISPATCH_QUEUE_INACTIVE | DISPATCH_QUEUE_ROLE_INNER)._ds;
    ds->dq_label = "source";
    ds->ds_refs = dr;
    dr->du_owner_wref = _dispatch_ptr2wref(ds);

    //没有传队列，获取root_queues
    if (unlikely(!dq)) {
        dq = _dispatch_get_default_queue(true);
    } else {
        _dispatch_retain((dispatch_queue_t _Nonnull)dq);
    }
    //目标队列为传进来的dq
    ds->do_targetq = dq;
    //是timer 并且设置了interval则调用dispatch_source_set_timer进行设置
    //也就是说type为timer的时候即使不设置timer也会默认设置。这里时间间隔设置为了handle
    if (dr->du_is_timer && (dr->du_timer_flags & DISPATCH_TIMER_INTERVAL)) {
        dispatch_source_set_timer(ds, DISPATCH_TIME_NOW, handle, UINT64_MAX);
    }
    _dispatch_object_debug(ds, "%s", __func__);
    //返回自己创建的source，source本身也是队列。
    return ds;
}

根据type创建对应的队列。add对应_dispatch_source_data_create，timer对应_dispatch_source_timer_create。
如果创建的时候没有传处理handle的队列，会默认获取root_queues中的队列。
设置目标队列为传进来的队列。
如果type为DISPATCH_SOURCE_TYPE_INTERVAL（应该是私有的）则主动调用一次dispatch_source_set_timer。
返回自己创建的source，source本身也是队列。

_dispatch_source_data_create

static dispatch_unote_t
_dispatch_source_data_create(dispatch_source_type_t dst, uintptr_t handle,
        uintptr_t mask)
{
    if (handle || mask) {
        return DISPATCH_UNOTE_NULL;
    }

    // bypass _dispatch_unote_create() because this is always "direct"
    // even when EV_UDATA_SPECIFIC is 0
    dispatch_unote_class_t du = _dispatch_calloc(1u, dst->dst_size);
    du->du_type = dst;
    du->du_filter = dst->dst_filter;
    du->du_is_direct = true;
    return (dispatch_unote_t){ ._du = du };
}

直接调用_dispatch_calloc创建返回。

_dispatch_source_timer_create

static dispatch_unote_t
_dispatch_source_timer_create(dispatch_source_type_t dst,
        uintptr_t handle, uintptr_t mask)
{
    dispatch_timer_source_refs_t dt;
    ......
    //创建
    dt = _dispatch_calloc(1u, dst->dst_size);
    dt->du_type = dst;
    dt->du_filter = dst->dst_filter;
    dt->du_is_timer = true;
    dt->du_timer_flags |= (uint8_t)(mask | dst->dst_timer_flags);
    dt->du_ident = _dispatch_timer_unote_idx(dt);
    dt->dt_timer.target = UINT64_MAX;
    dt->dt_timer.deadline = UINT64_MAX;
    dt->dt_timer.interval = UINT64_MAX;
    dt->dt_heap_entry[DTH_TARGET_ID] = DTH_INVALID_ID;
    dt->dt_heap_entry[DTH_DEADLINE_ID] = DTH_INVALID_ID;
    return (dispatch_unote_t){ ._dt = dt };
}

内部时间给的默认值是最大值。

4.2.2 dispatch_source_set_event_handler

void
dispatch_source_set_event_handler(dispatch_source_t ds,
        dispatch_block_t handler)
{
    _dispatch_source_set_handler(ds, handler, DS_EVENT_HANDLER, true);
}

调用_dispatch_source_set_handler传递的类型为DS_EVENT_HANDLER。

DISPATCH_NOINLINE
static void
_dispatch_source_set_handler(dispatch_source_t ds, void *func,
        uintptr_t kind, bool is_block)
{
    dispatch_continuation_t dc;
    //创建dc存储handler
    dc = _dispatch_source_handler_alloc(ds, func, kind, is_block);
    //挂起
    if (_dispatch_lane_try_inactive_suspend(ds)) {
        //替换
        _dispatch_source_handler_replace(ds, kind, dc);
        //恢复
        return _dispatch_lane_resume(ds, DISPATCH_RESUME);
    }
......
}

