前言
本文是上一篇：搞懂Objective-C中的ARC的延伸和補(bǔ)充
- 上一篇討論了下面幾個(gè)問題
1藻烤、iOS操作系統(tǒng)內(nèi)存分區(qū)
2、Objective-C中的指針
3、什么是ARC，引用計(jì)數(shù)存儲(chǔ)在哪里，哪些對(duì)象是通過引用計(jì)數(shù)來管理內(nèi)存的
4瑞佩、ARC下哪些對(duì)象是autorelease對(duì)象
- 本文將討論下面幾個(gè)問題
1、autorelease對(duì)象的釋放時(shí)機(jī)
2、為什么需要手動(dòng)添加autoreleasePool
3潭千、autoreleasePool的源碼實(shí)現(xiàn)
4、autoreleasePool為什么用雙向鏈表來實(shí)現(xiàn)呢
從線程池說起
操作系統(tǒng)實(shí)現(xiàn)了一個(gè)線程工廠稱為線程池借尿，還有一個(gè)配合工作的管理器稱為調(diào)度中心刨晴，iOS系統(tǒng)并沒有接口去直接操作線程池，線程的創(chuàng)建路翻、調(diào)度狈癞、銷毀全權(quán)交給線程池，當(dāng)我們?cè)噲D去創(chuàng)建一個(gè)線程的時(shí)候茂契，調(diào)度中心會(huì)檢查線程池的運(yùn)行狀態(tài)蝶桶、運(yùn)行線程數(shù)、運(yùn)行策略掉冶，決定接下來的執(zhí)行流程真竖，是直接申請(qǐng)線程執(zhí)行還是放入緩沖隊(duì)列中，待到執(zhí)行的時(shí)候厌小，線程池決定是開辟新的線程還是復(fù)用現(xiàn)有空閑線程恢共，長時(shí)間空閑的線程會(huì)被回收（一般10s左右）
iOS線程對(duì)應(yīng)的數(shù)據(jù)結(jié)構(gòu)
iOS操作系統(tǒng)為線程運(yùn)行配備了一套數(shù)據(jù)結(jié)構(gòu)，一個(gè)棧召锈、一個(gè)autoreleasePool旁振、一個(gè)runloop（懶加載）以及一些用于控制狀態(tài)的標(biāo)志位變量
- 棧：存放函數(shù)的參數(shù)值與局部變量的值等
- autoreleasePool：存放當(dāng)前線程產(chǎn)生的autorelease對(duì)象
- runloop：一個(gè)while循環(huán)，用于線程可持續(xù)運(yùn)行的一個(gè)循環(huán)，蘋果將它封裝成一個(gè)類拐袜，內(nèi)部注冊(cè)了一些觀察者吉嚣，實(shí)現(xiàn)了一套狀態(tài)機(jī)，動(dòng)可執(zhí)行蹬铺，靜可休眠
好的~我們回到問題本身
autorelease對(duì)象的釋放時(shí)機(jī)
autorelease對(duì)象何時(shí)釋放尝哆，主線程與非主線程的釋放時(shí)機(jī)有何不同？為什么需要手動(dòng)添加autoreleasePool甜攀？
這個(gè)問題是不是經(jīng)常被問到秋泄，有真正理解過嘛~
先說結(jié)論：
加入到autoreleasePool中的對(duì)象，會(huì)在線程銷毀前规阀，傾倒一次autoreleasePool恒序，取出每個(gè)對(duì)象，發(fā)送一次release消息谁撼，對(duì)象的引用計(jì)數(shù)-1歧胁，此時(shí)對(duì)象是否釋放取決與引用計(jì)數(shù)是否為0
主線程的運(yùn)行循環(huán)在app裝載、鏈接以后就會(huì)在main函數(shù)中啟動(dòng)厉碟，這是常規(guī)設(shè)計(jì)喊巍，所以主線程不會(huì)被銷毀直到app進(jìn)程退出之前，這就導(dǎo)致上面的策略無法釋放autorelease對(duì)象箍鼓，于是蘋果在runloop中加入這部分對(duì)象的釋放時(shí)機(jī)崭参，每次循環(huán)同步執(zhí)行完，在進(jìn)入休眠之前款咖，會(huì)傾倒一次autoreleasePool
通常我們執(zhí)行完現(xiàn)有任務(wù)就無需再次使用這個(gè)線程何暮，所以非主線程的runloop無需啟動(dòng)（特殊場(chǎng)景的需求除外），這就導(dǎo)致一個(gè)問題铐殃，非主線程的autorelease對(duì)象直到線程退出之前才會(huì)得到引用計(jì)數(shù)-1郭卫，當(dāng)這個(gè)線程有長任務(wù)的時(shí)候，內(nèi)存會(huì)一直漲背稼，這與峰值無關(guān)贰军，會(huì)一直漲到OOM
基于以上問題，蘋果公開了autoreleasePool的api蟹肘，在非主線程的場(chǎng)景下词疼，手動(dòng)添加@autoreleasepool{}，在autoreleasepool{}作用域結(jié)束前會(huì)傾倒一次
所謂嵌套autoreleasepool帘腹，不過是一個(gè)線程對(duì)應(yīng)了多個(gè)autoreleasepool贰盗，默認(rèn)有一個(gè)，其他的是手動(dòng)添加的阳欲，根本不存在嵌套一說舵盈，都只負(fù)責(zé)各自作用域產(chǎn)生的autorelease對(duì)象
主線程同樣有需要手動(dòng)添加autoreleasepool的場(chǎng)景陋率，可以降低內(nèi)存峰值，iOS屏幕的刷新幀率是60秽晚，即一秒刷新60次瓦糟，一次約16.7毫秒，如果在這個(gè)時(shí)間內(nèi)赴蝇，一次運(yùn)行循環(huán)還沒有執(zhí)行完的話就會(huì)出現(xiàn)卡頓現(xiàn)象菩浙，因?yàn)橄到y(tǒng)的vsync信號(hào)過來的時(shí)候，app沒有準(zhǔn)備好下一幀圖像句伶，所以看上去就是卡頓了劲蜻，根本原因是因?yàn)橹骶€程阻塞導(dǎo)致16.7毫秒內(nèi)沒有把圖像提交到緩沖區(qū)（說遠(yuǎn)了），基于以上OOM通常不會(huì)出現(xiàn)在主線程考余，因?yàn)橐呀?jīng)卡爆了先嬉，調(diào)試過程中就會(huì)發(fā)現(xiàn)，所以這里用于降低內(nèi)存峰值
再看個(gè)demo楚堤，這里只討論非主線程：
- (void)testRelease {
dispatch_queue_t queue = dispatch_queue_create("com.test.autorelease.queue", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
while (1) {
