一、队列是如何产生的？
1、串行与并发队列
**注：代码中有中文解析的地方为重点 ** (其他地方可以不看)
// 如何在创建的时候区分串行还是并发
// 通过结构体位域设置dqai的属性
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
return _dispatch_lane_create_with_target(label, attr,
DISPATCH_TARGET_QUEUE_DEFAULT, true);
}
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
_dispatch_lane_create_with_target(const char *label, dispatch_queue_attr_t dqa,
dispatch_queue_t tq, bool legacy)
{
// 串行队列会返回{ } 空
dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);// 下面有进去的代码
//
// Step 1: Normalize arguments (qos, overcommit, tq)
//
dispatch_qos_t qos = dqai.dqai_qos;
#if !HAVE_PTHREAD_WORKQUEUE_QOS
if (qos == DISPATCH_QOS_USER_INTERACTIVE) {
dqai.dqai_qos = qos = DISPATCH_QOS_USER_INITIATED;
}
if (qos == DISPATCH_QOS_MAINTENANCE) {
dqai.dqai_qos = qos = DISPATCH_QOS_BACKGROUND;
}
#endif // !HAVE_PTHREAD_WORKQUEUE_QOS
_dispatch_queue_attr_overcommit_t overcommit = dqai.dqai_overcommit;
if (overcommit != _dispatch_queue_attr_overcommit_unspecified && tq) {
if (tq->do_targetq) {
DISPATCH_CLIENT_CRASH(tq, "Cannot specify both overcommit and "
"a non-global target queue");
}
}
if (tq && dx_type(tq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE) {
// Handle discrepancies between attr and target queue, attributes win
if (overcommit == _dispatch_queue_attr_overcommit_unspecified) {
if (tq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) {
overcommit = _dispatch_queue_attr_overcommit_enabled;
} else {
overcommit = _dispatch_queue_attr_overcommit_disabled;
}
}
if (qos == DISPATCH_QOS_UNSPECIFIED) {
qos = _dispatch_priority_qos(tq->dq_priority);
}
tq = NULL;
} else if (tq && !tq->do_targetq) {
// target is a pthread or runloop root queue, setting QoS or overcommit
// is disallowed
if (overcommit != _dispatch_queue_attr_overcommit_unspecified) {
DISPATCH_CLIENT_CRASH(tq, "Cannot specify an overcommit attribute "
"and use this kind of target queue");
}
} else {
if (overcommit == _dispatch_queue_attr_overcommit_unspecified) {
// Serial queues default to overcommit!
overcommit = dqai.dqai_concurrent ?
_dispatch_queue_attr_overcommit_disabled :
_dispatch_queue_attr_overcommit_enabled;
}
}
if (!tq) {
tq = _dispatch_get_root_queue(
qos == DISPATCH_QOS_UNSPECIFIED ? DISPATCH_QOS_DEFAULT : qos,
overcommit == _dispatch_queue_attr_overcommit_enabled)->_as_dq;
// DISPATCH_QOS_DEFAULT 4
// _dispatch_queue_attr_overcommit_enabled yes no
// 第2步细讲--有详细代码
if (unlikely(!tq)) {
DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");
}
}
//
// Step 2: Initialize the queue
//
if (legacy) {
// if any of these attributes is specified, use non legacy classes
if (dqai.dqai_inactive || dqai.dqai_autorelease_frequency) {
legacy = false;
}
}
const void *vtable;
dispatch_queue_flags_t dqf = legacy ? DQF_MUTABLE : 0;
if (dqai.dqai_concurrent) // 通过dqai.dqai_concurrent区分串行和并发
{
vtable = DISPATCH_VTABLE(queue_concurrent);
//OS_dispatch_##name##_class → OS_dispatch_queue_concurrent_class
}
else
{
vtable = DISPATCH_VTABLE(queue_serial);
}
switch (dqai.dqai_autorelease_frequency) {
case DISPATCH_AUTORELEASE_FREQUENCY_NEVER:
dqf |= DQF_AUTORELEASE_NEVER;
break;
case DISPATCH_AUTORELEASE_FREQUENCY_WORK_ITEM:
dqf |= DQF_AUTORELEASE_ALWAYS;
break;
}
if (label) //label赋值
{
const char *tmp = _dispatch_strdup_if_mutable(label);
if (tmp != label) {
dqf |= DQF_LABEL_NEEDS_FREE;
label = tmp;
}
}
// 开辟内存，生成相应的对象：dq -- 队列对象 → dispatch_queue_create方法创建出对象
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
// 其中有 dispatch_object_t 最大的对象，其他对象都是此对象衍生的
// 构造方法 --- 如果是并发，给个最大宽度 0xffe 4094，如果是串行，宽度给个1
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
dq->dq_label = label;//label赋值
dq->dq_priority = _dispatch_priority_make((dispatch_qos_t)dqai.dqai_qos,
