iOS开发多线程GCD队列

dispatch_queue_t main = dispatch_get_main_queue(); //主队列
dispatch_queue_t global = dispatch_get_global_queue(0, 0); //全局队列
dispatch_queue_t serial = dispatch_queue_create("WT", DISPATCH_QUEUE_SERIAL); // 串行队列
dispatch_queue_t concurrent = dispatch_queue_create("WT", DISPATCH_QUEUE_CONCURRENT); //并发队列
// 主队列源码
dispatch_get_main_queue(void) { 
    return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
}
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,
};
// 全局队列源码
dispatch_get_global_queue(intptr_t priority, uintptr_t flags) {
    dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority);
    ……
    return _dispatch_get_root_queue(qos, flags & DISPATCH_QUEUE_OVERCOMMIT);
}
#define DISPATCH_QUEUE_WIDTH_FULL 0x1000ull
#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1)
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,
    ),
    ...省略部分...
    _DISPATCH_ROOT_QUEUE_ENTRY(USER_INTERACTIVE, DISPATCH_PRIORITY_FLAG_OVERCOMMIT,
        .dq_label = "com.apple.root.user-interactive-qos.overcommit",
        .dq_serialnum = 15,
    ),
};
// 串行队列源码
#define DISPATCH_QUEUE_SERIAL NULL
// 并发队列源码
#define DISPATCH_QUEUE_CONCURRENT DISPATCH_GLOBAL_OBJECT(dispatch_queue_attr_t,  _dispatch_queue_attr_concurrent)
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);
}
#if OS_OBJECT_USE_OBJC
#define DISPATCH_GLOBAL_OBJECT(type, object) ((OS_OBJECT_BRIDGE type)&(object))

#define DISPATCH_QUEUE_WIDTH_MAX  (DISPATCH_QUEUE_WIDTH_FULL - 2)
static dispatch_queue_t _dispatch_lane_create_with_target(const char *label, dispatch_queue_attr_t dqa, dispatch_queue_t tq, bool legacy) {
    // 串行队列dqai为{}
    dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);
    ...省略部分 - 根据dqai规范化参数...
    dispatch_lane_t dq = _dispatch_object_alloc(vtable, sizeof(struct dispatch_lane_s));
    // 初始化队列 并发队列 DISPATCH_QUEUE_WIDTH_MAX  串行队列 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));
    ...省略部分...
    return _dispatch_trace_queue_create(dq)._dq;
}
_dispatch_queue_attr_to_info(dispatch_queue_attr_t dqa) {
    dispatch_queue_attr_info_t dqai = { };
    // 串行队列直接返回 {}
    if (!dqa) return dqai; //
    #if DISPATCH_VARIANT_STATIC
        if (dqa == &_dispatch_queue_attr_concurrent) { // 并发队列
            dqai.dqai_concurrent = true;
            return dqai;
        }
    #endif
    ...一系列操作...
    return dqai;
}
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_resume
        if (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); // 串行队列DQF_WIDTH(1) -- 主队列  -- 串行队列
    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;
}