NSString *heapAreastring = [[NSString alloc] initWithFormat:@"堆區(qū)string-release"];
NSLog(@"當(dāng)前線程：%@, 值：%@", [NSThread currentThread], heapAreastring);
}
});
}
如上創(chuàng)建了一個(gè)串行隊(duì)列坝初，寫了個(gè)死循環(huán)，一直在創(chuàng)建堆區(qū)字符串钾军，但是內(nèi)存不會(huì)漲一絲一毫，原因上一篇文章搞懂Objective-C中的ARC講過绢要，這個(gè)對(duì)象直接會(huì)被release吏恭，不會(huì)加入autoreleasePool
修改一下：
- (void)testAutorelease {
dispatch_queue_t queue = dispatch_queue_create("com.test.autorelease.queue", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
while (1) {
NSString *stringAutorelease = [NSString stringWithFormat:@"堆區(qū)string-autorelease"];
NSLog(@"當(dāng)前線程：%@, 值：%@", [NSThread currentThread], heapAreastring);
}
});
}
會(huì)發(fā)現(xiàn)非主線程創(chuàng)建autorelease對(duì)象會(huì)導(dǎo)致內(nèi)存無休止的在漲，直到OOM
再次修改：
- (void)testAutoreleaseWithAutoreleasepool {
dispatch_queue_t queue = dispatch_queue_create("com.test.autorelease.queue", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
while (1) {
@autoreleasepool {
NSString *stringAutorelease = [NSString stringWithFormat:@"堆區(qū)string-autorelease"];
NSLog(@"當(dāng)前線程：%@, 值：%@", [NSThread currentThread], stringAutorelease);
}
}
});
}
結(jié)果是內(nèi)存也不會(huì)漲一絲一毫重罪，數(shù)據(jù)剛好能對(duì)應(yīng)上面的結(jié)論樱哼，對(duì)于有知其所以然需求的可以往下看
autoreleasePool的源碼實(shí)現(xiàn)
前輩這篇講得很好了，本文會(huì)基于最新源碼做個(gè)淺析（可能不是最新源碼剿配，作者開始看源碼時(shí)候的版本objc4-781.2）搅幅，順便講述下如何查閱源碼，源碼鏈接
在源碼中搜索
會(huì)發(fā)現(xiàn)AutoreleasePoolPage是一個(gè)c++的類呼胚，繼承AutoreleasePoolPageData茄唐，具體實(shí)現(xiàn)很長，這里省去部分函數(shù)實(shí)現(xiàn)
struct AutoreleasePoolPageData
{
magic_t const magic;
__unsafe_unretained id *next;
pthread_t const thread;
AutoreleasePoolPage * const parent;
AutoreleasePoolPage *child;
uint32_t const depth;
uint32_t hiwat;
AutoreleasePoolPageData(__unsafe_unretained id* _next, pthread_t _thread, AutoreleasePoolPage* _parent, uint32_t _depth, uint32_t _hiwat)
: magic(), next(_next), thread(_thread),
parent(_parent), child(nil),
depth(_depth), hiwat(_hiwat)
{
}
};
class AutoreleasePoolPage : private AutoreleasePoolPageData
{
id *add(id obj)
{
ASSERT(!full());
unprotect();
id *ret = next; // faster than `return next-1` because of aliasing
*next++ = obj;
protect();
return ret;
}
void releaseAll()
{
releaseUntil(begin());
}
void releaseUntil(id *stop)
{
// Not recursive: we don't want to blow out the stack
// if a thread accumulates a stupendous amount of garbage
while (this->next != stop) {
// Restart from hotPage() every time, in case -release
// autoreleased more objects
AutoreleasePoolPage *page = hotPage();
// fixme I think this `while` can be `if`, but I can't prove it
while (page->empty()) {
page = page->parent;
setHotPage(page);
}
page->unprotect();
id obj = *--page->next;
memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
page->protect();
if (obj != POOL_BOUNDARY) {
objc_release(obj);
}
}
setHotPage(this);
#if DEBUG
// we expect any children to be completely empty
for (AutoreleasePoolPage *page = child; page; page = page->child) {
ASSERT(page->empty());
}
#endif
}