dqai.dqai_relpri); // 优先级
if (overcommit == _dispatch_queue_attr_overcommit_enabled) {
dq->dq_priority |= DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
}
if (!dqai.dqai_inactive) {
_dispatch_queue_priority_inherit_from_target(dq, tq);
_dispatch_lane_inherit_wlh_from_target(dq, tq);
}
_dispatch_retain(tq);
dq->do_targetq = tq;
_dispatch_object_debug(dq, "%s", __func__);
return _dispatch_trace_queue_create(dq)._dq; // 下面有进去的代码
}
dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);
_dispatch_queue_attr_to_info(dispatch_queue_attr_t dqa)
{
dispatch_queue_attr_info_t dqai = { };
if (!dqa) return dqai;
#if DISPATCH_VARIANT_STATIC
if (dqa == &_dispatch_queue_attr_concurrent) {
dqai.dqai_concurrent = true;
return dqai;
}
#endif
if (dqa < _dispatch_queue_attrs ||
dqa >= &_dispatch_queue_attrs[DISPATCH_QUEUE_ATTR_COUNT]) {
DISPATCH_CLIENT_CRASH(dqa->do_vtable, "Invalid queue attribute");
}
// 苹果自己定的算法
size_t idx = (size_t)(dqa - _dispatch_queue_attrs);
// 位域写法：某几位代表某个意思-为了节省
// 例：0000 00000 0000000 0000 0000000 0000 00 0 00
dqai.dqai_inactive = (idx % DISPATCH_QUEUE_ATTR_INACTIVE_COUNT);
idx /= DISPATCH_QUEUE_ATTR_INACTIVE_COUNT;
dqai.dqai_concurrent = !(idx % DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT);
idx /= DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT;
dqai.dqai_relpri = -(idx % DISPATCH_QUEUE_ATTR_PRIO_COUNT);
idx /= DISPATCH_QUEUE_ATTR_PRIO_COUNT;
dqai.dqai_qos = idx % DISPATCH_QUEUE_ATTR_QOS_COUNT;
idx /= DISPATCH_QUEUE_ATTR_QOS_COUNT;
dqai.dqai_autorelease_frequency =
idx % DISPATCH_QUEUE_ATTR_AUTORELEASE_FREQUENCY_COUNT;
idx /= DISPATCH_QUEUE_ATTR_AUTORELEASE_FREQUENCY_COUNT;
dqai.dqai_overcommit = idx % DISPATCH_QUEUE_ATTR_OVERCOMMIT_COUNT;
idx /= DISPATCH_QUEUE_ATTR_OVERCOMMIT_COUNT;
return dqai;
}
_dispatch_trace_queue_create(dq)._dq
_dispatch_trace_queue_create(dispatch_queue_class_t dqu)
{
_dispatch_only_if_ktrace_enabled({
uint64_t dq_label[4] = {0}; // So that we get the right null termination
dispatch_queue_t dq = dqu._dq;
strncpy((char *)dq_label, (char *)dq->dq_label ?: "", sizeof(dq_label));
_dispatch_ktrace2(DISPATCH_QOS_TRACE_queue_creation_start,
dq->dq_serialnum,
_dispatch_priority_to_pp_prefer_fallback(dq->dq_priority));
_dispatch_ktrace4(DISPATCH_QOS_TRACE_queue_creation_end,
dq_label[0], dq_label[1], dq_label[2], dq_label[3]);
});
return _dispatch_introspection_queue_create(dqu); //直接跳进去
}
_dispatch_introspection_queue_create(dispatch_queue_t dq)
{
dispatch_queue_introspection_context_t dqic;
size_t sz = sizeof(struct dispatch_queue_introspection_context_s);
if (!_dispatch_introspection.debug_queue_inversions) {
sz = offsetof(struct dispatch_queue_introspection_context_s,
__dqic_no_queue_inversion);
}
dqic = _dispatch_calloc(1, sz);
dqic->dqic_queue._dq = dq;
if (_dispatch_introspection.debug_queue_inversions) {
LIST_INIT(&dqic->dqic_order_top_head);
LIST_INIT(&dqic->dqic_order_bottom_head);
}
dq->do_finalizer = dqic;
_dispatch_unfair_lock_lock(&_dispatch_introspection.queues_lock);
LIST_INSERT_HEAD(&_dispatch_introspection.queues, dqic, dqic_list);
_dispatch_unfair_lock_unlock(&_dispatch_introspection.queues_lock);
DISPATCH_INTROSPECTION_INTERPOSABLE_HOOK_CALLOUT(queue_create, dq);
// 注意DISPATCH_INTROSPECTION_HOOK_ENABLED
if (DISPATCH_INTROSPECTION_HOOK_ENABLED(queue_create))
{
_dispatch_introspection_queue_create_hook(dq);//直接进去
}
return upcast(dq)._dqu;
}
_dispatch_introspection_queue_create_hook(dispatch_queue_t dq)
{
dispatch_introspection_queue_s diq;
diq = dispatch_introspection_queue_get_info(dq);//直接进去
dispatch_introspection_hook_callout_queue_create(&diq);
}
dispatch_introspection_queue_get_info(dispatch_queue_t dq)
