GCD的源碼在我看來姜挺，一直是iOS源碼中最晦澀難懂一種，因為涉及到太多的宏定義和結構定義，并且沒有多少關于GCD源碼的文章作為參考咧党，我只能根據(jù)自己的理解來粗略進行分析，如果有哪些錯誤或者不足之處陨亡，希望以后看到本篇文章的大佬能夠留言指出傍衡，謝謝深员！
一. 從打印開始
我們先來看一段常寫代碼，并分析其屬性
- (void)testQueue{
dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL);
NSLog(@"serialQueue:%@",serialQueue);
dispatch_queue_t concurrentQueue = dispatch_queue_create("concurrentQueue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"concurrentQueue:%@",concurrentQueue);
dispatch_queue_t mainQueue = dispatch_get_main_queue();
NSLog(@"mainQueue:%@",mainQueue);
dispatch_queue_t default_globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSLog(@"default_globalQueue:%@",default_globalQueue);
dispatch_queue_t low_globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
NSLog(@"low_globalQueue:%@",low_globalQueue);
dispatch_queue_t high_globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
NSLog(@"high_globalQueue:%@",high_globalQueue);
}
打印結果
serialQueue:<OS_dispatch_queue_serial: serialQueue>
concurrentQueue:<OS_dispatch_queue_concurrent: concurrentQueue>
mainQueue:<OS_dispatch_queue_main: com.apple.main-thread>
default_globalQueue:<OS_dispatch_queue_global: com.apple.root.default-qos>
low_globalQueue:<OS_dispatch_queue_global: com.apple.root.utility-qos>
high_globalQueue:<OS_dispatch_queue_global: com.apple.root.user-initiated-qos>
我們在將斷點加在最后蛙埂，po一下具體的內(nèi)部屬性
po結果.png
我們知道倦畅，主隊列也是串行隊列，全局隊列也是并行隊列绣的，從po出的信息來看他們的共性
- 主隊列和串行隊列的
width都0x1,而并行隊列的width都大于1 - 全局隊列的target都是
[0x0]叠赐，串行隊列的target都顯示的是com.apple.root.default-qos.overcommit[0x103021f80]，并行隊列的target為com.apple.root.default-qos[0x103021f00]
據(jù)此屡江，我們可以初步得出結論：
- 串行隊列的寬度(width)為1芭概，并行隊列的寬度大于1
- 全局隊列的target為
[0x0]，其他隊列的target以com.apple.root.default-開頭 - 自己創(chuàng)建的串行隊列和并行隊列都使用自己創(chuàng)建時給的名字惩嘉，而主隊列和全局隊列罢洲，都有一個系統(tǒng)給的名字，主隊列名為
com.apple.main-thread宏怔，全局默認優(yōu)先級名為com.apple.root.default-qos奏路，全局低優(yōu)先級名為com.apple.root.utility-qos，全局高優(yōu)先級名為com.apple.root.user-initiated-qos
接下來我們從源碼開始分析
二臊诊、從隊列創(chuàng)建開始
我們從dispatch_queue_create函數(shù)作為切入開始查詢源碼
dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)
{
//調(diào)用_dispatch_lane_create_with_target
return _dispatch_lane_create_with_target(label, attr,
DISPATCH_TARGET_QUEUE_DEFAULT, true);
}
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)
{
//1. 第一步 設置dispatch_queue_attr_info_t屬性
/*
通過傳入的隊列屬性鸽粉，設置隊列信息
串行隊列 返回的是空字典
非串行隊列 表示叫
*/
dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);
/*
規(guī)范化各個參數(shù)，如(qos, overcommit, tq)
*/
dispatch_qos_t qos = dqai.dqai_qos;
#if !HAVE_PTHREAD_WORKQUEUE_QOS
//如果qos等于6抓艳，那么將值設置為5
if (qos == DISPATCH_QOS_USER_INTERACTIVE) {
dqai.dqai_qos = qos = DISPATCH_QOS_USER_INITIATED;
}
//如果qos等于1触机，那么將值設置為2
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;
if (unlikely(!tq)) {
DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");
}
}
/*
第2步，創(chuàng)建隊列
*/
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;
//DQF_MUTABLE = 0x00400000,