void kill()
{
// Not recursive: we don't want to blow out the stack
// if a thread accumulates a stupendous amount of garbage
AutoreleasePoolPage *page = this;
while (page->child) page = page->child;
AutoreleasePoolPage *deathptr;
do {
deathptr = page;
page = page->parent;
if (page) {
page->unprotect();
page->child = nil;
page->protect();
}
delete deathptr;
} while (deathptr != this);
}
static void tls_dealloc(void *p)
{
if (p == (void*)EMPTY_POOL_PLACEHOLDER) {
// No objects or pool pages to clean up here.
return;
}
// reinstate TLS value while we work
setHotPage((AutoreleasePoolPage *)p);
if (AutoreleasePoolPage *page = coldPage()) {
if (!page->empty()) objc_autoreleasePoolPop(page->begin()); // pop all of the pools
if (slowpath(DebugMissingPools || DebugPoolAllocation)) {
// pop() killed the pages already
} else {
page->kill(); // free all of the pages
}
}
// clear TLS value so TLS destruction doesn't loop
setHotPage(nil);
}
static inline void
pop(void *token)
{
AutoreleasePoolPage *page;
id *stop;
if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
// Popping the top-level placeholder pool.
page = hotPage();
if (!page) {
// Pool was never used. Clear the placeholder.
return setHotPage(nil);
}
// Pool was used. Pop its contents normally.
// Pool pages remain allocated for re-use as usual.
page = coldPage();
token = page->begin();
} else {
page = pageForPointer(token);
}
stop = (id *)token;
if (*stop != POOL_BOUNDARY) {
if (stop == page->begin() && !page->parent) {
// Start of coldest page may correctly not be POOL_BOUNDARY:
// 1. top-level pool is popped, leaving the cold page in place
// 2. an object is autoreleased with no pool
} else {
// Error. For bincompat purposes this is not
// fatal in executables built with old SDKs.
return badPop(token);
}
}
if (slowpath(PrintPoolHiwat || DebugPoolAllocation || DebugMissingPools)) {
return popPageDebug(token, page, stop);
}
return popPage<false>(token, page, stop);
}
template<bool allowDebug>
static void
popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
if (allowDebug && PrintPoolHiwat) printHiwat();
page->releaseUntil(stop);
// memory: delete empty children
if (allowDebug && DebugPoolAllocation && page->empty()) {
// special case: delete everything during page-per-pool debugging
AutoreleasePoolPage *parent = page->parent;
page->kill();
setHotPage(parent);
} else if (allowDebug && DebugMissingPools && page->empty() && !page->parent) {
// special case: delete everything for pop(top)
// when debugging missing autorelease pools
page->kill();
setHotPage(nil);
} else if (page->child) {
// hysteresis: keep one empty child if page is more than half full
if (page->lessThanHalfFull()) {
page->child->kill();
}
else if (page->child->child) {
page->child->child->kill();
}
}
}
}
還是很長是不是蝇更，但是對(duì)于理解原理都重要沪编，看看蘋果自己的注釋
/***********************************************************************
Autorelease pool implementation
A thread's autorelease pool is a stack of pointers.