{
if (dx_metatype(dq) == _DISPATCH_WORKLOOP_TYPE) {
return _dispatch_introspection_workloop_get_info(upcast(dq)._dwl); //进哪个都可以，进这个
}
return _dispatch_introspection_lane_get_info(upcast(dq)._dl);//进哪个都可以，进上面那个
}
_dispatch_introspection_workloop_get_info(dispatch_workloop_t dwl)
{
uint64_t dq_state = os_atomic_load2o(dwl, dq_state, relaxed);
// 将原来产生的队列进行了进一步的包装
dispatch_introspection_queue_s diq = {
.queue = dwl->_as_dq,
.target_queue = dwl->do_targetq, // 注意
.label = dwl->dq_label,
.serialnum = dwl->dq_serialnum,
.width = 1,
.suspend_count = 0,
.enqueued = _dq_state_is_enqueued(dq_state),
.barrier = _dq_state_is_in_barrier(dq_state),
.draining = 0,
.global = 0,
.main = 0,
};
return diq;
}
2、main和global
_dispatch_get_root_queue
_dispatch_get_root_queue(dispatch_qos_t qos, bool overcommit)// (4 , yes no)
{
if (unlikely(qos < DISPATCH_QOS_MIN || qos > DISPATCH_QOS_MAX)) {
DISPATCH_CLIENT_CRASH(qos, "Corrupted priority");
}
return &_dispatch_root_queues[2 * (qos - 1) + overcommit]; //取数组下标 6 或 7
// 下面进入_dispatch_root_queues[]--静态数组，装了很多信息：target_queue
}
struct dispatch_queue_global_s _dispatch_root_queues[] = {
#define _DISPATCH_ROOT_QUEUE_IDX(n, flags) \
((flags & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) ? \
DISPATCH_ROOT_QUEUE_IDX_##n##_QOS_OVERCOMMIT : \
DISPATCH_ROOT_QUEUE_IDX_##n##_QOS)
#define _DISPATCH_ROOT_QUEUE_ENTRY(n, flags, ...) \
[_DISPATCH_ROOT_QUEUE_IDX(n, flags)] = { \
DISPATCH_GLOBAL_OBJECT_HEADER(queue_global), \
.dq_state = DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE, \
.do_ctxt = _dispatch_root_queue_ctxt(_DISPATCH_ROOT_QUEUE_IDX(n, flags)), \
.dq_atomic_flags = DQF_WIDTH(DISPATCH_QUEUE_WIDTH_POOL), \
.dq_priority = flags | ((flags & DISPATCH_PRIORITY_FLAG_FALLBACK) ? \
_dispatch_priority_make_fallback(DISPATCH_QOS_##n) : \
_dispatch_priority_make(DISPATCH_QOS_##n, 0)), \
__VA_ARGS__ \
}
_DISPATCH_ROOT_QUEUE_ENTRY(MAINTENANCE, 0,
.dq_label = "com.apple.root.maintenance-qos",
.dq_serialnum = 4,
),
_DISPATCH_ROOT_QUEUE_ENTRY(MAINTENANCE, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.maintenance-qos.overcommit",
.dq_serialnum = 5,
),
_DISPATCH_ROOT_QUEUE_ENTRY(BACKGROUND, 0,
.dq_label = "com.apple.root.background-qos",
.dq_serialnum = 6,
),
_DISPATCH_ROOT_QUEUE_ENTRY(BACKGROUND, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.background-qos.overcommit",
.dq_serialnum = 7,
),
_DISPATCH_ROOT_QUEUE_ENTRY(UTILITY, 0,
.dq_label = "com.apple.root.utility-qos",
.dq_serialnum = 8,
),
_DISPATCH_ROOT_QUEUE_ENTRY(UTILITY, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.utility-qos.overcommit",
.dq_serialnum = 9,
),
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT, DISPATCH_PRIORITY_FLAG_FALLBACK,
.dq_label = "com.apple.root.default-qos",
.dq_serialnum = 10,
),
_DISPATCH_ROOT_QUEUE_ENTRY(DEFAULT,
DISPATCH_PRIORITY_FLAG_FALLBACK | DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.default-qos.overcommit",
.dq_serialnum = 11,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INITIATED, 0,
.dq_label = "com.apple.root.user-initiated-qos",
.dq_serialnum = 12,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INITIATED, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.user-initiated-qos.overcommit",
.dq_serialnum = 13,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INTERACTIVE, 0,
.dq_label = "com.apple.root.user-interactive-qos",
.dq_serialnum = 14,
),
_DISPATCH_ROOT_QUEUE_ENTRY(USER_INTERACTIVE, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
.dq_label = "com.apple.root.user-interactive-qos.overcommit",
.dq_serialnum = 15,
),
};
2.1 dispatch_get_global_queue --- 全局并发
dispatch_get_global_queue(long priority, unsigned long flags)