dispatch_queue_flags_t dqf = legacy ? DQF_MUTABLE : 0;
if (dqai.dqai_concurrent) {
//通過dqai.dqai_concurrent的這個屬性來區(qū)分并發(fā)和串行
//這個是并行 生成名為OS_dispatch_queue_concurrent_class的類
vtable = DISPATCH_VTABLE(queue_concurrent);
} else {
//這個是串行 生成名為OS_dispatch_queue_serial_class的類
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;
}
}
//開辟內(nèi)存 - 生成響應的對象 queue
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
/*
#define DISPATCH_QUEUE_WIDTH_FULL 0x1000ull 也就是4096
#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1) 0xfff 也就是4095
#define DISPATCH_QUEUE_WIDTH_MAX (DISPATCH_QUEUE_WIDTH_FULL - 2) 0xffe 也就是4094
*/
//構造方法玷或，初始化隊列屬性
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
//設置隊列標簽
dq->dq_label = label;
//設置優(yōu)先級
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;
}
上面的源碼多數(shù)地方已經(jīng)給了注釋儡首，我們來捋一捋步驟：
1. 通過傳入的參數(shù) 配置隊列屬性
dispatch_queue_attr_info_t dqai = _dispatch_queue_attr_to_info(dqa);
我們來看看dispatch_queue_attr_info_t是個什么結構
typedef uint32_t dispatch_qos_t;
typedef uint32_t dispatch_priority_t;
typedef struct dispatch_queue_attr_info_s {
//服務質(zhì)量，優(yōu)先級
dispatch_qos_t dqai_qos : 8;
int dqai_relpri : 8;
//枚舉類型偏友，有三個值蔬胯，分別為不確定、可以復用位他、不可以復用
uint16_t dqai_overcommit:2;
uint16_t dqai_autorelease_frequency:2;
//1為并行氛濒，0位串行
uint16_t dqai_concurrent:1;
uint16_t dqai_inactive:1;
} dispatch_queue_attr_info_t;
這是一個很明顯的位域結構，其他屬性我們先不管鹅髓，我們第一眼可以看到dqai_concurrent屬性舞竿，占一位，取值在0和1之間窿冯，我們可以初步推斷dqai_concurrent是用來標識隊列是串行還是并行骗奖。
接下來我們繼續(xù)查看_dispatch_queue_attr_to_info函數(shù)的實現(xiàn)
dispatch_queue_attr_info_t
_dispatch_queue_attr_to_info(dispatch_queue_attr_t dqa)
{
dispatch_queue_attr_info_t dqai = { };
//串行隊列直接返回空
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");
}
size_t idx = (size_t)(dqa - _dispatch_queue_attrs);
//設置dispatch_queue_attr_info_t的各個屬性值
//對2進行取余
dqai.dqai_inactive = (idx % DISPATCH_QUEUE_ATTR_INACTIVE_COUNT);
//除以2，下降一位，接下來就好設置dqai_concurrent的值了
idx /= DISPATCH_QUEUE_ATTR_INACTIVE_COUNT;
//對2進行取余执桌，然后再取反鄙皇，得到1
dqai.dqai_concurrent = !(idx % DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT);
idx /= DISPATCH_QUEUE_ATTR_CONCURRENCY_COUNT;
//對16取余
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;
}
這個函數(shù)里面具體操作如下：
- 如果是串行隊列，直接返回空
- 如果是并行隊列鼻吮，則進行位操作育苟，逐個設置位域內(nèi)部值
設置好隊列屬性初始值之后较鼓，我們接著來規(guī)范化各個參數(shù)
/*
規(guī)范化各個參數(shù)椎木，如(qos, overcommit, tq)
*/
dispatch_qos_t qos = dqai.dqai_qos;
#if !HAVE_PTHREAD_WORKQUEUE_QOS
//如果qos等于6，那么將值設置為5
if (qos == DISPATCH_QOS_USER_INTERACTIVE) {
dqai.dqai_qos = qos = DISPATCH_QOS_USER_INITIATED;
}
//如果qos等于1博烂，那么將值設置為2
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;
if (unlikely(!tq)) {
DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");
}
}
這個里面香椎，先設置qos的值，而系統(tǒng)定義了多個宏來表示qos的值禽篱，我們可以看到實際的qos取值在0-6之間畜伐，0表示未確定，還有一個15表示飽和的躺率，而且從這些宏定義的英語取名和我們之前的打印結果可以看出玛界，qos就是關于優(yōu)先級的值，而且還可以看出悼吱，高優(yōu)先級的qos為5慎框，低優(yōu)先級的qos為3，默認優(yōu)先級的qos為4后添，而后臺優(yōu)先級的qos為2笨枯，所以我們可以假定推斷：后臺優(yōu)先級的優(yōu)先級比低優(yōu)先級還低
