iOS 底层原理之—dyld 与 objc 的关联

前言

 

在dyld加载过程中，我们知道会调用_objc_init方法，那么在_objc_init方法中究竟做了什么呢？我们来探究下。

 

_objc_init方法

 

_objc_init方法实现

 

 

 

 

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

}

 

 

 

 

从_objc_init实现中我们分析下该方法主要做了什么

 

environ_init()

 

该方法主要是读取运行时的环境变量，我们可以通过设置DYLD_PRINT_STATISTICS = YES来打印APP启动到main()函数之前的时长，进而可以进行APP启动优化。具体的environ_init()简介可参考博客iOS-底层原理 16：dyld与objc的关联中有关nviron_init()部分的介绍

 

tls_init()

 

主要用于关于线程key的绑定，比如每线程数据的析构函数。

 

 

 

 

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++静态构造函数

 

 

 

 

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()

 

主要是运行时的初始化，主要分为两部分：分类初始化和类的表初始化

 

 

 

 

void runtime_init(void)

{

objc::unattachedCategories.init(32);

objc::allocatedClasses.init();

}

复制代码

 

 

 

 

exception_init()

 

初始化libobjc异常处理

 

 

 

 

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

* 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()

 

主要是缓存初始化

 

 

 

 

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()

 

主要用来启动机制回调

 

 

 

 

/// 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与objc关联

 

_dyld_objc_notify_register(&map_images, load_images, unmap_image)

 

主要是dyld注册 实际代码实现

 

 

 

 

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);

}

 

 

 

 

从上文正中我们可以看出

 

• mapped即map_images

• init即load_images

• unmapped即unmap_image

 

map_images()函数分析

 

 

 

 

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

* 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);

}

 

 

 

 

从map_images函数中我们发现map_images_nolock函数是重点，我们进入map_images_nolock函数

 

map_images_nolock

 

我们查看代码实现

 

从截图中我们可以看出_read_images是我们要重点研究的方法

 

_read_images函数分析

 

是否是第一次加载

 

修复预编译时@selector的错乱问题

 

错误类处理，通过readClass读取出来类的信息

 

重新设置映射镜像

 

消息处理

 

类中如果有协议，读取协议

 

映射协议

 

加载分类

 

注意在分类处理中主要是通过load_categories_nolock处理，我们进入load_categories_nolock函数中

 

load_categories_nolock函数

 

 

 

 

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));

}

 

 

 

 

从load_categories_nolock函数实现中，我们可以看到该函数将类、实例方法、协议、属性、类方法等再次链接了一次。

 

非懒加载类处理

 

处理没有使用的类

 

dyld与objc关联总结

 

• dyld_start调用_objc_init来初始化，_objc_init中通过dyld调用_dyld_objc_notify_register函数，传入map_images跟load_images这两个参数来处理

• map_images通过map_images_nolock函数调用_read_images函数

• 在_read_images函数中处理类信息、属性、协议、分类等

• 当一切准备妥当，则再次返回dyld_start中，此时dyld跟objc关联了起来

 

资料推荐

 

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