{
dispatch_assert(countof(_dispatch_root_queues) ==
DISPATCH_ROOT_QUEUE_COUNT);
if (flags & ~(unsigned long)DISPATCH_QUEUE_OVERCOMMIT) {
return DISPATCH_BAD_INPUT;
}
dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority);
#if !HAVE_PTHREAD_WORKQUEUE_QOS
if (qos == QOS_CLASS_MAINTENANCE) {
qos = DISPATCH_QOS_BACKGROUND;
} else if (qos == QOS_CLASS_USER_INTERACTIVE) {
qos = DISPATCH_QOS_USER_INITIATED;
}
#endif
if (qos == DISPATCH_QOS_UNSPECIFIED) {
return DISPATCH_BAD_INPUT;
}
return _dispatch_get_root_queue(qos, flags & DISPATCH_QUEUE_OVERCOMMIT);//上面那个方法
}
width = 0xfff
2.2 dispatch_get_main_queue --- 主队列 - 串行
一般代码：
dispatch_get_main_queue(void)
{
return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
//进入dispatch_queue_main_t
}
DISPATCH_DECL_SUBCLASS(dispatch_queue_main, dispatch_queue_serial);
#define DISPATCH_DECL_SUBCLASS(name, base) OS_OBJECT_DECL_SUBCLASS(name, base)
// name --- 标准类
// base --- 基类
源码：
#define OS_OBJECT_DECL_SUBCLASS(name, super) \
OS_OBJECT_DECL_IMPL(name, <OS_OBJECT_CLASS(super)>)
//意思是：name(标准类) 是 base(基类) 的重写
//即：dispatch_queue_main_t 是 _dispatch_main_q 的重写 --- 函数重写
struct dispatch_queue_static_s _dispatch_main_q = {
DISPATCH_GLOBAL_OBJECT_HEADER(queue_main),
#if !DISPATCH_USE_RESOLVERS
.do_targetq = _dispatch_get_default_queue(true),
#endif
.dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(1) |
DISPATCH_QUEUE_ROLE_BASE_ANON,
.dq_label = "com.apple.main-thread",
.dq_atomic_flags = DQF_THREAD_BOUND | DQF_WIDTH(1),
.dq_serialnum = 1,
};
dispatch_get_main_queue(void)
{
return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
}
#define DISPATCH_GLOBAL_OBJECT(type, object) ((OS_OBJECT_BRIDGE type)&(object))
二、同步函数与异步函数
1、同步函数 ---- dispatch_sync
dispatch_sync(dispatch_queue_t dq, dispatch_block_t work)
{
uintptr_t dc_flags = DC_FLAG_BLOCK;
if (unlikely(_dispatch_block_has_private_data(work))) {
return _dispatch_sync_block_with_privdata(dq, work, dc_flags);
}
_dispatch_sync_f(dq, work, _dispatch_Block_invoke(work), dc_flags);
}
_dispatch_sync_f_inline(dispatch_queue_t dq, void *ctxt,
dispatch_function_t func, uintptr_t dc_flags)
{
if (likely(dq->dq_width == 1)) //串行走这
{
// 同步函数和barrier函数都走这
return _dispatch_barrier_sync_f(dq, ctxt, func, dc_flags);
}
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;
// Global concurrent queues and queues bound to non-dispatch threads
// always fall into the slow case, see DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE
if (unlikely(!_dispatch_queue_try_reserve_sync_width(dl))) {
return _dispatch_sync_f_slow(dl, ctxt, func, 0, dl, dc_flags);
}
if (unlikely(dq->do_targetq->do_targetq)) {
return _dispatch_sync_recurse(dl, ctxt, func, dc_flags);
}
_dispatch_introspection_sync_begin(dl);
_dispatch_sync_invoke_and_complete(dl, ctxt, func DISPATCH_TRACE_ARG(
_dispatch_trace_item_sync_push_pop(dq, ctxt, func, dc_flags)));
}
_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(); // 从这里进入 → 标记A
//获取线程 -- 通过 mach 中的 pthread api 获取线程id
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;
// The more correct thing to do would be to merge the qos of the thread
// that just acquired the barrier lock into the queue state.
//
// However this is too expensive for the fast path, so skip doing it.
// The chosen tradeoff is that if an enqueue on a lower priority thread
// contends with this fast path, this thread may receive a useless override.
//
// Global concurrent queues and queues bound to non-dispatch threads
// always fall into the slow case, see DISPATCH_ROOT_QUEUE_STATE_INIT_VALUE
// 死锁判断
if (unlikely(!_dispatch_queue_try_acquire_barrier_sync(dl, tid))) // 从这里进入 → 标记B
{
return _dispatch_sync_f_slow(dl, ctxt, func, DC_FLAG_BARRIER, dl,