typedef uint32_t dispatch_qos_t;
#define DISPATCH_QOS_UNSPECIFIED ((dispatch_qos_t)0)
#define DISPATCH_QOS_MAINTENANCE ((dispatch_qos_t)1)
#define DISPATCH_QOS_BACKGROUND ((dispatch_qos_t)2)
#define DISPATCH_QOS_UTILITY ((dispatch_qos_t)3)
#define DISPATCH_QOS_DEFAULT ((dispatch_qos_t)4)
#define DISPATCH_QOS_USER_INITIATED ((dispatch_qos_t)5)
#define DISPATCH_QOS_USER_INTERACTIVE ((dispatch_qos_t)6)
#define DISPATCH_QOS_MIN DISPATCH_QOS_MAINTENANCE
#define DISPATCH_QOS_MAX DISPATCH_QOS_USER_INTERACTIVE
#define DISPATCH_QOS_SATURATED ((dispatch_qos_t)15)
然后設置overcommit的值，從下面我們可以看到overcommit是一個枚舉
typedef enum {
//不明確
_dispatch_queue_attr_overcommit_unspecified = 0,
//可以復用
_dispatch_queue_attr_overcommit_enabled,
//不可以復用
_dispatch_queue_attr_overcommit_disabled,
} _dispatch_queue_attr_overcommit_t;
再接下來是設置tq的值遇西，由于調(diào)用這個函數(shù)我們傳入的tq為NULL馅精，所以會進入這段代碼
if (!tq) {
tq = _dispatch_get_root_queue(
qos == DISPATCH_QOS_UNSPECIFIED ? DISPATCH_QOS_DEFAULT : qos,
overcommit == _dispatch_queue_attr_overcommit_enabled)->_as_dq;
if (unlikely(!tq)) {
DISPATCH_CLIENT_CRASH(qos, "Invalid queue attribute");}
}
這段代碼調(diào)用了_dispatch_get_root_queue函數(shù)，傳入兩個參數(shù)粱檀，第一個參數(shù)傳入的值洲敢，如果qos的值不為0，則傳入qos的值茄蚯，否則傳入默認值4压彭；第二個參數(shù)傳入的0或者1，如果可復用則傳入的為1第队，否則傳入0
DISPATCH_ALWAYS_INLINE DISPATCH_CONST
static inline dispatch_queue_global_t
_dispatch_get_root_queue(dispatch_qos_t qos, bool overcommit)
{
if (unlikely(qos < DISPATCH_QOS_MIN || qos > DISPATCH_QOS_MAX)) {
DISPATCH_CLIENT_CRASH(qos, "Corrupted priority");
}
return &_dispatch_root_queues[2 * (qos - 1) + overcommit];
}
我們關注返回值&_dispatch_root_queues[2 * (qos - 1) + overcommit];哮塞，這句的意思是從一個叫_dispatch_root_queues的數(shù)組中取出值，賦值給tq凳谦。
我們來看看這個數(shù)組是什么：
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,
),
};
我們可以看出這個數(shù)組一共定義了12個成員忆畅，取得它們其中某一個，然后取地址，就是我們的tq的值家凯。而下標2 * (qos - 1) + overcommit就是根據(jù)我們傳入的值計算得來缓醋，按照默認傳入的default來看，就是2*3+0/1绊诲，也就是6或者7送粱，我們可以看到下標為6的隊列名為"com.apple.root.default-qos"，和我們在外面打印出來的默認優(yōu)先級的全局隊列一致
從這里我們大致可以猜想：
默認創(chuàng)建了12個靜態(tài)隊列掂之，包含了全局隊列抗俄，我們獲取全局隊列的時候，是直接在數(shù)組中讀取的
2. 創(chuàng)建隊列
/*
第2步世舰，創(chuàng)建隊列
*/
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;
//DQF_MUTABLE = 0x00400000,
dispatch_queue_flags_t dqf = legacy ? DQF_MUTABLE : 0;
if (dqai.dqai_concurrent) {
//通過dqai.dqai_concurrent的這個屬性來區(qū)分并發(fā)和串行
//這個是并行 生成名為OS_dispatch_queue_concurrent_class的類
vtable = DISPATCH_VTABLE(queue_concurrent);
} else {
//這個是串行 生成名為OS_dispatch_queue_serial_class的類
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;
}
}
//開辟內(nèi)存 - 生成響應的對象 queue
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
/*
#define DISPATCH_QUEUE_WIDTH_FULL 0x1000ull 也就是4096
#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1) 0xfff 也就是4095
#define DISPATCH_QUEUE_WIDTH_MAX (DISPATCH_QUEUE_WIDTH_FULL - 2) 0xffe 也就是4094
*/
//構造方法动雹，初始化隊列屬性
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
//設置隊列標簽
dq->dq_label = label;
