前言
在dyld加載程序中，我們知道會呼叫_objc_init方法，那么在_objc_init方法中究竟做了什么呢？我們來探究下，
_objc_init方法
_objc_init方法實作
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實作中我們分析下該方法主要做了什么
environ_init()
該方法主要是讀取運行時的環境變數，我們可以通過設定DYLD_PRINT_STATISTICS = YES來列印APP啟動到main()函式之前的時長，進而可以進行APP啟動優化，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()
主要用于關于執行緒key的系結，比如每執行緒資料的解構式，
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++靜態建構式
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，主要分為兩部分：分類初始化和類的表初始化
void runtime_init(void) { objc::unattachedCategories.init(32); objc::allocatedClasses.init(); } 復制代碼
exception_init()
初始化libobjc例外處理
/*********************************************************************** * 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()
主要是快取初始化
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()
主要用來啟動機制回呼
/// 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注冊 實際代碼實作
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); }
從上文正中我們可以看出
-
mapped即map_images -
init即load_images -
unmapped即unmap_image
map_images()函式分析
/*********************************************************************** * 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函式中我們發現map_images_nolock函式是重點，我們進入map_images_nolock函式
map_images_nolock
我們查看代碼實作
從截圖中我們可以看出_read_images是我們要重點研究的方法
_read_images函式分析
是否是第一次加載
修復預編譯時@selector的錯亂問題
錯誤類處理，通過readClass讀取出來類的資訊
重新設定映射鏡像
訊息處理
類中如果有協議，讀取協議
映射協議
加載分類
注意在分類處理中主要是通過load_categories_nolock處理，我們進入load_categories_nolock函式中
load_categories_nolock函式
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函式實作中，我們可以看到該函式將類、實體方法、協議、屬性、類方法等再次鏈接了一次，
非懶加載類處理
處理沒有使用的類
dyld與objc關聯總結
-
dyld_start呼叫_objc_init來初始化，_objc_init中通過dyld呼叫_dyld_objc_notify_register函式，傳入map_images跟load_images這兩個引數來處理 -
map_images通過map_images_nolock函式呼叫_read_images函式 -
在
_read_images函式中處理類資訊、屬性、協議、分類等 -
當一切準備妥當，則再次回傳
dyld_start中，此時dyld跟objc關聯了起來
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