创建_dispatch_source_handler_alloc存储handler，内部会进行标记非DS_EVENT_HANDLER会标记为DC_FLAG_CONSUME。
_dispatch_lane_try_inactive_suspend挂起队列。
_dispatch_source_handler_replace替换handler。

static inline void
_dispatch_source_handler_replace(dispatch_source_t ds, uintptr_t kind,
        dispatch_continuation_t dc)
{
    //handler目标回调为空释放handler
    if (!dc->dc_func) {
        _dispatch_continuation_free(dc);
        dc = NULL;
    } else if (dc->dc_flags & DC_FLAG_FETCH_CONTEXT) {
        dc->dc_ctxt = ds->do_ctxt;
    }
    //保存
    dc = os_atomic_xchg(&ds->ds_refs->ds_handler[kind], dc, release);
    if (dc) _dispatch_source_handler_dispose(dc);
}

_dispatch_lane_resume恢复队列，调用队列对应的awake。

image.png

先调用_dispatch_lane_resume_activate（这也就是set后立马调用的原因）：

static void
_dispatch_lane_resume_activate(dispatch_lane_t dq)
{
    if (dx_vtable(dq)->dq_activate) {
        dx_vtable(dq)->dq_activate(dq);
    }

    _dispatch_lane_resume(dq, DISPATCH_ACTIVATION_DONE);
}

再调用_dispatch_lane_resume。

4.2.3 dispatch_source_merge_data

void
dispatch_source_merge_data(dispatch_source_t ds, uintptr_t val)
{
    dispatch_queue_flags_t dqf = _dispatch_queue_atomic_flags(ds);
    dispatch_source_refs_t dr = ds->ds_refs;

    if (unlikely(dqf & (DSF_CANCELED | DQF_RELEASED))) {
        return;
    }
    //根据类型存值
    switch (dr->du_filter) {
    case DISPATCH_EVFILT_CUSTOM_ADD:
        //有累加
        os_atomic_add2o(dr, ds_pending_data, val, relaxed);
        break;
    case DISPATCH_EVFILT_CUSTOM_OR:
        os_atomic_or2o(dr, ds_pending_data, val, relaxed);
        break;
    case DISPATCH_EVFILT_CUSTOM_REPLACE:
        os_atomic_store2o(dr, ds_pending_data, val, relaxed);
        break;
    default:
        DISPATCH_CLIENT_CRASH(dr->du_filter, "Invalid source type");
    }
    //唤醒执行回调
    dx_wakeup(ds, 0, DISPATCH_WAKEUP_MAKE_DIRTY);
}

根据类型对值进行处理，处理完之后唤醒队列执行。

对于主线程会执行_dispatch_main_queue_wakeup，其中会取到dispatch_queue获取到dc，最后进行handler的调用。

4.2.4 dispatch_source_get_data

uintptr_t
dispatch_source_get_data(dispatch_source_t ds)
{
    dispatch_source_refs_t dr = ds->ds_refs;
#if DISPATCH_USE_MEMORYSTATUS
    if (dr->du_vmpressure_override) {
        return NOTE_VM_PRESSURE;
    }
#if TARGET_OS_SIMULATOR
    if (dr->du_memorypressure_override) {
        return NOTE_MEMORYSTATUS_PRESSURE_WARN;
    }
#endif
#endif // DISPATCH_USE_MEMORYSTATUS
    //获取数据
    uint64_t value = os_atomic_load2o(dr, ds_data, relaxed);
    return (unsigned long)(dr->du_has_extended_status ?
            DISPATCH_SOURCE_GET_DATA(value) : value);
}