//設置優(yōu)先級
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__);
一開始是設置并行/串行的隊列類名，使用DISPATCH_VTABLE來設置跟压，我們可以看出
- 串行隊列類名以
OS_dispatch_queue_serial開頭 - 并行隊列類名以
OS_dispatch_queue_concurrent開頭
這和我們一開始打印出來的類名一致
#define DISPATCH_VTABLE(name) DISPATCH_OBJC_CLASS(name)
#define DISPATCH_OBJC_CLASS(name) (&DISPATCH_CLASS_SYMBOL(name))
#define DISPATCH_CLASS_SYMBOL(name) OS_dispatch_##name##_class
接下來是設置隊列名label
if (label) {
const char *tmp = _dispatch_strdup_if_mutable(label);
if (tmp != label) {
dqf |= DQF_LABEL_NEEDS_FREE;
label = tmp;
}
}
接著根據(jù)設置好的類名胰蝠，分配內(nèi)存，生成queue,我們也可以看到dispatch_lane_t結構體的結構震蒋，這一步我們暫不做深究茸塞，以后研究更多了來補上
//開辟內(nèi)存 - 生成響應的對象 queue
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
#define DISPATCH_LANE_CLASS_HEADER(x) \
struct dispatch_queue_s _as_dq[0]; \
DISPATCH_QUEUE_CLASS_HEADER(x, \
struct dispatch_object_s *volatile dq_items_tail); \
dispatch_unfair_lock_s dq_sidelock; \
struct dispatch_object_s *volatile dq_items_head; \
uint32_t dq_side_suspend_cnt
typedef struct dispatch_lane_s {
DISPATCH_LANE_CLASS_HEADER(lane);
/* 32bit hole on LP64 */
} DISPATCH_ATOMIC64_ALIGN *dispatch_lane_t;
#define DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__) \
_DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__); \
/* LP64 global queue cacheline boundary */ \
unsigned long dq_serialnum; \
const char *dq_label; \
DISPATCH_UNION_LE(uint32_t volatile dq_atomic_flags, \
const uint16_t dq_width, \
const uint16_t __dq_opaque2 \
); \
dispatch_priority_t dq_priority; \
union { \
struct dispatch_queue_specific_head_s *dq_specific_head; \
struct dispatch_source_refs_s *ds_refs; \
struct dispatch_timer_source_refs_s *ds_timer_refs; \
struct dispatch_mach_recv_refs_s *dm_recv_refs; \
struct dispatch_channel_callbacks_s const *dch_callbacks; \
}; \
int volatile dq_sref_cnt
struct dispatch_queue_s {
DISPATCH_QUEUE_CLASS_HEADER(queue, void *__dq_opaque1);
/* 32bit hole on LP64 */
} DISPATCH_ATOMIC64_ALIGN;
然后我們調(diào)用構造方法，對隊列進行初始化
/*
#define DISPATCH_QUEUE_WIDTH_FULL 0x1000ull 也就是4096
#define DISPATCH_QUEUE_WIDTH_POOL (DISPATCH_QUEUE_WIDTH_FULL - 1) 0xfff 也就是4095
#define DISPATCH_QUEUE_WIDTH_MAX (DISPATCH_QUEUE_WIDTH_FULL - 2) 0xffe 也就是4094
*/
//構造方法查剖，初始化隊列屬性
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
從這段代碼我們可以看出钾虐，我們自己創(chuàng)建的串行隊列的width為1，自己創(chuàng)建的并行隊列width為0xffe,也就是(0x1000-0x2),這也和我們之前打印出來的一致
接下是設置隊列標簽(名字)梗搅，設置隊列優(yōu)先級等禾唁，然后調(diào)用_dispatch_retain(tq);將隊列的引用計數(shù)加1
_dispatch_retain(tq);
最后調(diào)用_dispatch_trace_queue_create(dq)._dq來返回創(chuàng)建的隊列
我們看到_dispatch_trace_queue_create調(diào)用了_dispatch_introspection_queue_create，然后_dispatch_introspection_queue_create又調(diào)用了_dispatch_introspection_queue_create最后return upcast(dq)._dqu;將dq強轉(zhuǎn)為根父類dispatch_object_t返回其_dqu值无切，這就是我們創(chuàng)建的queue
DISPATCH_ALWAYS_INLINE
static inline dispatch_queue_class_t
_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);
}
