一彩掐、objc

查看objc源碼的時候看到了void _objc_init(void)函數(shù)

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
}

那這個方法什么時候進來的呢,我們打一個斷點,通過lldb的bt命令看一下堆棧

image.png

結(jié)合這張圖我們得知_objc_init調(diào)用流程大致為:

dyld的doModInitFunctions方法調(diào)用libSystem.B.dylib的libSystem_initializer方法毛仪；接著初始化了libdispatch; libdispatch又調(diào)用了_os_object_int,最終來到了_objc_init

1叛买、這里其實是各種初始化：環(huán)境初始化；靜態(tài)變量初始化飞涂；運行時初始化赶舆；異常初始化踪栋；cache初始化等等
2敛腌、這里最重要的是_dyld_objc_notify_register方法,他這里就是dyld的注冊監(jiān)聽方法卧土，用來跟objc關(guān)聯(lián)的惫皱。

二像樊、dyld和objc關(guān)聯(lián)過程

1、_dyld_objc_notify_register解析

1.1 在objc代碼中我們看到它的聲明

//
// Note: only for use by objc runtime
// Register handlers to be called when objc images are mapped, unmapped, and initialized.
// Dyld will call back the "mapped" function with an array of images that contain an objc-image-info section.
// Those images that are dylibs will have the ref-counts automatically bumped, so objc will no longer need to
// call dlopen() on them to keep them from being unloaded.  During the call to _dyld_objc_notify_register(),
// dyld will call the "mapped" function with already loaded objc images.  During any later dlopen() call,
// dyld will also call the "mapped" function.  Dyld will call the "init" function when dyld would be called
// initializers in that image.  This is when objc calls any +load methods in that image.
//
void _dyld_objc_notify_register(_dyld_objc_notify_mapped    mapped,
                                _dyld_objc_notify_init      init,
                                _dyld_objc_notify_unmapped  unmapped);

不知道大家有沒有注意到源碼中的注釋+load()旅敷，這是一個小細節(jié)生棍，初始化鏡像的時候會調(diào)用在鏡像中的+load()方法.

1.2 在這個注釋中說dyld將會調(diào)用mapped、unmapped媳谁、initialized這些方法涂滴；既然是dyld調(diào)用那這些方法就應(yīng)該會傳到dyld中啊友酱，順著這條思路應(yīng)該能想到_dyld_objc_notify_register的實現(xiàn)應(yīng)該在dyld中,果不其然,在dyld3中

void _dyld_objc_notify_register(_dyld_objc_notify_mapped    mapped,
                                _dyld_objc_notify_init      init,
                                _dyld_objc_notify_unmapped  unmapped)
{
    log_apis("_dyld_objc_notify_register(%p, %p, %p)\n", mapped, init, unmapped);

    gAllImages.setObjCNotifiers(mapped, init, unmapped);
}

2、setObjCNotifiers解析

void AllImages::setObjCNotifiers(_dyld_objc_notify_mapped map, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmap)
{
    _objcNotifyMapped   = map;
    _objcNotifyInit     = init;
    _objcNotifyUnmapped = unmap;
   //.....此處省略
}

這三個函數(shù)指針會賦值給_objcNotifyMapped柔纵、_objcNotifyInit缔杉、_objcNotifyUnmapped這三個變量,其實這里就是寫入注冊函數(shù)

3、map_images解析

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);
}

接著進入map_images_nolock函數(shù)看看搁料，這里的核心代碼是_read_images方法

    if (hCount > 0) {
        _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
    }

3.1: 條件控制進行一次的加載
3.2: 修復(fù)預(yù)編譯階段的 @selector 的混亂問題
3.3: 查找類或详，錯誤混亂的類處理
3.4: 修復(fù)重映射一些沒有被鏡像文件加載進來的類
3.5: 修復(fù)一些消息!
3.6: 當我們類里面有協(xié)議的時候 : readProtocol
3.7: 修復(fù)沒有被加載的協(xié)議
3.8: 分類處理
3.9: 類的加載處理
3.10: 沒有被處理的類優(yōu)化那些被侵犯的

4、readClass解析
在_read_images中有一個readClass方法的調(diào)用;

Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();
    
    if (missingWeakSuperclass(cls)) {
        // No superclass (probably weak-linked). 
        // Disavow any knowledge of this subclass.
        if (PrintConnecting) {
            _objc_inform("CLASS: IGNORING class '%s' with "
                         "missing weak-linked superclass", 
                         cls->nameForLogging());
        }
        addRemappedClass(cls, nil);
        cls->superclass = nil;
        return nil;
    }
    
    cls->fixupBackwardDeployingStableSwift();

    Class replacing = nil;
    if (Class newCls = popFutureNamedClass(mangledName)) {
        // This name was previously allocated as a future class.
        // Copy objc_class to future class's struct.
        // Preserve future's rw data block.
        
        if (newCls->isAnySwift()) {
            _objc_fatal("Can't complete future class request for '%s' "
                        "because the real class is too big.", 
                        cls->nameForLogging());
        }
        
        class_rw_t *rw = newCls->data();
        const class_ro_t *old_ro = rw->ro();
        memcpy(newCls, cls, sizeof(objc_class));
        rw->set_ro((class_ro_t *)newCls->data());
        newCls->setData(rw);
        freeIfMutable((char *)old_ro->name);
        free((void *)old_ro);
        
        addRemappedClass(cls, newCls);
        
        replacing = cls;
        cls = newCls;
    }
    
    if (headerIsPreoptimized  &&  !replacing) {
        // class list built in shared cache
        // fixme strict assert doesn't work because of duplicates
        // ASSERT(cls == getClass(name));
        ASSERT(getClassExceptSomeSwift(mangledName));
    } else {
        addNamedClass(cls, mangledName, replacing);
        addClassTableEntry(cls);
    }

    // for future reference: shared cache never contains MH_BUNDLEs
    if (headerIsBundle) {
        cls->data()->flags |= RO_FROM_BUNDLE;
        cls->ISA()->data()->flags |= RO_FROM_BUNDLE;
    }
    
    return cls;
}

這一步會把class信息從二進制里面讀出來郭计，

將newCls->data()取出來作rw 霸琴；
將newCls->data()再取出來強轉(zhuǎn)為class_ro_t *放到到rw的ro部分
addClassTableEntry這是將類插入到類的集合表中，為了后面調(diào)用的快速查找

三昭伸、總結(jié)

dyld和objc中是存在著一些交互的過程梧乘，需要關(guān)聯(lián)起來進行研究學(xué)習(xí)。

dyld和objc的關(guān)聯(lián)

dyld和objc的關(guān)聯(lián)

一彩掐、objc

二像樊、dyld和objc關(guān)聯(lián)過程

三昭伸、總結(jié)