DC_FLAG_BARRIER | dc_flags);// 从这里进入 → 标记C
}
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)));
}
// 标记A
#define _dispatch_tid_self() ((dispatch_tid)_dispatch_thread_port())
// 标记A
#define _dispatch_thread_port() pthread_mach_thread_np(_dispatch_thread_self())
// 线程端口 -- 从当前线程中获取当前的线程id
// 标记B
_dispatch_queue_try_acquire_barrier_sync(dispatch_queue_class_t dq, uint32_t tid)
{
return _dispatch_queue_try_acquire_barrier_sync_and_suspend(dq._dl, tid, 0);
}
// suspend -- 挂起
// 标记B
_dispatch_queue_try_acquire_barrier_sync_and_suspend(dispatch_lane_t dq,
uint32_t tid, uint64_t suspend_count)
{
uint64_t init = DISPATCH_QUEUE_STATE_INIT_VALUE(dq->dq_width);
uint64_t value = DISPATCH_QUEUE_WIDTH_FULL_BIT | DISPATCH_QUEUE_IN_BARRIER |
_dispatch_lock_value_from_tid(tid) |
(suspend_count * DISPATCH_QUEUE_SUSPEND_INTERVAL);
uint64_t old_state, new_state;
// 从底层获取关于当前队列所依赖线程的状态信息
return os_atomic_rmw_loop2o(dq, dq_state, old_state, new_state, acquire, {
uint64_t role = old_state & DISPATCH_QUEUE_ROLE_MASK;
if (old_state != (init | role)) {
os_atomic_rmw_loop_give_up(break);
}
new_state = value | role;
});
}
// 标记C
_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);
}
pthread_priority_t pp = _dispatch_get_priority();
struct dispatch_sync_context_s dsc = {
.dc_flags = DC_FLAG_SYNC_WAITER | dc_flags,
.dc_func = _dispatch_async_and_wait_invoke,
.dc_ctxt = &dsc,
.dc_other = top_dq,
.dc_priority = pp | _PTHREAD_PRIORITY_ENFORCE_FLAG,
.dc_voucher = _voucher_get(),
.dsc_func = func,
.dsc_ctxt = ctxt,
.dsc_waiter = _dispatch_tid_self(),
};
// 将队列安排进线程
// 队列进来后通过push将任务压到相应的队列中
/*
死锁:
假设某队列：1、要执行block 2、执行block后执行任务4 3、block要执行任务3
则：1、该队列因为block所以要执行任务3，然而任务3在等任务4执行，任务4又在等任务block执行
*/
_dispatch_trace_item_push(top_dq, &dsc);
__DISPATCH_WAIT_FOR_QUEUE__(&dsc, dq); // 从这里进入
if (dsc.dsc_func == NULL) {
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);// 将任务一个个pop出来
_dispatch_sync_invoke_and_complete_recurse(top_dq, ctxt, func,top_dc_flags
DISPATCH_TRACE_ARG(&dsc)); // 调用 func(就是最外层我们自己写的block)
// *** 执行完毕后给mach(mac底层)发送信息
// ↑ 通过暴露hook函数，监听complete ↑
}
// 标记C
__DISPATCH_WAIT_FOR_QUEUE__(dispatch_sync_context_t dsc, dispatch_queue_t dq)
{
uint64_t dq_state = _dispatch_wait_prepare(dq);
if (unlikely(_dq_state_drain_locked_by(dq_state, dsc->dsc_waiter))) // 从这里进入 标记C
{
DISPATCH_CLIENT_CRASH((uintptr_t)dq_state,
"dispatch_sync called on queue "
"already owned by current thread");
}
// Blocks submitted to the main thread MUST run on the main thread, and
// dispatch_async_and_wait also executes on the remote context rather than
// the current thread.
//
// For both these cases we need to save the frame linkage for the sake of
// _dispatch_async_and_wait_invoke
_dispatch_thread_frame_save_state(&dsc->dsc_dtf);
if (_dq_state_is_suspended(dq_state) ||
_dq_state_is_base_anon(dq_state)) {
dsc->dc_data = DISPATCH_WLH_ANON;
} else if (_dq_state_is_base_wlh(dq_state)) {
dsc->dc_data = (dispatch_wlh_t)dq;
} else {
_dispatch_wait_compute_wlh(upcast(dq)._dl, dsc);
}
if (dsc->dc_data == DISPATCH_WLH_ANON) {
dsc->dsc_override_qos_floor = dsc->dsc_override_qos =
(uint8_t)_dispatch_get_basepri_override_qos_floor();
_dispatch_thread_event_init(&dsc->dsc_event);
}
dx_push(dq, dsc, _dispatch_qos_from_pp(dsc->dc_priority));
_dispatch_trace_runtime_event(sync_wait, dq, 0);
if (dsc->dc_data == DISPATCH_WLH_ANON) {
_dispatch_thread_event_wait(&dsc->dsc_event); // acquire
} else {
_dispatch_event_loop_wait_for_ownership(dsc);
}
if (dsc->dc_data == DISPATCH_WLH_ANON) {
_dispatch_thread_event_destroy(&dsc->dsc_event);
// If _dispatch_sync_waiter_wake() gave this thread an override,
// ensure that the root queue sees it.