再看_dispatch_introspection_queue_create函數(shù)的實現(xiàn)
dispatch_queue_class_t
_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);
if (DISPATCH_INTROSPECTION_HOOK_ENABLED(queue_create)) {
_dispatch_introspection_queue_create_hook(dq);
}
//強轉(zhuǎn)為dispatch_object_t類型 然后返回
return upcast(dq)._dqu;
}
最后我們來看一看dispatch_object_t和dispatch_queue_class_t的結構
typedef struct dispatch_object_s {
private:
dispatch_object_s();
~dispatch_object_s();
dispatch_object_s(const dispatch_object_s &);
void operator=(const dispatch_object_s &);
} *dispatch_object_t;
typedef union {
struct dispatch_queue_s *_dq;
struct dispatch_workloop_s *_dwl;
struct dispatch_lane_s *_dl;
struct dispatch_queue_static_s *_dsq;
struct dispatch_queue_global_s *_dgq;
struct dispatch_queue_pthread_root_s *_dpq;
struct dispatch_source_s *_ds;
struct dispatch_channel_s *_dch;
struct dispatch_mach_s *_dm;
dispatch_lane_class_t _dlu;
#ifdef __OBJC__
id<OS_dispatch_queue> _objc_dq;
#endif
} dispatch_queue_class_t DISPATCH_TRANSPARENT_UNION;
三荡短、主隊列
我們從dispatch_get_main_queue()入手
DISPATCH_INLINE DISPATCH_ALWAYS_INLINE DISPATCH_CONST DISPATCH_NOTHROW
dispatch_queue_main_t
dispatch_get_main_queue(void)
{
return DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);
}
源碼中調(diào)用了DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q);，我們先看看DISPATCH_GLOBAL_OBJECT是什么東西
#define DISPATCH_GLOBAL_OBJECT(type, object) ((OS_OBJECT_BRIDGE type)&(object))
然后我們再看看兩個參數(shù),我們先看看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)
#define OS_OBJECT_DECL_SUBCLASS(name, super) \
OS_OBJECT_DECL_IMPL(name, <OS_OBJECT_CLASS(super)>)
#define OS_OBJECT_DECL_IMPL(name, ...) \
OS_OBJECT_DECL_PROTOCOL(name, __VA_ARGS__) \
typedef NSObject<OS_OBJECT_CLASS(name)> \
* OS_OBJC_INDEPENDENT_CLASS name##_t
從上面的源碼我們可以看出哆键，主隊列是串行隊列的子類掘托，通過對父類方法的重寫，得到了dispatch_queue_main_t
我們再看第二個參數(shù)_dispatch_main_q籍嘹，找到
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,
};
這個結構體設置了主線程的各個屬性闪盔，從結構體中我們可以看到dq_label = "com.apple.main-thread"，這表示是我們的主線程辱士，這也和我們的打印一致
四泪掀、全局隊列
我們也從dispatch_get_global_queue函數(shù)出發(fā)
dispatch_queue_global_t
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);
}
這個函數(shù)最后調(diào)用_dispatch_get_root_queue函數(shù)
DISPATCH_ALWAYS_INLINE DISPATCH_CONST
static inline dispatch_queue_global_t
_dispatch_get_root_queue(dispatch_qos_t qos, bool overcommit)
{
if (unlikely(qos < DISPATCH_QOS_MIN || qos > DISPATCH_QOS_MAX)) {
DISPATCH_CLIENT_CRASH(qos, "Corrupted priority");
}
return &_dispatch_root_queues[2 * (qos - 1) + overcommit];
}
在第二段創(chuàng)建隊列中我們可以看到，_dispatch_get_root_queue函數(shù)的返回值是從_dispatch_root_queues數(shù)組中取出颂碘，我們再次看看這個數(shù)組的定義：
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,
),
};
我們可以看到异赫，根據(jù)我們傳入的不同的優(yōu)先級，全局隊列從_dispatch_root_queues數(shù)組中取出預先寫好的隊列來返回。
而從.dq_atomic_flags = DQF_WIDTH(DISPATCH_QUEUE_WIDTH_POOL)這句中可以看到塔拳，全局隊列的寬度為0xfff鼠证，寬度大于1，說明全局隊列是一個并發(fā)隊列靠抑，由前面可知量九，我們自己創(chuàng)建的并行隊列寬度為0xffe,這就是全局隊列和自創(chuàng)的并發(fā)隊列的區(qū)別
