OC底層原理16-objc_init初探 & dyld和objc關(guān)聯(lián)

引入
在 OC底層原理15-dyld加載流程中奈懒，我們初步窺探了dyld的加載流程：_dyld_start -> dyldbootstrap -> dyld::_main奕短，在dyld::_main流程的最后一步尋找主程序入口益眉，我們進(jìn)入了recursiveInitialization方法，首先調(diào)用了context.notifySingle方法進(jìn)行單個通知注入承疲，調(diào)用回調(diào)函數(shù)load_images残揉，調(diào)用doInitialization方法的時候，會從dyld -> libSystem -> libdispatch -> objc中的_objc_init疲扎，_objc_init又調(diào)用了dyld中實現(xiàn)的_dyld_objc_notify_register注冊回調(diào)函數(shù)，這樣就構(gòu)成了跨庫閉環(huán)捷雕，如下圖

recursiveInitialization流程.png

一、_objc_init源碼初探

image.png

1.1壹甥、environ_init()

讀取影響運行時的環(huán)境變量

進(jìn)入 objc可編譯源碼
修改void environ_init(void)方法如下救巷，打印所以設(shè)置的環(huán)境變量
運行打印環(huán)境變量

void environ_init(void) 
{
    for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
        const option_t *opt = &Settings[i];
        _objc_inform("%s: %s", opt->env, opt->help);
        _objc_inform("%s is set", opt->env);
    }
}

打印如下：由于太多，只展示部分
objc[11514]: OBJC_PRINT_IMAGES: log image and library names as they are loaded
objc[11514]: OBJC_PRINT_IMAGES is set
objc[11514]: OBJC_PRINT_IMAGE_TIMES: measure duration of image loading steps
objc[11514]: OBJC_PRINT_IMAGE_TIMES is set
...

環(huán)境變量的設(shè)置 Product -> Scheme -> Edit Scheme

環(huán)境變量的設(shè)置.png
以OBJC_PRINT_LOAD_METHODS為例設(shè)置成YES句柠，打印所有實現(xiàn)了load方法的類

打印結(jié)果.png

1.2浦译、environ_init()

關(guān)于線程key的綁定

void tls_init(void)
{
#if SUPPORT_DIRECT_THREAD_KEYS
//: -- init方法
    pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);
#else
//: -- 析構(gòu)
    _objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);
#endif
}

1.3棒假、static_init()

初始化objc庫里面的構(gòu)造函數(shù)，比dyld的析構(gòu)c++還早精盅，主動調(diào)用

1.4帽哑、runtime_init()

運行時初始化

void runtime_init(void)
{
//: -- 分類處理的初始化
    objc::unattachedCategories.init(32);
//: -- 初始化class表
    objc::allocatedClasses.init();
}

1.5、exception_init()

初始化libobjc的異常處理系統(tǒng)

crash不是崩潰叹俏，是違反系統(tǒng)的規(guī)定妻枕，系統(tǒng)給出的信號
系統(tǒng)會調(diào)用一個默認(rèn)回調(diào)函數(shù)，中斷你的進(jìn)程粘驰，即崩潰
我們可以指定一個回調(diào)函數(shù)屡谐，攔截中斷，就可以攔截崩潰

1.6蝌数、cache_init()

緩存條件初始化

1.7愕掏、_imp_implementationWithBlock_init()

通常這不會做什么，因為所有的初始化都是惰性的顶伞，但是對于某些進(jìn)程饵撑，我們會迫不及待地加載trampolines dylib

1.8、_dyld_objc_notify_register(&map_images, load_images, unmap_image)

注冊回調(diào)函數(shù)

二唆貌、dyld和objc關(guān)聯(lián)

我們發(fā)現(xiàn)objc的源碼中滑潘，并沒有實現(xiàn)_dyld_objc_notify_register函數(shù)，但是在dyld的源碼中挠锥，我們找到了_dyld_objc_notify_register的實現(xiàn)众羡，這里是跨庫調(diào)用

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);
}

map_images：映射鏡像文件，管理文件中和動態(tài)庫中所有的符號
load_images：加載鏡像文件
unmap_image：移除鏡像文件
注意：map_images前面有&蓖租，屬于引用類型粱侣，外界條件變，它就會跟著變蓖宦，要傳遞值
注意：load_images屬于值類型齐婴，不傳遞值

調(diào)用registerObjCNotifiers

void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmapped)
{
    // record functions to call
    sNotifyObjCMapped   = mapped;
    sNotifyObjCInit     = init;
    sNotifyObjCUnmapped = unmapped;

    // call 'mapped' function with all images mapped so far
    try {
        notifyBatchPartial(dyld_image_state_bound, true, NULL, false, true);
    }
    catch (const char* msg) {
        // ignore request to abort during registration
    }