if (dsc->dsc_override_qos > dsc->dsc_override_qos_floor) {
_dispatch_set_basepri_override_qos(dsc->dsc_override_qos);
}
}
}
// 标记C
_dispatch_lock_is_locked_by(dispatch_lock lock_value, dispatch_tid tid)
{
// equivalent to _dispatch_lock_owner(lock_value) == tid
// lock_value 和 tid 进行 异或操作(两个相同就会输出0)
// 说明 等待的和要被调度 的任务 是同一个
return ((lock_value ^ tid) & DLOCK_OWNER_MASK) == 0;
}
2、异步并发
2.1、如何创建线程
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); // 这里进入
_dispatch_continuation_async(dq, dc, qos, dc->dc_flags);// 这里进入标签A--创建线程的步骤
}
_dispatch_continuation_init(dispatch_continuation_t dc,
dispatch_queue_class_t dqu, dispatch_block_t work,
dispatch_block_flags_t flags, uintptr_t dc_flags)
{
void *ctxt = _dispatch_Block_copy(work);
dc_flags |= DC_FLAG_BLOCK | DC_FLAG_ALLOCATED;
if (unlikely(_dispatch_block_has_private_data(work))) {
dc->dc_flags = dc_flags;
dc->dc_ctxt = ctxt;
// will initialize all fields but requires dc_flags & dc_ctxt to be set
return _dispatch_continuation_init_slow(dc, dqu, flags);
}
dispatch_function_t func = _dispatch_Block_invoke(work); // 调用
if (dc_flags & DC_FLAG_CONSUME) {
func = _dispatch_call_block_and_release;
/*
注意：
_dispatch_call_block_and_release(void *block)
{
void (^b)(void) = block;
b();
Block_release(b);
}
*/
}
return _dispatch_continuation_init_f(dc, dqu, ctxt, func, flags, dc_flags);// 此处进入
}
_dispatch_continuation_init_f(dispatch_continuation_t dc,
dispatch_queue_class_t dqu, void *ctxt, dispatch_function_t f,
dispatch_block_flags_t flags, uintptr_t dc_flags)
{
pthread_priority_t pp = 0;
dc->dc_flags = dc_flags | DC_FLAG_ALLOCATED;
dc->dc_func = f; // 任务存入
dc->dc_ctxt = ctxt;
// in this context DISPATCH_BLOCK_HAS_PRIORITY means that the priority
// should not be propagated, only taken from the handler if it has one
if (!(flags & DISPATCH_BLOCK_HAS_PRIORITY)) {
pp = _dispatch_priority_propagate();
}
_dispatch_continuation_voucher_set(dc, flags);
return _dispatch_continuation_priority_set(dc, dqu, pp, flags);
}
// 标签A
_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);
// dx_push 宏 → dx_vtable → dq_push
// 并发 .dq_push = _dispatch_root_queue_push // 进入 标签A
}
// 标签A
_dispatch_root_queue_push(dispatch_queue_global_t rq, dispatch_object_t dou,
dispatch_qos_t qos)
{
#if DISPATCH_USE_KEVENT_WORKQUEUE
dispatch_deferred_items_t ddi = _dispatch_deferred_items_get();
if (unlikely(ddi && ddi->ddi_can_stash)) {
dispatch_object_t old_dou = ddi->ddi_stashed_dou;
dispatch_priority_t rq_overcommit;
rq_overcommit = rq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
if (likely(!old_dou._do || rq_overcommit)) {
dispatch_queue_global_t old_rq = ddi->ddi_stashed_rq;
dispatch_qos_t old_qos = ddi->ddi_stashed_qos;
ddi->ddi_stashed_rq = rq;
ddi->ddi_stashed_dou = dou;
ddi->ddi_stashed_qos = qos;
_dispatch_debug("deferring item %p, rq %p, qos %d",
dou._do, rq, qos);
if (rq_overcommit) {
ddi->ddi_can_stash = false;
}
if (likely(!old_dou._do)) {
return;
}
// push the previously stashed item
qos = old_qos;
rq = old_rq;
dou = old_dou;
}
}
#endif
#if HAVE_PTHREAD_WORKQUEUE_QOS
if (_dispatch_root_queue_push_needs_override(rq, qos)) {
return _dispatch_root_queue_push_override(rq, dou, qos);
}
#else
(void)qos;
#endif
_dispatch_root_queue_push_inline(rq, dou, dou, 1); // 进入 标签A
}
//标签A
_dispatch_root_queue_push_inline(dispatch_queue_global_t dq,
dispatch_object_t _head, dispatch_object_t _tail, int n)
{
struct dispatch_object_s *hd = _head._do, *tl = _tail._do;
if (unlikely(os_mpsc_push_list(os_mpsc(dq, dq_items), hd, tl, do_next))) {
return _dispatch_root_queue_poke(dq, n, 0); // 进入 标签A
}
}
//标签A
_dispatch_root_queue_poke(dispatch_queue_global_t dq, int n, int floor)
{
if (!_dispatch_queue_class_probe(dq)) {
return;
}
#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
if (likely(dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE))
#endif
{
if (unlikely(!os_atomic_cmpxchg2o(dq, dgq_pending, 0, n, relaxed))) {
_dispatch_root_queue_debug("worker thread request still pending "
"for global queue: %p", dq);
return;
}
}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
return _dispatch_root_queue_poke_slow(dq, n, floor); // 进入 标签A
}
//标签A
_dispatch_root_queue_poke_slow(dispatch_queue_global_t dq, int n, int floor)
{
int remaining = n;
int r = ENOSYS;
_dispatch_root_queues_init();
_dispatch_debug_root_queue(dq, __func__);
_dispatch_trace_runtime_event(worker_request, dq, (uint64_t)n);
#if !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (dx_type(dq) == DISPATCH_QUEUE_GLOBAL_ROOT_TYPE)
#endif
{
_dispatch_root_queue_debug("requesting new worker thread for global "
"queue: %p", dq);
r = _pthread_workqueue_addthreads(remaining,
_dispatch_priority_to_pp_prefer_fallback(dq->dq_priority));
// 此处直接往工作队列中添加线程。进不去，看不到更进一步的源码
(void)dispatch_assume_zero(r);
return;
}
#endif // !DISPATCH_USE_INTERNAL_WORKQUEUE
#if DISPATCH_USE_PTHREAD_POOL
dispatch_pthread_root_queue_context_t pqc = dq->do_ctxt;
if (likely(pqc->dpq_thread_mediator.do_vtable)) {