Each pointer is either an object to release, or POOL_BOUNDARY which is
an autorelease pool boundary.
A pool token is a pointer to the POOL_BOUNDARY for that pool. When
the pool is popped, every object hotter than the sentinel is released.
The stack is divided into a doubly-linked list of pages. Pages are added
and deleted as necessary.
Thread-local storage points to the hot page, where newly autoreleased
objects are stored.
**********************************************************************/
/***********************************************************************
Autorelease pool 的實(shí)現(xiàn)
線程的Autorelease pool是一系列聲明在棧上的指針。
每個(gè)指針要么是一個(gè)需要釋放的對(duì)象年扩，要么是 POOL_BOUNDARY蚁廓，它是
Autorelease pool邊界。
token是指向該池的 POOL_BOUNDARY 的指針厨幻。 什么時(shí)候
池被彈出相嵌，每個(gè)比哨兵更熱的對(duì)象都被釋放腿时。
棧被分成一個(gè)雙向鏈接的AutoreleasePoolPage列表。 Pages根據(jù)需要饭宾，添加
和刪除批糟。
線程本地存儲(chǔ)指向最新被存儲(chǔ)到自動(dòng)釋池所在的熱點(diǎn)頁面
。
**********************************************************************/
看完這個(gè)注釋捏雌，我覺得都不用往下講了??跃赚，一個(gè)字，清晰透徹
autorelease對(duì)象加入到自動(dòng)釋放池的流程
id *add(id obj)
{
ASSERT(!full());
unprotect();
id *ret = next; // faster than `return next-1` because of aliasing
*next++ = obj;
protect();
return ret;
}
next指針作為游標(biāo)指向棧頂最新add進(jìn)來的autorelease對(duì)象的下一個(gè)位置性湿，當(dāng)有新的obj加入autoreleasePool的時(shí)候取到next指針的地址存儲(chǔ)obj纬傲，next指針向棧頂方向移動(dòng)8個(gè)字節(jié)
runloop沒有啟動(dòng)的線程 autorelease對(duì)象的釋放流程
每個(gè)autoreleasePool初始化的時(shí)候都會(huì)綁定當(dāng)前線程，如下是AutoreleasePoolPage的初始化函數(shù)
static void init()
{
int r __unused = pthread_key_init_np(AutoreleasePoolPage::key,
AutoreleasePoolPage::tls_dealloc);
ASSERT(r == 0);
}
第一個(gè)參數(shù)是AutoreleasePoolPage的標(biāo)識(shí)肤频，第二個(gè)參數(shù)是一個(gè)函數(shù)指針叹括，線程退出之前會(huì)執(zhí)行這個(gè)函數(shù)，就是tls_dealloc宵荒，調(diào)用棧如下：
1汁雷、tls_dealloc中會(huì)取AutoreleasePoolPage *page = coldPage()
2、page調(diào)用objc_autoreleasePoolPop(page->begin())函數(shù)
3报咳、調(diào)用棧如下objc_autoreleasePoolPop->pop->popPage->releaseUntil
4侠讯、releaseUntil中取hotPage()，然后通過一個(gè)循環(huán)暑刃，取next游標(biāo)指針id obj = *--page->next厢漩，調(diào)用objc_release(obj)，然后游標(biāo)指針向棧底移動(dòng)8個(gè)字節(jié)
5岩臣、所有autorelease對(duì)象清理干凈溜嗜，最后調(diào)用kill()函數(shù)遍歷釋放所有autoreleasePage
runloop啟動(dòng)的線程 autorelease對(duì)象的釋放流程
線程休眠之前，runloop會(huì)調(diào)用objc_autoreleasePoolPop架谎，上面流程除了第一步炸宵，后面一樣
autoreleasePool為什么用雙向鏈表來實(shí)現(xiàn)呢
單向的可以嗎？可以呀谷扣，但是查詢速度會(huì)以指數(shù)級(jí)別降低土全，刪除棧頂page后想刪除的倒數(shù)第二個(gè)page，怎么辦会涎，從頭開始遍歷唄涯曲，遍歷n-1次，然后遍歷n-2次.......
所以為什么設(shè)計(jì)成雙向鏈表在塔，效率高幻件，如此而已
