iOS-GCD源码解析(一)数据结构

1、dispatch_queue_t

https://opensource.apple.com/tarballs/libdispatch/

根据
#define DISPATCH_DECL(name) typedef struct name##_s *name##_t
dispatch_queue_t 就是 dispatch_queue_s

我们再看看dispatch_queue_s定义（源码：libdispatch/Project Headers/queue_internal.h）

struct dispatch_queue_s {
    DISPATCH_STRUCT_HEADER(dispatch_queue_s, dispatch_queue_vtable_s);
    DISPATCH_QUEUE_HEADER;
    char dq_label[DISPATCH_QUEUE_MIN_LABEL_SIZE]; // must be last
    char _dq_pad[DISPATCH_QUEUE_CACHELINE_PAD]; // for static queues only
};

#define DISPATCH_QUEUE_HEADER \
    uint32_t volatile dq_running; \
    uint32_t dq_width; \
    struct dispatch_object_s *volatile dq_items_tail; \
    struct dispatch_object_s *volatile dq_items_head; \
    unsigned long dq_serialnum; \
    dispatch_queue_t dq_specific_q;

#define DISPATCH_STRUCT_HEADER(x, y) \
    const struct y *do_vtable; \
    struct x *volatile do_next; \
    unsigned int do_ref_cnt; \
    unsigned int do_xref_cnt; \
    unsigned int do_suspend_cnt; \
    struct dispatch_queue_s *do_targetq; \
    void *do_ctxt; \
    void *do_finalizer;

根据上面的宏定义，可以得出最终的 struct dispatch_queue_t 定义

struct dispatch_queue_s {
    //封装了dispose、debug等方法和变量
    const struct dispatch_queue_vtable_s *do_vtable;
    struct dispatch_queue_s *volatile do_next;
    unsigned int do_ref_cnt;
    unsigned int do_xref_cnt;
    unsigned int do_suspend_cnt;
    //main queue和user queue指向global queue中的一个
    struct dispatch_queue_s *do_targetq;
    void *do_ctxt;
    void *do_finalizer;
    //1 2 0
    uint32_t volatile dq_running;
    //串行:1 并行:UINT_MAX
    uint32_t dq_width;
    struct dispatch_object_s *volatile dq_items_tail;
    struct dispatch_object_s *volatile dq_items_head;
    //queue的序列号
    unsigned long dq_serialnum;
    dispatch_queue_t dq_specific_q;
    //queue名字
    char dq_label[DISPATCH_QUEUE_MIN_LABEL_SIZE]; // must be last
    char _dq_pad[DISPATCH_QUEUE_CACHELINE_PAD]; // for static queues only
};

2、dispatch_get_main_queue()

#define dispatch_get_main_queue() (&_dispatch_main_q)

struct dispatch_queue_s _dispatch_main_q = {
#if !DISPATCH_USE_RESOLVERS
    .do_vtable = &_dispatch_queue_vtable,
    .do_targetq = &_dispatch_root_queues[
            DISPATCH_ROOT_QUEUE_IDX_DEFAULT_OVERCOMMIT_PRIORITY],
#endif
    .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
    .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
    .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
    .dq_label = "com.apple.main-thread",
    .dq_running = 1,
    .dq_width = 1,
    .dq_serialnum = 1,
};

3、dispatch_get_global_queue()

//第一个参数 priority 是优先级
//第二个参数为预留参数 flag，填0
dispatch_queue_t
dispatch_get_global_queue(long priority, unsigned long flags)
{
    if (flags & ~DISPATCH_QUEUE_OVERCOMMIT) {
        return NULL;
    }
    return _dispatch_get_root_queue(priority,
            flags & DISPATCH_QUEUE_OVERCOMMIT);
}

//所有dispatch_get_global_queue 相当于调用下面这个方法
//_dispatch_get_root_queue(priority, overcommit = false)
static inline dispatch_queue_t
_dispatch_get_root_queue(long priority, bool overcommit)
{
    if (overcommit) switch (priority) {
    case DISPATCH_QUEUE_PRIORITY_LOW:
        return &_dispatch_root_queues[
                DISPATCH_ROOT_QUEUE_IDX_LOW_OVERCOMMIT_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_DEFAULT:
        return &_dispatch_root_queues[
                DISPATCH_ROOT_QUEUE_IDX_DEFAULT_OVERCOMMIT_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_HIGH:
        return &_dispatch_root_queues[
                DISPATCH_ROOT_QUEUE_IDX_HIGH_OVERCOMMIT_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_BACKGROUND:
        return &_dispatch_root_queues[
                DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_OVERCOMMIT_PRIORITY];
    }
    switch (priority) {
    case DISPATCH_QUEUE_PRIORITY_LOW:
        return &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_IDX_LOW_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_DEFAULT:
        return &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_IDX_DEFAULT_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_HIGH:
        return &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_IDX_HIGH_PRIORITY];
    case DISPATCH_QUEUE_PRIORITY_BACKGROUND:
        return &_dispatch_root_queues[
                DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_PRIORITY];
    default:
        return NULL;
    }
}
//所有的globle queue
struct dispatch_queue_s _dispatch_root_queues[] = {
    [DISPATCH_ROOT_QUEUE_IDX_LOW_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_LOW_PRIORITY],

