iOS 底層原理之—dyld 與 objc 的關聯

前言

 

在dyld加載過程當中，咱們知道會調用_objc_init方法，那麼在_objc_init方法中究竟作了什麼呢？咱們來探究下。

 

_objc_init方法

 

_objc_init方法實現

 

 

 

 

void _objc_init(void)

{

static bool initialized = false;

if (initialized) return;

initialized = true;

 

// fixme defer initialization until an objc-using image is found?

environ_init();

tls_init();

static_init();

runtime_init();

exception_init();

cache_init();

_imp_implementationWithBlock_init();

 

_dyld_objc_notify_register(&map_images, load_images, unmap_image);

 

#if __OBJC2__

didCallDyldNotifyRegister = true;

#endif

}

 

 

 

 

從_objc_init實現中咱們分析下該方法主要作了什麼

 

environ_init()

 

該方法主要是讀取運行時的環境變量，咱們能夠經過設置DYLD_PRINT_STATISTICS = YES來打印APP啓動到main()函數以前的時長，進而能夠進行APP啓動優化。具體的environ_init()簡介可參考博客iOS-底層原理 16：dyld與objc的關聯中有關nviron_init()部分的介紹

 

tls_init()

 

主要用於關於線程key的綁定，好比每線程數據的析構函數。

 

 

 

 

void tls_init(void)

{

#if SUPPORT_DIRECT_THREAD_KEYS

pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);

#else

_objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);

#endif

}

 

 

 

 

static_init()

 

主要是C++靜態構造函數

 

 

 

 

static void static_init()

{

size_t count;

auto inits = getLibobjcInitializers(&_mh_dylib_header, &count);

for (size_t i = 0; i < count; i++) {

inits[i]();

}

}

 

 

 

 

runtime_init()

 

主要是運行時的初始化，主要分爲兩部分：分類初始化和類的表初始化

 

 

 

 

void runtime_init(void)

{

objc::unattachedCategories.init(32);

objc::allocatedClasses.init();

}

複製代碼

 

 

 

 

exception_init()

 

初始化libobjc異常處理

 

 

 

 

/***********************************************************************

* exception_init

* Initialize libobjc's exception handling system.

* Called by map_images().

**********************************************************************/

void exception_init(void)

{

old_terminate = std::set_terminate(&_objc_terminate);

}

 

 

 

 

cache_init()

 

主要是緩存初始化

 

 

 

 

void cache_init()

{

#if HAVE_TASK_RESTARTABLE_RANGES

mach_msg_type_number_t count = 0;

kern_return_t kr;

 

while (objc_restartableRanges[count].location) {

count++;

}

 

kr = task_restartable_ranges_register(mach_task_self(),

objc_restartableRanges, count);

if (kr == KERN_SUCCESS) return;

_objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",

kr, mach_error_string(kr));

#endif // HAVE_TASK_RESTARTABLE_RANGES

}

 

 

 

 

_imp_implementationWithBlock_init()

 

主要用來啓動機制回調

 

 

 

 

/// everything is initialized lazily, but for certain processes we eagerly load

/// the trampolines dylib.

void

_imp_implementationWithBlock_init(void)

{

#if TARGET_OS_OSX

// Eagerly load libobjc-trampolines.dylib in certain processes. Some

// programs (most notably QtWebEngineProcess used by older versions of

// embedded Chromium) enable a highly restrictive sandbox profile which

// blocks access to that dylib. If anything calls

// imp_implementationWithBlock (as AppKit has started doing) then we'll

// crash trying to load it. Loading it here sets it up before the sandbox

// profile is enabled and blocks it.

//

// This fixes EA Origin (rdar://problem/50813789)

// and Steam (rdar://problem/55286131)

if (__progname &&

(strcmp(__progname, "QtWebEngineProcess") == 0 ||

strcmp(__progname, "Steam Helper") == 0)) {

Trampolines.Initialize();

}

#endif

}

 

 

 

 

dyld與objc關聯

 

_dyld_objc_notify_register(&map_images, load_images, unmap_image)

 

主要是dyld註冊 實際代碼實現

 

 

 

 

void _dyld_objc_notify_register(_dyld_objc_notify_mapped mapped,

_dyld_objc_notify_init init,

_dyld_objc_notify_unmapped unmapped)

{

dyld::registerObjCNotifiers(mapped, init, unmapped);

}

 

 

 

 

從上文正中咱們能夠看出

 

• mapped即map_images

• init即load_images

• unmapped即unmap_image

 

map_images()函數分析

 

 

 

 

/***********************************************************************

* map_images

* Process the given images which are being mapped in by dyld.

