在了解了我们要做什么的之前，先做一下知识点的补充，因为在realizeClassWithoutSwift中有一些知识点，需要我们提前了解一下。

• ro：属于clean memory，即；加载后不会改变内存。
• rw：属于dirty memory，即；可以向类中添加属性、方法等，是在运行时可以改变的内存。
• rwe：相当于类的额外信息，因为在使用过程中，只有很少的类会真正改变其内容，所以为了避免资源浪费就有了rwe。

realizeClassWithoutSwift

同样的，我们首先来看一下源码是怎么写的：

/***********************************************************************
* realizeClassWithoutSwift
* Performs first-time initialization on class cls, 对类 cls 执行首次初始化
* including allocating its read-write data. 包括分配其读写数据
* Does not perform any Swift-side initialization. 不执行任何 Swift 端初始化
* Returns the real class structure for the class. 返回类的真实类结构
* Locking: runtimeLock must be write-locked by the caller 锁定：runtimeLock 必须由调用者写锁定
**********************************************************************/
static Class realizeClassWithoutSwift(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    class_rw_t *rw;
    Class supercls;
    Class metacls;

    if (!cls) return nil;
    if (cls->isRealized()) {
        validateAlreadyRealizedClass(cls);
        return cls;
    }
    ASSERT(cls == remapClass(cls));

    // fixme verify class is not in an un-dlopened part of the shared cache?

    auto ro = (const class_ro_t *)cls->data();
    auto isMeta = ro->flags & RO_META;
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro();
        ASSERT(!isMeta);
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        rw = objc::zalloc<class_rw_t>();
        rw->set_ro(ro);
        rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
        cls->setData(rw);
    }

    cls->cache.initializeToEmptyOrPreoptimizedInDisguise();

#if FAST_CACHE_META
    if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif

    // Choose an index for this class.
    // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
    cls->chooseClassArrayIndex();

    if (PrintConnecting) {
        _objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
                     cls->nameForLogging(), isMeta ? " (meta)" : "", 
                     (void*)cls, ro, cls->classArrayIndex(),
                     cls->isSwiftStable() ? "(swift)" : "",
                     cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
    }

    // Realize superclass and metaclass, if they aren't already.
    // This needs to be done after RW_REALIZED is set above, for root classes.
    // This needs to be done after class index is chosen, for root metaclasses.
    // This assumes that none of those classes have Swift contents,
    //   or that Swift's initializers have already been called.
    //   fixme that assumption will be wrong if we add support
    //   for ObjC subclasses of Swift classes.
    supercls = realizeClassWithoutSwift(remapClass(cls->getSuperclass()), nil);
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);

#if SUPPORT_NONPOINTER_ISA
    if (isMeta) {
        // Metaclasses do not need any features from non pointer ISA
        // This allows for a faspath for classes in objc_retain/objc_release.
        cls->setInstancesRequireRawIsa();
    } else {
        // Disable non-pointer isa for some classes and/or platforms.
        // Set instancesRequireRawIsa.
        bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
        bool rawIsaIsInherited = false;
        static bool hackedDispatch = false;

        if (DisableNonpointerIsa) {
            // Non-pointer isa disabled by environment or app SDK version
            instancesRequireRawIsa = true;
        }
        else if (!hackedDispatch  &&  0 == strcmp(ro->getName(), "OS_object"))
        {
            // hack for libdispatch et al - isa also acts as vtable pointer
            hackedDispatch = true;
            instancesRequireRawIsa = true;
        }
        else if (supercls  &&  supercls->getSuperclass()  &&
                 supercls->instancesRequireRawIsa())
        {
            // This is also propagated by addSubclass()
            // but nonpointer isa setup needs it earlier.
            // Special case: instancesRequireRawIsa does not propagate
            // from root class to root metaclass
            instancesRequireRawIsa = true;
            rawIsaIsInherited = true;
        }

        if (instancesRequireRawIsa) {
            cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
        }
    }
// SUPPORT_NONPOINTER_ISA
#endif

    // Update superclass and metaclass in case of remapping
    cls->setSuperclass(supercls);
    cls->initClassIsa(metacls);

    // Reconcile instance variable offsets / layout.
    // This may reallocate class_ro_t, updating our ro variable.
    if (supercls  &&  !isMeta) reconcileInstanceVariables(cls, supercls, ro);

    // Set fastInstanceSize if it wasn't set already.
    cls->setInstanceSize(ro->instanceSize);

    // Copy some flags from ro to rw
    if (ro->flags & RO_HAS_CXX_STRUCTORS) {
        cls->setHasCxxDtor();
        if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
            cls->setHasCxxCtor();
        }
    }
    
