iOS进阶-08应用程序加载-map_Images&Load_Images

上一章介绍了应用加载的基本流程，即函数void _objc_init(void)内部的实现，其内部最主要的函数_dyld_objc_notify_register(&map_images, load_images, unmap_image);只是说了注册方法指针回调，用于程序调用，那这个方法内部的map_images, load_images,unmap_image又是做了一些什么呢？

map_images

我将选取部分源码中的函数进行说明，在函数中重要的位置会有相应的注释,后面不再一一解释

map_images()

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);
}

map_images_nolock()

void 
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
                  const struct mach_header * const mhdrs[])
{
    static bool firstTime = YES;
    header_info *hList[mhCount];
    uint32_t hCount;
    size_t selrefCount = 0;
...
...
    if (hCount > 0) {
        _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
    }
    firstTime = NO;
}

_read_images源码简化后的代码

void _read_images {
    
    // 1:第一次进来 - 开始创建表
    // gdb_objc_realized_classes : 所有类的表 - 包括实现的和没有实现的
    // allocatedClasses: 包含用objc_allocateClassPair分配的所有类（和元类）的表。(已分配)
    if (!doneOnce) {
           doneOnce = YES;
        // namedClasses
        // Preoptimized classes don't go in this table.
        // 4/3 is NXMapTable's load factor
        int namedClassesSize =
            (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
        gdb_objc_realized_classes =
            NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
        
        allocatedClasses = NXCreateHashTable(NXPtrPrototype, 0, nil);
    }
    
    // 2:类处理
    for (i = 0; i < count; i++) {
      Class cls = (Class)classlist[i];
      Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
    }
    
    // 3: 方法编号处理
    for (EACH_HEADER) {
        SEL *sels = _getObjc2SelectorRefs(hi, &count);
        UnfixedSelectors += count;
        for (i = 0; i < count; i++) {
          const char *name = sel_cname(sels[i]);
          sels[i] = sel_registerNameNoLock(name, isBundle);
        }
    }

    // 4: 协议处理
    for (EACH_HEADER) {
        extern objc_class OBJC_CLASS_$_Protocol;
        Class cls = (Class)&OBJC_CLASS_$_Protocol;
        NXMapTable *protocol_map = protocols();
        protocol_t **protolist = _getObjc2ProtocolList(hi, &count);
        for (i = 0; i < count; i++) {
            readProtocol(protolist[i], cls, protocol_map,
                         isPreoptimized, isBundle);
        }
    }
    
    // 5: 非懒加载类处理
    for (EACH_HEADER) {
      classref_t *classlist =
          _getObjc2NonlazyClassList(hi, &count);
      addClassTableEntry(cls);
      realizeClassWithoutSwift(cls);
    }
    
    // 6: 待处理的类
    if (resolvedFutureClasses) {
        for (i = 0; i < resolvedFutureClassCount; i++) {
            Class cls = resolvedFutureClasses[i];
            if (cls->isSwiftStable()) {
                _objc_fatal("Swift class is not allowed to be future");
            }
            realizeClassWithoutSwift(cls);
            cls->setInstancesRequireRawIsa(false/*inherited*/);
        }
        free(resolvedFutureClasses);
    }
    
    // 7:分类处理
   for (EACH_HEADER) {
       category_t **catlist =
           _getObjc2CategoryList(hi, &count);
       bool hasClassProperties = hi->info()->hasCategoryClassProperties();
       for (i = 0; i < count; i++) {
           category_t *cat = catlist[i];
           Class cls = remapClass(cat->cls);
       }
   }

}

_read_images 之 readClass()

Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();
    if (missingWeakSuperclass(cls)) {
        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)) {
        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->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) {
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;
}

readClass()方法中主要做了这几件事：
1.判断下是不是后期要处理的类，读取Class的data() - 设置ro/rw

2. addNamedClass(cls, mangledName, replacing);
3. addClassTableEntry(cls);
   4.插入到表中

_read_images 之 static Class realizeClassWithoutSwift(Class cls)

static Class realizeClassWithoutSwift(Class cls)
{
    runtimeLock.assertLocked();

    const class_ro_t *ro;
    class_rw_t *rw;
    Class supercls;
    Class metacls;
    bool isMeta;

    if (!cls) return nil;
    if (cls->isRealized()) return cls;
    assert(cls == remapClass(cls));

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

    ro = (const class_ro_t *)cls->data();
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro;
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
        rw->ro = ro;
        rw->flags = RW_REALIZED|RW_REALIZING;
        cls->setData(rw);
    }

    isMeta = ro->flags & RO_META;

    rw->version = isMeta ? 7 : 0;  // old runtime went up to 6


    // 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->superclass));
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()));

