上一篇我们介绍了多线程的一些概念,本篇我们主要探究iOS开发中经常会使用到的多线程技术GCD
。
GCD的概念
- GCD 是苹果公司为多核的并行运算提出的解决方案
- GCD 会自动利用更多的CPU内核(比如双核、四核)
- GCD 会自动管理线程的生命周期(创建线程、调度任务、销毁线程)
- 程序员只需要告诉 GCD 想要执行什么任务,不需要编写任何线程管理代码。 总结来说GCD就是将任务添加到队列,并指定任务执行的函数。
GCD的基本使用
一般情况下我们会这样使用GCD
//创建任务block
dispatch_block_t block = ^{
NSLog(@"这是任务");
};
//创建串行队列
dispatch_queue_t queue = dispatch_queue_create("com.lg.cn", NULL);
//执行任务
dispatch_async(queue, block);
总结来看就是三部:
-
创建任务块
dispatch_block_t
-
创建队列
dispatch_queue_t
-
将任务添加到队列并执行任务函数
dispatch_async
或dispatch_sync
还有两个概念其实我们也很熟悉了就是
函数
和队列
。- 函数包括
同步函数(dispatch_sync)
和异步函数(dispatch_async)
。 - 队列包括
串行队列(DISPATCH_QUEUE_SERIAL)
和并行队列(DISPATCH_QUEUE_CONCURRENT)
- 函数包括
主队列
主队列(dispatch_queue_main_t
)是我们运行程序就会启动的一个队列,它是主线程所在的队列,会贯穿我们应用运行的始终。通过我们dispatch_get_main_queue
函数的注释我们看到主队列是一个串行队列,这也不难理解,因为串行队列里的任务会逐个顺序执行,而我们主线程上的任务也符合这一特性。
//下面这行注释说明它是一个串行队列,但不完全是一个标准的串行队列。
* Because the main queue doesn't behave entirely like a regular serial queue,
* it may have unwanted side-effects when used in processes that are not UI apps
* (daemons). For such processes, the main queue should be avoided.
*
* @see dispatch_queue_main_t
*
* @result
* Returns the main queue. This queue is created automatically on behalf of
* the main thread before main() is called.
*/
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);
}
我们下载libdispatch
的源码,看一下dispatch_get_main_queue
的源码,调用的是DISPATCH_GLOBAL_OBJECT(dispatch_queue_main_t, _dispatch_main_q)
,它是一个宏
#define DISPATCH_GLOBAL_OBJECT(type, object) (static_cast<type>(&(object)))
可以看到第一个参数type
是类型,第二个参数object
参数真正的参数也就是_dispatch_main_q
,我们全局搜索_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,
};
发现_dispatch_main_q
是一个结构体。可以看到dispatch_queue_main_t
是一个结构体dispatch_queue_static_s
。
串行队列和并发队列源码上的区分
上面我们已经知道,gcd的队列的本质是dispatch_queue_static_s
结构体,结构体中那个成员标示的是串行还是并行队列呢?我们源码中找答案。我们的队列是通过dispatch_queue_create
函数创建的,它的第二个参数传入的是队列的类型,我们源码中找dispatch_queue_create
函数的定义:
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);
}
跟着调用继续查找_dispatch_lane_create_with_target
函数:
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)
//
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;
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) {
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;
}
}
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
_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;
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;
}
这个函数比较长,按照管理我们还是先看返回值_dispatch_trace_queue_create(dq)._dq
。重点看dq
怎么创建的。所以我们主要dq
的创建及成员赋值的过程。
dispatch_lane_t dq = _dispatch_object_alloc(vtable,
sizeof(struct dispatch_lane_s));
_dispatch_queue_init(dq, dqf, dqai.dqai_concurrent ?