    // <rdar://problem/32209809> call 'init' function on all images already init'ed (below libSystem)
    for (std::vector<ImageLoader*>::iterator it=sAllImages.begin(); it != sAllImages.end(); it++) {
        ImageLoader* image = *it;
        if ( (image->getState() == dyld_image_state_initialized) && image->notifyObjC() ) {
            dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
            (*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
        }
    }
}

objc把回調(diào)函數(shù)map_images、load_images稠茂、unmap_image分別傳給了dyld的sNotifyObjCMapped柠偶、sNotifyObjCInit、sNotifyObjCUnmapped

現(xiàn)在研究sNotifyObjCMapped睬关，sNotifyObjCInit诱担，sNotifyObjCUnmapped是何時被調(diào)用的
全局搜索sNotifyObjCMapped，發(fā)現(xiàn)在notifyBatchPartial方法中調(diào)用了sNotifyObjCMapped

static void notifyBatchPartial(dyld_image_states state, bool orLater, dyld_image_state_change_handler onlyHandler, bool preflightOnly, bool onlyObjCMappedNotification)
{
...
//: -- sNotifyObjCMapped 調(diào)用
(*sNotifyObjCMapped)(objcImageCount, paths, mhs);
...
}

在上面的registerObjCNotifiers方法中就對notifyBatchPartial進(jìn)行了調(diào)用电爹，即objc中調(diào)用_dyld_objc_notify_register進(jìn)入dyld后蔫仙，就會回調(diào)map_images，進(jìn)行鏡像文件的映射

全局搜索sNotifyObjCInit丐箩，發(fā)現(xiàn)在notifySingle方法中調(diào)用了sNotifyObjCInit

static void notifySingle(dyld_image_states state, const ImageLoader* image, ImageLoader::InitializerTimingList* timingInfo)
{
    ...
    if ( (state == dyld_image_state_dependents_initialized) && (sNotifyObjCInit != NULL) && image->notifyObjC() ) {
        uint64_t t0 = mach_absolute_time();
        dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
//: -- sNotifyObjCInit 調(diào)用
        (*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
        uint64_t t1 = mach_absolute_time();
        uint64_t t2 = mach_absolute_time();
        uint64_t timeInObjC = t1-t0;
        uint64_t emptyTime = (t2-t1)*100;
        if ( (timeInObjC > emptyTime) && (timingInfo != NULL) ) {
            timingInfo->addTime(image->getShortName(), timeInObjC);
        }
    }
    ...
}

當(dāng)dyld_image_states == dyld_image_state_dependents_initialized時摇邦，會開始調(diào)用sNotifyObjCInit

在文章開頭的引入中恤煞，我們分析了recursiveInitialization方法，里面調(diào)用了``

void ImageLoader::recursiveInitialization(const LinkContext& context, mach_port_t this_thread, const char* pathToInitialize,
                                          InitializerTimingList& timingInfo, UninitedUpwards& uninitUps)
{
    ...
    // let objc know we are about to initialize this image
    uint64_t t1 = mach_absolute_time();
    fState = dyld_image_state_dependents_initialized;
    oldState = fState;
//: -- 調(diào)用notifySingle施籍，并且`dyld_image_states` == `dyld_image_state_dependents_initialized `
    context.notifySingle(dyld_image_state_dependents_initialized, this, &timingInfo);
            
    // initialize this image
    bool hasInitializers = this->doInitialization(context);

    // let anyone know we finished initializing this image
    fState = dyld_image_state_initialized;
    oldState = fState;
    context.notifySingle(dyld_image_state_initialized, this, NULL);
    ...
}

全局搜索sNotifyObjCUnmapped居扒，發(fā)現(xiàn)在removeImage方法中調(diào)用了sNotifyObjCUnmapped

void removeImage(ImageLoader* image)
{
    ...
    (*sNotifyObjCUnmapped)(image->getRealPath(), image->machHeader());
    ...
}

dyld的加載流程從_dyld_start -> dyldbootstrap -> dyld::_main，在dyld::_main流程的最后一步尋找主程序入口丑慎，進(jìn)入了recursiveInitialization方法喜喂，首先調(diào)用了context.notifySingle方法進(jìn)行單個通知注入，調(diào)用回調(diào)函數(shù)load_images立哑，調(diào)用doInitialization方法的時候夜惭，會從dyld -> libSystem -> libdispatch -> objc中的_objc_init，_objc_init又調(diào)用了dyld中實現(xiàn)的_dyld_objc_notify_register注冊回調(diào)函數(shù)铛绰，調(diào)用dyld中的_dyld_objc_notify_register的時候诈茧，又調(diào)用了回調(diào)函數(shù)map_images，這樣就構(gòu)成了跨庫閉環(huán)捂掰，就把objc和dyld進(jìn)行了關(guān)聯(lián)敢会。