与merge_data相反，一个存一个取。

4.2.5 dispatch_resume

void
dispatch_resume(dispatch_object_t dou)
{
    DISPATCH_OBJECT_TFB(_dispatch_objc_resume, dou);
    if (unlikely(_dispatch_object_is_global(dou) ||
            _dispatch_object_is_root_or_base_queue(dou))) {
        return;
    }
    if (dx_cluster(dou._do) == _DISPATCH_QUEUE_CLUSTER) {
        _dispatch_lane_resume(dou._dl, DISPATCH_RESUME);
    }
}

经过调试走的是_dispatch_lane_resume逻辑，与_dispatch_source_set_handler中调用的一致。awake队列。

4.2.6 dispatch_suspend

void
dispatch_suspend(dispatch_object_t dou)
{
    DISPATCH_OBJECT_TFB(_dispatch_objc_suspend, dou);
    if (unlikely(_dispatch_object_is_global(dou) ||
            _dispatch_object_is_root_or_base_queue(dou))) {
        return;
    }
    if (dx_cluster(dou._do) == _DISPATCH_QUEUE_CLUSTER) {
        return _dispatch_lane_suspend(dou._dl);
    }
}

调用_dispatch_lane_suspend挂起队列。

4.2.7 dispatch_source_cancel

dispatch_source_cancel(dispatch_source_t ds)
{
    _dispatch_object_debug(ds, "%s", __func__);

    _dispatch_retain_2(ds);

    if (_dispatch_queue_atomic_flags_set_orig(ds, DSF_CANCELED) & DSF_CANCELED){
        _dispatch_release_2_tailcall(ds);
    } else {
        //_dispatch_workloop_wakeup
        dx_wakeup(ds, 0, DISPATCH_WAKEUP_MAKE_DIRTY | DISPATCH_WAKEUP_CONSUME_2);
    }
}

调用_dispatch_workloop_wakeup：

image.png
cancel内部会对状态进行判断，如果是挂起状态会报错。所以需要在运行状态下取消。
调用_dispatch_release_2_tailcall进行释放操作。

4.2.8 dispatch_source_set_timer

void
dispatch_source_set_timer(dispatch_source_t ds, dispatch_time_t start,
        uint64_t interval, uint64_t leeway)
{
    dispatch_timer_source_refs_t dt = ds->ds_timer_refs;
    dispatch_timer_config_t dtc;

    if (unlikely(!dt->du_is_timer)) {
        DISPATCH_CLIENT_CRASH(ds, "Attempt to set timer on a non-timer source");
    }
    //根据type配置timer和interval
    if (dt->du_timer_flags & DISPATCH_TIMER_INTERVAL) {
        dtc = _dispatch_interval_config_create(start, interval, leeway, dt);
    } else {
        dtc = _dispatch_timer_config_create(start, interval, leeway, dt);
    }
    if (_dispatch_timer_flags_to_clock(dt->du_timer_flags) != dtc->dtc_clock &&
            dt->du_filter == DISPATCH_EVFILT_TIMER_WITH_CLOCK) {
        DISPATCH_CLIENT_CRASH(0, "Attempting to modify timer clock");
    }
    //跟踪配置
    _dispatch_source_timer_telemetry(ds, dtc->dtc_clock, &dtc->dtc_timer);
    dtc = os_atomic_xchg2o(dt, dt_pending_config, dtc, release);
    if (dtc) free(dtc);
    //唤醒
    dx_wakeup(ds, 0, DISPATCH_WAKEUP_MAKE_DIRTY);
}

4.2.9 dispatch_source_set_registration_handler

void
dispatch_source_set_registration_handler(dispatch_source_t ds,
        dispatch_block_t handler)
{
    _dispatch_source_set_handler(ds, handler, DS_REGISTN_HANDLER, true);
}