while (dispatch_semaphore_signal(&pqc->dpq_thread_mediator)) {
_dispatch_root_queue_debug("signaled sleeping worker for "
"global queue: %p", dq);
if (!--remaining) {
return;
}
}
}
bool overcommit = dq->dq_priority & DISPATCH_PRIORITY_FLAG_OVERCOMMIT;
if (overcommit) {
os_atomic_add2o(dq, dgq_pending, remaining, relaxed);
} else {
if (!os_atomic_cmpxchg2o(dq, dgq_pending, 0, remaining, relaxed)) {
_dispatch_root_queue_debug("worker thread request still pending for "
"global queue: %p", dq);
return;
}
}
int can_request, t_count;
// seq_cst with atomic store to tail <rdar://problem/16932833>
t_count = os_atomic_load2o(dq, dgq_thread_pool_size, ordered); // 获取缓存池的大小
do {
can_request = t_count < floor ? 0 : t_count - floor; // 可以产生的线程数
if (remaining > can_request) // 想要产生的线程数 > 能够产生的线程数
{
_dispatch_root_queue_debug("pthread pool reducing request from %d to %d",
remaining, can_request);
os_atomic_sub2o(dq, dgq_pending, remaining - can_request, relaxed);
remaining = can_request;
}
if (remaining == 0) {
_dispatch_root_queue_debug("pthread pool is full for root queue: "
"%p", dq);
return;
}
} while (!os_atomic_cmpxchgvw2o(dq, dgq_thread_pool_size, t_count,
t_count - remaining, &t_count, acquire)); // 判断缓存池的大小
pthread_attr_t *attr = &pqc->dpq_thread_attr;
pthread_t tid, *pthr = &tid;
#if DISPATCH_USE_MGR_THREAD && DISPATCH_USE_PTHREAD_ROOT_QUEUES
if (unlikely(dq == &_dispatch_mgr_root_queue)) {
pthr = _dispatch_mgr_root_queue_init();
}
#endif
// remaining > 0 切通过以上判断说明没有问题则开始开辟线程
do {
_dispatch_retain(dq); // released in _dispatch_worker_thread
while ((r = pthread_create(pthr, attr, _dispatch_worker_thread, dq))) // 创建线程
{
if (r != EAGAIN) {
(void)dispatch_assume_zero(r);
}
_dispatch_temporary_resource_shortage();
}
} while (--remaining);// 当remaining为0时说明开辟完毕
#else
(void)floor;
#endif // DISPATCH_USE_PTHREAD_POOL
}
2.2、如何执行任务
给block打断点 bt 查看堆栈
看 ： _dispatch_worker_thread2
_dispatch_worker_thread2(pthread_priority_t pp)
{
bool overcommit = pp & _PTHREAD_PRIORITY_OVERCOMMIT_FLAG;
dispatch_queue_global_t dq;
pp &= _PTHREAD_PRIORITY_OVERCOMMIT_FLAG | ~_PTHREAD_PRIORITY_FLAGS_MASK;
_dispatch_thread_setspecific(dispatch_priority_key, (void *)(uintptr_t)pp);
dq = _dispatch_get_root_queue(_dispatch_qos_from_pp(pp), overcommit);
_dispatch_introspection_thread_add();
_dispatch_trace_runtime_event(worker_unpark, dq, 0);
int pending = os_atomic_dec2o(dq, dgq_pending, relaxed);
dispatch_assert(pending >= 0);
_dispatch_root_queue_drain(dq, dq->dq_priority,
DISPATCH_INVOKE_WORKER_DRAIN | DISPATCH_INVOKE_REDIRECTING_DRAIN); //这里进入
_dispatch_voucher_debug("root queue clear", NULL);
_dispatch_reset_voucher(NULL, DISPATCH_THREAD_PARK);
_dispatch_trace_runtime_event(worker_park, NULL, 0);
}
_dispatch_root_queue_drain(dispatch_queue_global_t dq,
dispatch_priority_t pri, dispatch_invoke_flags_t flags)
{
#if DISPATCH_DEBUG
dispatch_queue_t cq;
if (unlikely(cq = _dispatch_queue_get_current())) {
DISPATCH_INTERNAL_CRASH(cq, "Premature thread recycling");
}
#endif
_dispatch_queue_set_current(dq);
_dispatch_init_basepri(pri);
_dispatch_adopt_wlh_anon();
struct dispatch_object_s *item;
bool reset = false;
dispatch_invoke_context_s dic = { };
#if DISPATCH_COCOA_COMPAT
_dispatch_last_resort_autorelease_pool_push(&dic);
#endif // DISPATCH_COCOA_COMPAT
_dispatch_queue_drain_init_narrowing_check_deadline(&dic, pri);
_dispatch_perfmon_start();
while (likely(item = _dispatch_root_queue_drain_one(dq))) {
if (reset) _dispatch_wqthread_override_reset();
_dispatch_continuation_pop_inline(item, &dic, flags, dq);// 这里进入
reset = _dispatch_reset_basepri_override();
if (unlikely(_dispatch_queue_drain_should_narrow(&dic))) {
break;
}
}
// overcommit or not. worker thread
if (pri & DISPATCH_PRIORITY_FLAG_OVERCOMMIT) {
_dispatch_perfmon_end(perfmon_thread_worker_oc);
} else {
_dispatch_perfmon_end(perfmon_thread_worker_non_oc);
}
#if DISPATCH_COCOA_COMPAT
_dispatch_last_resort_autorelease_pool_pop(&dic);
#endif // DISPATCH_COCOA_COMPAT
_dispatch_reset_wlh();
_dispatch_clear_basepri();
_dispatch_queue_set_current(NULL);
}
_dispatch_continuation_pop_inline(dispatch_object_t dou,
dispatch_invoke_context_t dic, dispatch_invoke_flags_t flags,
dispatch_queue_class_t dqu)
{
dispatch_pthread_root_queue_observer_hooks_t observer_hooks =
_dispatch_get_pthread_root_queue_observer_hooks();
if (observer_hooks) observer_hooks->queue_will_execute(dqu._dq);
flags &= _DISPATCH_INVOKE_PROPAGATE_MASK;
if (_dispatch_object_has_vtable(dou)) {
dx_invoke(dou._dq, dic, flags);
} else {
_dispatch_continuation_invoke_inline(dou, flags, dqu); // 这里进入
}
if (observer_hooks) observer_hooks->queue_did_execute(dqu._dq);
}
_dispatch_continuation_invoke_inline(dispatch_object_t dou,
dispatch_invoke_flags_t flags, dispatch_queue_class_t dqu)
{
dispatch_continuation_t dc = dou._dc, dc1;
dispatch_invoke_with_autoreleasepool(flags, {
uintptr_t dc_flags = dc->dc_flags;
// Add the item back to the cache before calling the function. This
// allows the 'hot' continuation to be used for a quick callback.