        .dq_label = "com.apple.root.low-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 4,
    },
    [DISPATCH_ROOT_QUEUE_IDX_LOW_OVERCOMMIT_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_LOW_OVERCOMMIT_PRIORITY],

        .dq_label = "com.apple.root.low-overcommit-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 5,
    },
    [DISPATCH_ROOT_QUEUE_IDX_DEFAULT_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_DEFAULT_PRIORITY],

        .dq_label = "com.apple.root.default-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 6,
    },
    [DISPATCH_ROOT_QUEUE_IDX_DEFAULT_OVERCOMMIT_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_DEFAULT_OVERCOMMIT_PRIORITY],

        .dq_label = "com.apple.root.default-overcommit-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 7,
    },
    [DISPATCH_ROOT_QUEUE_IDX_HIGH_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_HIGH_PRIORITY],

        .dq_label = "com.apple.root.high-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 8,
    },
    [DISPATCH_ROOT_QUEUE_IDX_HIGH_OVERCOMMIT_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_HIGH_OVERCOMMIT_PRIORITY],

        .dq_label = "com.apple.root.high-overcommit-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 9,
    },
    [DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_PRIORITY],

        .dq_label = "com.apple.root.background-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 10,
    },
    [DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_OVERCOMMIT_PRIORITY] = {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[
                DISPATCH_ROOT_QUEUE_IDX_BACKGROUND_OVERCOMMIT_PRIORITY],

        .dq_label = "com.apple.root.background-overcommit-priority",
        .dq_running = 2,
        .dq_width = UINT32_MAX,
        .dq_serialnum = 11,
    },
};

4、dipatch_queue_create()

创建一个用户queue
dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
    dispatch_queue_t dq;
    size_t label_len;

    if (!label) {
        label = "";
    }

    label_len = strlen(label);
    if (label_len < (DISPATCH_QUEUE_MIN_LABEL_SIZE - 1)) {
        label_len = (DISPATCH_QUEUE_MIN_LABEL_SIZE - 1);
    }

    // XXX switch to malloc()
    dq = calloc(1ul, sizeof(struct dispatch_queue_s) -
            DISPATCH_QUEUE_MIN_LABEL_SIZE - DISPATCH_QUEUE_CACHELINE_PAD +
            label_len + 1);
    if (slowpath(!dq)) {
        return dq;
    }

    _dispatch_queue_init(dq);
    strcpy(dq->dq_label, label);

    if (fastpath(!attr)) {
        return dq;
    }
    if (fastpath(attr == DISPATCH_QUEUE_CONCURRENT)) {
        dq->dq_width = UINT32_MAX;
        dq->do_targetq = _dispatch_get_root_queue(0, false);
    } else {
        dispatch_debug_assert(!attr, "Invalid attribute");
    }
    return dq;
}
//初始化用户queue
static inline void
_dispatch_queue_init(dispatch_queue_t dq)
{
    dq->do_vtable = &_dispatch_queue_vtable;
    dq->do_next = DISPATCH_OBJECT_LISTLESS;
    dq->do_ref_cnt = 1;
    dq->do_xref_cnt = 1;
    // Default target queue is overcommit!
    dq->do_targetq = _dispatch_get_root_queue(0, true);
    dq->dq_running = 0;
    dq->dq_width = 1;
    dq->dq_serialnum = dispatch_atomic_inc(&_dispatch_queue_serial_numbers) - 1;
}

5、我们先整理出dispatch_get_main_queue、dispatch_get_global_queue、dispatch_queue_create中变量的区别，以便后面分析方法

dispatch_queue_t	main queue	global queue	create queue
do_vtable	&_dispatch_queue_vtable	&_dispatch_queue_root_vtable	&_dispatch_queue_vtable
do_next	/	/	DISPATCH_OBJECT_LISTLESS=0x89abcdef
do_ref_cnt	~0u	~0u	1
do_xref_cnt	~0u	~0u	1
do_suspend_cnt	1	1	/
do_targetq	DISPATCH_ROOT_QUEUE _IDX_DEFAULT_OVERCOMMIT_PRIORITY	/	串行:DISPATCH_ROOT_QUEUE_IDX_DEFAULT_OVERCOMMIT_PRIORITY 并行:DISPATCH_QUEUE_PRIORITY_DEFAULT
do_ctxt	/	self	/
do_finalizer	/	/	/
dq_running	1	2	0
dq_width	1	UINT32_MAX	串行:1 并行:UINT32_MAX
dq_items_tail	/	/	/
dq_items_head	/	/	/
dq_serialnum	1	4~11	当前最大序列号+1
dq_specific_q	/	/	/
dq_label	com.apple.main-thread	队列名	自定义队列名
_dq_pad	/	/	/