dispatch_queue_create创建源码分析

1、创建队列的时候采用 dispatch_queue_create、底层内部源码为

2、调用源码

dispatch_queue_t dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{return _dispatch_lane_create_with_target(label, attr, DISPATCH_TARGET_QUEUE_DEFAULT, true);
}

3、_dispatch_lane_create_with_target实现过程为

1. 优先级判断
2. 异常处理
3. 串并行队列判断、串为_dispatch_queue_attr_overcommit_enabled、并发为_dispatch_queue_attr_overcommit_disabled
4. 初始化queue
5. 开辟内存-生成响应的对象queue
6. 构造方法
7. 标签赋值
8. 优先级处理

DISPATCH_NOINLINE
static dispatch_queue_t
_dispatch_lane_create_with_target(const char *label, dispatch_queue_attr_t dqa,dispatch_queue_t tq, bool legacy)
{dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);//// Step 1: Normalize arguments (qos, overcommit, tq)////优先级判断dispatch_qos_t qos = dqai.dqai_qos;
#if !HAVE_PTHREAD_WORKQUEUE_QOSif (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//异常处理_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 winif (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 disallowedif (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! 并发disabled 串行enabledovercommit = 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,//DISPATCH_QOS_DEFAULT = 4overcommit == _dispatch_queue_attr_overcommit_enabled)->_as_dq;//串1 并0if (unlikely(!tq)) {DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");}}//// Step 2: Initialize the queue// 初始化 queueif (legacy) {// if any of these attributes is specified, use non legacy classesif (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) {vtable = DISPATCH_VTABLE(queue_concurrent);} 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) {const char *tmp = _dispatch_strdup_if_mutable(label);if (tmp != label) {dqf |= DQF_LABEL_NEEDS_FREE;label = tmp;}}//开辟内存-生成响应的对象 queuedispatch_lane_t dq = _dispatch_object_alloc(vtable,sizeof(struct dispatch_lane_s));//构造方法_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));//标签dq->dq_label = label;//优先级dq->dq_priority = _dispatch_priority_make((dispatch_qos_t)dqai.dqai_qos,dqai.dqai_relpri);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;
}

4、其中 _dispatch_queue_attr_to_info(dqa);调用时dispatch_queue_attr_info_t 结构体为位域、那么该函数的作用为赋值。

//位域 赋值
typedef struct dispatch_queue_attr_info_s {dispatch_qos_t dqai_qos : 8;int      dqai_relpri : 8;uint16_t dqai_overcommit:2;uint16_t dqai_autorelease_frequency:2;uint16_t dqai_concurrent:1;uint16_t dqai_inactive:1;
} dispatch_queue_attr_info_t;

赋值操作

dispatch_queue_attr_info_t
_dispatch_queue_attr_to_info(dispatch_queue_attr_t dqa)
{dispatch_queue_attr_info_t dqai = { };//当前空的结构体指针、则直接返回if (!dqa) return dqai;
#if DISPATCH_VARIANT_STATICif (dqa == &_dispatch_queue_attr_concurrent) {dqai.dqai_concurrent = true;return dqai;}
#endifif (dqa < _dispatch_queue_attrs ||dqa >= &_dispatch_queue_attrs[DISPATCH_QUEUE_ATTR_COUNT]) {DISPATCH_CLIENT_CRASH(dqa->do_vtable, "Invalid queue attribute");}size_t idx = (size_t)(dqa - _dispatch_queue_attrs);dqai.dqai_inactive = (idx % DISPATCH_QUEUE_ATTR_INACTIVE_COUNT);idx /= DISPATCH_QUEUE_ATTR_INACTIVE_COUNT;//并发队列dqai.dqai_concurrent = !(idx % DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT);idx /= DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT;dqai.dqai_relpri = -(int)(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;
}

5、_dispatch_queue_init方法

// Note to later developers: ensure that any initialization changes are
// made for statically allocated queues (i.e. _dispatch_main_q).
static inline dispatch_queue_class_t
_dispatch_queue_init(dispatch_queue_class_t dqu, dispatch_queue_flags_t dqf,uint16_t width, uint64_t initial_state_bits)
{uint64_t dq_state = DISPATCH_QUEUE_STATE_INIT_VALUE(width);dispatch_queue_t dq = dqu._dq;dispatch_assert((initial_state_bits & ~(DISPATCH_QUEUE_ROLE_MASK |DISPATCH_QUEUE_INACTIVE)) == 0);if (initial_state_bits & DISPATCH_QUEUE_INACTIVE) {dq->do_ref_cnt += 2; // rdar://8181908 see _dispatch_lane_resumeif (dx_metatype(dq) == _DISPATCH_SOURCE_TYPE) {dq->do_ref_cnt++; // released when DSF_DELETED is set}}dq_state |= initial_state_bits;dq->do_next = DISPATCH_OBJECT_LISTLESS;dqf |= DQF_WIDTH(width);os_atomic_store2o(dq, dq_atomic_flags, dqf, relaxed);dq->dq_state = dq_state;dq->dq_serialnum =os_atomic_inc_orig(&_dispatch_queue_serial_numbers, relaxed);return dqu;
}

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