也是直接调用的_dispatch_source_set_handler，参数是DS_REGISTN_HANDLER。

4.2.10 dispatch_source_set_cancel_handler

void
dispatch_source_set_cancel_handler(dispatch_source_t ds,
        dispatch_block_t handler)
{
    _dispatch_source_set_handler(ds, handler, DS_CANCEL_HANDLER, true);
}

直接调用的_dispatch_source_set_handler，参数是DS_CANCEL_HANDLER。
会根据DS_REGISTN_HANDLER、DS_CANCEL_HANDLER、DS_EVENT_HANDLER进行handler的获取和释放，因为这三者可能同时存在。

那么就有个问题设置timer类型后我们没有主动调用dispatch_source_merge_data，那么它是在什么时机调用的呢？在回调中bt:

    frame #2: 0x000000010b6a29c8 libdispatch.dylib`_dispatch_client_callout + 8
    frame #3: 0x000000010b6a5316 libdispatch.dylib`_dispatch_continuation_pop + 557
    frame #4: 0x000000010b6b8e8b libdispatch.dylib`_dispatch_source_invoke + 2205
    frame #5: 0x000000010b6b4508 libdispatch.dylib`_dispatch_root_queue_drain + 351
    frame #6: 0x000000010b6b4e6d libdispatch.dylib`_dispatch_worker_thread2 + 135
    frame #7: 0x00007fff611639f7 libsystem_pthread.dylib`_pthread_wqthread + 220
    frame #8: 0x00007fff61162b77 libsystem_pthread.dylib`start_wqthread + 15

搜索_dispatch_source_invoke只找到了：

DISPATCH_VTABLE_INSTANCE(source,
    .do_type        = DISPATCH_SOURCE_KEVENT_TYPE,
    .do_dispose     = _dispatch_source_dispose,
    .do_debug       = _dispatch_source_debug,
    .do_invoke      = _dispatch_source_invoke,

    .dq_activate    = _dispatch_source_activate,
    .dq_wakeup      = _dispatch_source_wakeup,
    .dq_push        = _dispatch_lane_push,
);

也就是调用的source的do_invoke，调用逻辑为_dispatch_root_queue_drain -> _dispatch_continuation_pop_inline -> dx_invoke：

void
_dispatch_source_invoke(dispatch_source_t ds, dispatch_invoke_context_t dic,
        dispatch_invoke_flags_t flags)
{
    _dispatch_queue_class_invoke(ds, dic, flags,
            DISPATCH_INVOKE_DISALLOW_SYNC_WAITERS, _dispatch_source_invoke2);

#if DISPATCH_EVENT_BACKEND_KEVENT
    if (flags & DISPATCH_INVOKE_WORKLOOP_DRAIN) {
        dispatch_workloop_t dwl = (dispatch_workloop_t)_dispatch_get_wlh();
        dispatch_timer_heap_t dth = dwl->dwl_timer_heap;
        if (dth && dth[0].dth_dirty_bits) {
            //调用
            _dispatch_event_loop_drain_timers(dwl->dwl_timer_heap,
                    DISPATCH_TIMER_WLH_COUNT);
        }
    }
#endif // DISPATCH_EVENT_BACKEND_KEVENT
}

4.3 Dispatch Source 封装 Timer

目标是封装一个类似NSTimer的工具。

void
dispatch_source_set_timer(dispatch_source_t source,
    dispatch_time_t start,
    uint64_t interval,
    uint64_t leeway);

source：事件源。
start：控制计时器第一次触发的时刻。
- 参数类型是 dispatch_time_t（opaque类型），不能直接操作它。需要 dispatch_time 和 dispatch_walltime 函数来创建。
- 常量 DISPATCH_TIME_NOW 和 DISPATCH_TIME_FOREVER 很常用。
- 当使用dispatch_time 或者 DISPATCH_TIME_NOW 时，系统会使用默认时钟来进行计时。然而当系统休眠的时候，默认时钟是不走的，也就会导致计时器停止。使用 dispatch_walltime 可以让计时器按照真实时间间隔进行计时。
interval：回调间隔时间。
leeway：计时器触发的精准程度，就算指定为0系统也无法保证完全精确的触发时间，只是会尽可能满足这个需求。