* Calls ABI-agnostic code after taking ABI-specific locks.

*

* Locking: write-locks runtimeLock

**********************************************************************/

void

map_images(unsigned count, const char * const paths[],

const struct mach_header * const mhdrs[])

{

mutex_locker_t lock(runtimeLock);

return map_images_nolock(count, paths, mhdrs);

}

 

 

 

 

從map_images函數中咱們發現map_images_nolock函數是重點，咱們進入map_images_nolock函數

 

map_images_nolock

 

咱們查看代碼實現

 

從截圖中咱們能夠看出_read_images是咱們要重點研究的方法

 

_read_images函數分析

 

是不是第一次加載

 

修復預編譯時@selector的錯亂問題

 

錯誤類處理，經過readClass讀取出來類的信息

 

從新設置映射鏡像

 

消息處理

 

類中若是有協議，讀取協議

 

映射協議

 

加載分類

 

注意在分類處理中主要是經過load_categories_nolock處理，咱們進入load_categories_nolock函數中

 

load_categories_nolock函數

 

 

 

 

static void load_categories_nolock(header_info *hi) {

bool hasClassProperties = hi->info()->hasCategoryClassProperties();

 

size_t count;

auto processCatlist = [&](category_t * const *catlist) {

for (unsigned i = 0; i < count; i++) {

category_t *cat = catlist[i];

Class cls = remapClass(cat->cls);

locstamped_category_t lc{cat, hi};

 

if (!cls) {

// Category's target class is missing (probably weak-linked).

// Ignore the category.

if (PrintConnecting) {

_objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "

"missing weak-linked target class",

cat->name, cat);

}

continue;

}

 

// Process this category.

if (cls->isStubClass()) {

// Stub classes are never realized. Stub classes

// don't know their metaclass until they're

// initialized, so we have to add categories with

// class methods or properties to the stub itself.

// methodizeClass() will find them and add them to

// the metaclass as appropriate.

if (cat->instanceMethods ||

cat->protocols ||

cat->instanceProperties ||

cat->classMethods ||

cat->protocols ||

(hasClassProperties && cat->_classProperties))

{

objc::unattachedCategories.addForClass(lc, cls);

}

} else {

// First, register the category with its target class.

// Then, rebuild the class's method lists (etc) if

// the class is realized.

if (cat->instanceMethods || cat->protocols

|| cat->instanceProperties)

{

if (cls->isRealized()) {

attachCategories(cls, &lc, 1, ATTACH_EXISTING);

} else {

objc::unattachedCategories.addForClass(lc, cls);

}

}

 

if (cat->classMethods || cat->protocols

|| (hasClassProperties && cat->_classProperties))

{

if (cls->ISA()->isRealized()) {

attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);

} else {

objc::unattachedCategories.addForClass(lc, cls->ISA());

}

}

}

}

};

 

processCatlist(_getObjc2CategoryList(hi, &count));

processCatlist(_getObjc2CategoryList2(hi, &count));

}

 

 

 

 

從load_categories_nolock函數實現中，咱們能夠看到該函數將類、實例方法、協議、屬性、類方法等再次連接了一次。

 

非懶加載類處理

 

處理沒有使用的類

 

dyld與objc關聯總結

 

• dyld_start調用_objc_init來初始化，_objc_init中經過dyld調用_dyld_objc_notify_register函數，傳入map_images跟load_images這兩個參數來處理

• map_images經過map_images_nolock函數調用_read_images函數

• 在_read_images函數中處理類信息、屬性、協議、分類等

• 當一切準備穩當，則再次返回dyld_start中，此時dyld跟objc關聯了起來

 

資料推薦

 

若是你正在跳槽或者正準備跳槽不妨動動小手，添加一下我們的交流羣1012951431來獲取一份詳細的大廠面試資料爲你的跳槽多添一份保障。