    // Propagate the associated objects forbidden flag from ro or from
    // the superclass.
    if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
        (supercls && supercls->forbidsAssociatedObjects()))
    {
        rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
    }

    // Connect this class to its superclass's subclass lists
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

    // Attach categories
    methodizeClass(cls, previously);

    return cls;
}

大家注意看官方对该函数的注释，这里给出了翻译。realizeClassWithoutSwift方法的主要所用就是实现类，官方注释写的很明白Returns the real class structure for the class，返回一个真实的类结构。

主要有以下几个步骤：

• 1、读取data数据，设置ro，rw。
• 2、递归realizeClassWithoutSwift（这里回忆一下isa走位图）。
• 3、设置类的一些相关信息，子类，父类等等。
• 4、methodizeClass，附加类别。

1、读取data数据，设置ro，rw

在上面我们已经介绍过ro和rw的相关定义。

2、递归realizeClassWithoutSwift

这里会递归调用realizeClassWithoutSwift，对于当前类的父类，元类进行初始化。

3、设置类的一些相关信息，子类，父类等等

在得到了父类和元类之后，对当前类进行配置。当然，上面有递归调用，所以，或有if判断，看一下当前类的类型。

4、methodizeClass，附加类别

我们跟踪进methodizeClass之后，首先还是要阅读官方注释；通过过官方注释我们得知，这个方法是修正cls的方法列表、协议列表、属性列表。

/***********************************************************************
* methodizeClass
* Fixes up cls's method list, protocol list, and property list.
* Attaches any outstanding categories.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void methodizeClass(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    bool isMeta = cls->isMetaClass();
    auto rw = cls->data();
    auto ro = rw->ro();
    auto rwe = rw->ext();

    // Methodizing for the first time
    if (PrintConnecting) {
        _objc_inform("CLASS: methodizing class '%s' %s", 
                     cls->nameForLogging(), isMeta ? "(meta)" : "");
    }

    // Install methods and properties that the class implements itself.
    method_list_t *list = ro->baseMethods();
    if (list) {
        prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls), nullptr);
        if (rwe) rwe->methods.attachLists(&list, 1);
    }

    property_list_t *proplist = ro->baseProperties;
    if (rwe && proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }

    protocol_list_t *protolist = ro->baseProtocols;
    if (rwe && protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }

    // Root classes get bonus method implementations if they don't have 
    // them already. These apply before category replacements.
    if (cls->isRootMetaclass()) {
        // root metaclass
        addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
    }

    // Attach categories.
    if (previously) {
        if (isMeta) {
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_METACLASS);
        } else {
            // When a class relocates, categories with class methods
            // may be registered on the class itself rather than on
            // the metaclass. Tell attachToClass to look for those.
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_CLASS_AND_METACLASS);
        }
    }
    objc::unattachedCategories.attachToClass(cls, cls,
                                             isMeta ? ATTACH_METACLASS : ATTACH_CLASS);

#if DEBUG
    // Debug: sanity-check all SELs; log method list contents
    for (const auto& meth : rw->methods()) {
        if (PrintConnecting) {
            _objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(meth.name()));
        }
        ASSERT(sel_registerName(sel_getName(meth.name())) == meth.name());
    }
#endif
}

• 这里虽然官方注释是Attach categories，但是大家不要认为只是附加分类中的方法。因为在源码中有这样一段注释，大家仔细看一下：
  

  

  
  看一看到这里对于方法，属性的处理，是包含类本身的方法，属性等信息的。对于协议也是一样的，因为源码中也获取了baseProtocols:
  

  

最终，方法列表、属性列表、协议列表，都附加到了rwe中。

4.1 prepareMethodLists

上面我们看到，对于方法列表的处理，并不是直接添加到调用rwe->methods.attachLists(&list, 1);。在附加之前，还调用了prepareMethodLists函数；那么这个函数又是做什么的呢？我们跟进去看一下，最终发现了这样一段代码。（注意：这里list传入的是&list）

也就是说，方法列表在附加到rwe中之前，做了一次排序。（通过sore可以推测，注意上图中红圈。）

4.2 fixupMethodList

上面我们追踪到了fixupMethodList，推测到这是一个对方法列表进行排序的函数。那究竟是怎么排序的呢？我们进入函数看一下：

同时，在排序之前，根据if的判断，还会进行一次name(返回的SEL)的设置。(看图中，红框上方的代码块。)

• 这个地方我们可以通过打印验证一下。我们在fixupMethodList中，打印一下，排序前后的methodlist:
  

  

• 我们自定一个类，并调用一下，（注意：这里要调用一下load方法，不然系统会默认为懒加载）

• 接下来，运行工程(源码工程)，在控制台的打印里面搜索我们自定义的方法（因为有系统的一些打印，会比较多。），可以看到方法按照地址排了序：