首先实现一个最简单的封装：

- (instancetype)initTimerWithTimeInterval:(NSTimeInterval)interval queue:(dispatch_queue_t)queue leeway:(NSTimeInterval)leeway repeats:(BOOL)repeats handler:(dispatch_block_t)handler {    
    if (self == [super init]) {
        self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
        dispatch_source_set_timer(self.timer, dispatch_walltime(NULL, 0), interval * NSEC_PER_SEC, leeway * NSEC_PER_SEC);
        //解决与handler互相持有
        __weak typeof(self) weakSelf = self;

        //事件回调，这个函数在执行完之后 block 会立马执行一遍。后面隔一定时间间隔再执行一次。
        dispatch_source_set_event_handler(self.timer, ^{
            if (handler) {
                handler();
            }
            if (!repeats) {
                //repeats 为 NO 执行一次后取消
                [weakSelf cancel];
            }
        });
    }
    return self;
}

这样就满足了最基本的要求，由于handler的调用在设置和恢复后会立马调用，所以需要过滤需改handler实现如下：

//忽略 handler 设置完马上回调
if (weakSelf.isAutoFirstCallback) {
    @synchronized(weakSelf) {
        weakSelf.isAutoFirstCallback = NO;
    }
    return;
}
//忽略挂起恢复后的立马回调
if (!weakSelf.resumeCallbackEnable && weakSelf.isResumeCallback) {
    @synchronized(weakSelf) {
        weakSelf.isResumeCallback = NO;
    }
    return;
}

if (handler) {
    handler();
}

if (!repeats) {
    //repeats 为 NO 执行一次后取消
    [weakSelf cancel];
}

为了更灵活对注册以及取消source逻辑也进行暴露：

dispatch_source_set_registration_handler(self.timer, ^{
    if (weakSelf.startBlock) {
        weakSelf.startBlock();
    }
});
//取消回调
dispatch_source_set_cancel_handler(self.timer, ^{
    if (weakSelf.cancelBlock) {
        weakSelf.cancelBlock();
    }
});

由于source本身提供了挂起和恢复的功能，同样对其封装。并且需要进行释放操作，所以提供cancel功能：

- (void)start {
    //为了与isResumeCallback区分开
    @synchronized(self) {
        if (!self.isStarted && self.timerStatus == HPTimerSuspend) {
            self.isStarted = YES;
            self.timerStatus = HPTimerResume;
            dispatch_resume(self.timer);
        }
    }
}

- (void)suspend {
    //挂起，挂起的时候不能设置timer为nil
    @synchronized(self) {
        if (self.timerStatus == HPTimerResume) {
            self.timerStatus = HPTimerSuspend;
            dispatch_suspend(self.timer);
        }
    }
}

- (void)resume {
    //恢复
    @synchronized(self) {
        if (self.timerStatus == HPTimerSuspend) {
            self.isResumeCallback = YES;
            self.timerStatus = HPTimerResume;
            dispatch_resume(self.timer);
        }
    }
}

- (void)cancel {
    //取消
    @synchronized(self) {
        if (self.timerStatus != HPTimerCanceled) {
            //先恢复再取消
            if (self.timerStatus == HPTimerSuspend) {
                [self resume];
            }
            self.timerStatus = HPTimerCanceled;
            dispatch_source_cancel(self.timer);
            _timer = nil;
        }
    }
}

- (void)dealloc {
    [self cancel];
}

dealloc中主动进行cancel调用方可以不必在自己的dealloc中调用。

这样再暴露一些简单接口就可以直接调用了（调用方需要持有timer）：

self.timer = [HPTimer scheduledTimerWithTimeInterval:3 handler:^{
    NSLog(@"timer 回调");
}];