//
// The ccache version is per-thread.
// Therefore, the object has not been reused yet.
// This generates better assembly.
_dispatch_continuation_voucher_adopt(dc, dc_flags);
if (!(dc_flags & DC_FLAG_NO_INTROSPECTION)) {
_dispatch_trace_item_pop(dqu, dou);
}
if (dc_flags & DC_FLAG_CONSUME) {
dc1 = _dispatch_continuation_free_cacheonly(dc);
} else {
dc1 = NULL;
}
if (unlikely(dc_flags & DC_FLAG_GROUP_ASYNC)) //
{
_dispatch_continuation_with_group_invoke(dc);
} else {
_dispatch_client_callout(dc->dc_ctxt, dc->dc_func); // 这里进入
_dispatch_trace_item_complete(dc);
}
if (unlikely(dc1)) {
_dispatch_continuation_free_to_cache_limit(dc1);
}
});
_dispatch_perfmon_workitem_inc();
}
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
_dispatch_get_tsd_base();
void *u = _dispatch_get_unwind_tsd();
if (likely(!u)) return f(ctxt);
_dispatch_set_unwind_tsd(NULL);
f(ctxt); // 执行任务函数
_dispatch_free_unwind_tsd();
_dispatch_set_unwind_tsd(u);
}
三、单例
static dispatch_once_t onceToken;
// dispatch_once_t == typedef long
dispatch_once(&onceToken, ^{
<#code to be executed once#>
});
dispatch_once(dispatch_once_t *val, dispatch_block_t block)
{
dispatch_once_f(val, block, _dispatch_Block_invoke(block));
}
dispatch_once_f(dispatch_once_t *val, void *ctxt, dispatch_function_t func)
{
dispatch_once_gate_t l = (dispatch_once_gate_t)val;
// l 就是 onceToken
#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)) // 如果来过一次就不来了
{
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
// 满足条件
// 试图进去
if (_dispatch_once_gate_tryenter(l)) // 进入这里 标记A
{
// 单例调用
return _dispatch_once_callout(l, ctxt, func); // 进入这里 标记B
}
return _dispatch_once_wait(l);
}
//标记A
_dispatch_once_gate_tryenter(dispatch_once_gate_t l)
{
// 对象是否在OS底层存储过
// 如果没存货则进行unlock
return os_atomic_cmpxchg(&l->dgo_once, DLOCK_ONCE_UNLOCKED,
(uintptr_t)_dispatch_lock_value_for_self(), relaxed);
// DLOCK_ONCE_UNLOCKED --- 标记为没有锁住
}
// 标记B
_dispatch_once_callout(dispatch_once_gate_t l, void *ctxt,
dispatch_function_t func)
{
// block()的执行
_dispatch_client_callout(ctxt, func);
_dispatch_once_gate_broadcast(l); // 进入这里 标记B
}
// 标记B
_dispatch_once_gate_broadcast(dispatch_once_gate_t l)
{
dispatch_lock value_self = _dispatch_lock_value_for_self();// 拿到self对象
uintptr_t v;
#if DISPATCH_ONCE_USE_QUIESCENT_COUNTER
v = _dispatch_once_mark_quiescing(l);
#else
v = _dispatch_once_mark_done(l); // 因为是从执行完毕过来的，所以走这里 标记B
#endif
if (likely((dispatch_lock)v == value_self)) return;
_dispatch_gate_broadcast_slow(&l->dgo_gate, (dispatch_lock)v);
}
// 标记B
_dispatch_once_mark_done(dispatch_once_gate_t dgo)
{
return os_atomic_xchg(&dgo->dgo_once, DLOCK_ONCE_DONE, release);
// 直接给当前对象给一个DLOCK_ONCE_DONE，所以下次走dispatch_once_f的时候就直接return了
}
