iOS 底層原理之—dyld 與 objc 的關聯
阿新 • • 發佈：2020-10-20
前言
在dyld載入過程中，我們知道會呼叫_objc_init方法，那麼在_objc_init方法中究竟做了什麼呢？我們來探究下。
_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();從_objc_init實現中我們分析下該方法主要做了什麼
environ_init()
該方法主要是讀取執行時的環境變數，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()簡介可參考部落格iOS-底層原理 16：dyld與objc的關聯中有關 nviron_init()部分的介紹
tls_init()
主要用於關於執行緒key的繫結，比如每執行緒資料的解構函式。
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);static_init()
主要是C++靜態建構函式
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()
主要是執行時的初始化，主要分為兩部分：分類初始化和類的表初始化
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撥
/// 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 實際程式碼實現
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
我們檢視程式碼實現
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
是否是第一次載入
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
重新設定對映映象
訊息處理
類中如果有協議，讀取協議
對映協議
載入分類
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函式實現中，我們可以看到該函式將類、例項方法、協議、屬性、類方法等再次連結了一次。
非懶載入類處理
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，則再次返回
dyld_start中，此時dyld跟objc關聯了起來
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