完整代码详见 HPTimer

具体的更多介绍和用法可以参考：
iOS多线程——Dispatch Source
timer 注意事项

五、延迟函数(dispatch_after)

void
dispatch_after(dispatch_time_t when, dispatch_queue_t queue,
        dispatch_block_t work)
{
    _dispatch_after(when, queue, NULL, work, true);
}

直接调用_dispatch_after：

static inline void
_dispatch_after(dispatch_time_t when, dispatch_queue_t dq,
        void *ctxt, void *handler, bool block)
{
    dispatch_timer_source_refs_t dt;
    dispatch_source_t ds;
    uint64_t leeway, delta;
    //FOREVER 直接返回什么也不做
    if (when == DISPATCH_TIME_FOREVER) {
#if DISPATCH_DEBUG
        DISPATCH_CLIENT_CRASH(0, "dispatch_after called with 'when' == infinity");
#endif
        return;
    }

    delta = _dispatch_timeout(when);
    if (delta == 0) {
        if (block) {
            //时间为0直接执行handler
            return dispatch_async(dq, handler);
        }
        return dispatch_async_f(dq, ctxt, handler);
    }
    //精度 = 间隔 / 10
    leeway = delta / 10; // 
    //<1 毫秒 的时候设置最小值为1毫秒
    if (leeway < NSEC_PER_MSEC) leeway = NSEC_PER_MSEC;
    //大于60s的时候设置为60s，也就是  1ms <= leeway <= 1min
    if (leeway > 60 * NSEC_PER_SEC) leeway = 60 * NSEC_PER_SEC;

    // this function can and should be optimized to not use a dispatch source
    //创建 type 为 after 的 source
    ds = dispatch_source_create(&_dispatch_source_type_after, 0, 0, dq);
    dt = ds->ds_timer_refs;

    dispatch_continuation_t dc = _dispatch_continuation_alloc();
    if (block) {
        //包装handler
        _dispatch_continuation_init(dc, dq, handler, 0, 0);
    } else {
        _dispatch_continuation_init_f(dc, dq, ctxt, handler, 0, 0);
    }
    // reference `ds` so that it doesn't show up as a leak
    dc->dc_data = ds;
    _dispatch_trace_item_push(dq, dc);
    //存储handler
    os_atomic_store2o(dt, ds_handler[DS_EVENT_HANDLER], dc, relaxed);
    dispatch_clock_t clock;
    uint64_t target;
    _dispatch_time_to_clock_and_value(when, false, &clock, &target);
    if (clock != DISPATCH_CLOCK_WALL) {
        leeway = _dispatch_time_nano2mach(leeway);
    }
    dt->du_timer_flags |= _dispatch_timer_flags_from_clock(clock);
    dt->dt_timer.target = target;
    dt->dt_timer.interval = UINT64_MAX;
    dt->dt_timer.deadline = target + leeway;
    dispatch_activate(ds);
}

延时时间设置为DISPATCH_TIME_FOREVER直接返回什么也不做。
延时时间为0直接调用dispatch_async执行handler。
精度：1ms <= leeway <= 1min要在这个范围，否则会修正。
创建_dispatch_source_type_after类型的source。
包装存储handler。
调用_dispatch_time_to_clock_and_value进行target设置。

本质上 dispatch_after 也是对 source的封装。

时间单位

#define NSEC_PER_SEC 1000000000ull      1秒 = 10亿纳秒              
#define NSEC_PER_MSEC 1000000ull       1毫秒  =   100万纳秒 
#define USEC_PER_SEC 1000000ull        1秒  = 100万微秒   
#define NSEC_PER_USEC 1000ull           1微秒   = 1000 纳秒

1s = 1000ms = 100万us = 10亿ns
1ms = 1000us
1us = 1000ns

GCD底层分析（三）：栅栏、信号量、调度组以及source