DISPATCH_QUEUE_WIDTH_MAX : 1, DISPATCH_QUEUE_ROLE_INNER |
(dqai.dqai_inactive ? DISPATCH_QUEUE_INACTIVE : 0));
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);
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;
}
看到第三个参数width
复制的地方是dqf |= DQF_WIDTH(width);
即:
- width = 1表示串行队列
- width = DISPATCH_QUEUE_WIDTH_MAX表示并行队列,其中
DISPATCH_QUEUE_WIDTH_MAX
的定义如下。#define DISPATCH_QUEUE_WIDTH_FULL 0x1000ull #define DISPATCH_QUEUE_WIDTH_MAX (DISPATCH_QUEUE_WIDTH_FULL - 2)
dispatch_queue_t的继承链
dispatch_queue_t
的继承链是什么样子呢,我们在代码中按cmd
+dispatch_queue_t
会跳转到DISPATCH_DECL(dispatch_queue);
代码,它是dispatch_queue_t
的定义。我们在libdispatch源码中搜索DISPATCH_DECL(
找定义的地方,根据上下文if
判断下面这行是oc
情况下的定义:
#define DISPATCH_DECL(name) \
typedef struct name##_s : public dispatch_object_s {} *name##_t
///dispatch_queue_t -> dispatch_queue_s -> dispatch_object_s
可以看到继承链为dispatch_queue_t -> dispatch_queue_s -> dispatch_object_s
我们观察一下的机构
struct dispatch_queue_s {
DISPATCH_QUEUE_CLASS_HEADER(queue, void *__dq_opaque1);
/* 32bit hole on LP64 */
} DISPATCH_ATOMIC64_ALIGN;
继续看DISPATCH_QUEUE_CLASS_HEADER
结构体
#define _DISPATCH_QUEUE_CLASS_HEADER(x, __pointer_sized_field__) \
DISPATCH_OBJECT_HEADER(x); \
__pointer_sized_field__; \
DISPATCH_UNION_LE(uint64_t volatile dq_state, \
dispatch_lock dq_state_lock, \
uint32_t dq_state_bits \
)
#endif
继承于DISPATCH_OBJECT_HEADER
继续搜索:
#define _DISPATCH_OBJECT_HEADER(x) \
struct _os_object_s _as_os_obj[0]; \
OS_OBJECT_STRUCT_HEADER(dispatch_##x); \
struct dispatch_##x##_s *volatile do_next; \
struct dispatch_queue_s *do_targetq; \
void *do_ctxt; \
union { \
dispatch_function_t DISPATCH_FUNCTION_POINTER do_finalizer; \
void *do_introspection_ctxt; \
}
最后继承的是_os_object_s
,所以完整继承链就是
dispatch_queue_t -> dispatch_queue_s -> dispatch_object_s -> _os_object_s
函数的调用时机
dispatch_sync(dispatch_get_global_queue(0, 0), ^{
NSLog(@"函数调用了");
});
我们本小节探究函数的block
参数是什么时候调用的,我们以同步函数为例,全局搜索dispatch_sync
DISPATCH_NOINLINE
void
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);
}
work
为我们传入的block
,所以我们看和work
参数相关的代码
_dispatch_Block_invoke
函数的实现:
#define _dispatch_Block_invoke(bb) \
((dispatch_function_t)((struct Block_layout *)bb)->invoke)
可以看到_dispatch_Block_invoke函数主要是调用了work
的invoke
方法。
我们再看_dispatch_sync_f
的实现:
static void
_dispatch_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func,
uintptr_t dc_flags)
{
_dispatch_sync_f_inline(dq, ctxt, func, dc_flags);
}
继续跟踪_dispatch_sync_f_inline
函数:
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_sync_f_inline(dispatch_queue_t dq, void *ctxt,
dispatch_function_t func, uintptr_t dc_flags)
{
if (likely(dq->dq_width == 1)) {
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_sync_f_inline
函数的ctxt
和func
参数是和block相关的参数,调用的地方比较多,我们在demo工程打一个符号断点看一下,到底执行了哪个方法:
我们发现实际调用的是_dispatch_sync_f_slow
函数
所以我们继续看_dispatch_sync_f_slow
的实现
DISPATCH_NOINLINE
static void
_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(),
};
_dispatch_trace_item_push(top_dq, &dsc);
__DISPATCH_WAIT_FOR_QUEUE__(&dsc, dq);
if (dsc.dsc_func == NULL) {
// dsc_func being cleared means that the block ran on another thread ie.
// case (2) as listed in _dispatch_async_and_wait_f_slow.
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);
_dispatch_sync_invoke_and_complete_recurse(top_dq, ctxt, func,top_dc_flags
DISPATCH_TRACE_ARG(&dsc));
}
关注的参数依然是ctxt
和func
,和上一步骤类似,我们继续打符号断点_dispatch_sync_invoke_and_complete_recurse
和_dispatch_sync_function_invoke
来看具体执行的代码。
实际调用了_dispatch_sync_function_invoke
函数:
static void
_dispatch_sync_function_invoke(dispatch_queue_class_t dq, void *ctxt,
dispatch_function_t func)
{
_dispatch_sync_function_invoke_inline(dq, ctxt, func);
}
ctxt
和func
的调用在_dispatch_client_callout
函数,有多个实现:
DISPATCH_ALWAYS_INLINE
static inline void
_dispatch_sync_function_invoke_inline(dispatch_queue_class_t dq, void *ctxt,
dispatch_function_t func)
{
dispatch_thread_frame_s dtf;
_dispatch_thread_frame_push(&dtf, dq);
_dispatch_client_callout(ctxt, func);
_dispatch_perfmon_workitem_inc();
_dispatch_thread_frame_pop(&dtf);
}
ctxt
和func
的调用在_dispatch_client_callout
函数,有多个实现:
DISPATCH_NOINLINE
void
_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);
_dispatch_free_unwind_tsd();
_dispatch_set_unwind_tsd(u);
}
#undef _dispatch_client_callout
void
_dispatch_client_callout(void *ctxt, dispatch_function_t f)
{
@try {
return f(ctxt);
}
@catch (...) {
objc_terminate();
}
}
虽然多处实现,但是调用block
的代码都是f(ctxt)
。
所以block
的调用链是:dispatch_sync
->_dispatch_sync_f
->_dispatch_sync_f_inline
->_dispatch_sync_f_slow
->_dispatch_sync_function_invoke
->_dispatch_client_callout
->f(ctxt)
。
同理异步函数dispatch_async
用相同的方法也能探究出一个调用链,最后调用的也是f(ctxt)
,感兴趣的童鞋可以探究一下。