#if SUPPORT_NONPOINTER_ISA
    // 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  &&  !(ro->flags & RO_META)  &&  
             0 == strcmp(ro->name, "OS_object")) 
    {
        // hack for libdispatch et al - isa also acts as vtable pointer
        hackedDispatch = true;
        instancesRequireRawIsa = true;
    }
    else if (supercls  &&  supercls->superclass  &&  
             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->setInstancesRequireRawIsa(rawIsaIsInherited);
    }
// SUPPORT_NONPOINTER_ISA
#endif

    // Update superclass and metaclass in case of remapping
    cls->superclass = 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);
    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;
    }
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }
    methodizeClass(cls);
    return cls;
}

1. 读取class的data（）
2. ro/rw的创建
   3.父类和元类的实现内部(递归)分别是：supercls = realizeClassWithoutSwift(remapClass(cls->superclass));和
   metacls = realizeClassWithoutSwift(remapClass(cls->ISA()));
   4.配置父类和元类的归属关系：;cls->superclass = supercls;;和
   ;cls->initClassIsa(metacls);;
   5.将此类连接到其超类的子类列表

if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

realizeClassWithoutSwift

之methodizeClass(cls);

static void methodizeClass(Class cls)
{
    runtimeLock.assertLocked();

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

    // 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));
        rw->methods.attachLists(&list, 1);
    }

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

    protocol_list_t *protolist = ro->baseProtocols;
    if (protolist) {
        rw->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, SEL_initialize, (IMP)&objc_noop_imp, "", NO);
    }

    // Attach categories.
    category_list *cats = unattachedCategoriesForClass(cls, true /*realizing*/);
    attachCategories(cls, cats, false /*don't flush caches*/);

    if (PrintConnecting) {
        if (cats) {
            for (uint32_t i = 0; i < cats->count; i++) {
                _objc_inform("CLASS: attached category %c%s(%s)", 
                             isMeta ? '+' : '-', 
                             cls->nameForLogging(), cats->list[i].cat->name);
            }
        }
    }
    
    if (cats) free(cats);

#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
}

把ro数据写到rw中：method_list_t、property_list_t、protocol_list_t

methodizeClass 之 attachLists()

void attachLists(List* const * addedLists, uint32_t addedCount) {
        if (addedCount == 0) return;

        if (hasArray()) {
            // 多对多
            uint32_t oldCount = array()->count;
            uint32_t newCount = oldCount + addedCount;
            setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
            array()->count = newCount;
            memmove(array()->lists + addedCount, array()->lists, 
                    oldCount * sizeof(array()->lists[0]));
            memcpy(array()->lists, addedLists, 
                   addedCount * sizeof(array()->lists[0]));
        }
        else if (!list  &&  addedCount == 1) {
            // 0对多
            list = addedLists[0];
        } 
        else {
            // 1对多
            List* oldList = list;
            uint32_t oldCount = oldList ? 1 : 0;
            uint32_t newCount = oldCount + addedCount;
            setArray((array_t *)malloc(array_t::byteSize(newCount)));
            array()->count = newCount;
            if (oldList) array()->lists[addedCount] = oldList;
            memcpy(array()->lists, addedLists, 
                   addedCount * sizeof(array()->lists[0]));
        }
    }

1.贴的时候是从源内存地址的起始位置开始复制的，并且是二维数组的形式；

array_t原理图

2.是一个个先move然后copy

memmove(array()->lists + addedCount, array()->lists, 
                    oldCount * sizeof(array()->lists[0]));
memcpy(array()->lists, addedLists, 
                   addedCount * sizeof(array()->lists[0]));

3.其他地方也会用到attachLists()eg:

• 类的加载，处理方法-属性-协议-methodizeClass
• 方法的添加 addMethod
• 添加属性_class_addProperty
• 添加协议class_addProtocol
• 分类的加载attachCategories
  为什么？因为都有相同二位数组结构：entsize_list_tt

map_images小节

map_images主要做了

• 初始化类：realiseClass 设置rw、ro
• 处理分类：把分类中的方法协议、属性添加到类中
• 加载到相应的哈希表中

懒加载类和非懒加载类

懒加载类和非懒加载类的表面区别在于是否实现了load方法,如果

• 实现了load方法就是非懒加载类
• 未实现了load方法就是懒加载类
  但内层区在于属非懒加载类直接走上节课的realizeClassWithoutSwift 去加载数据，懒加载类 在lookupimpOrforward中 有
  if (!cls->isRealized())判断条件，如果用到时候没有实现过则进入实现流程，如果实现过则跳过判断，进行下面的流程。

if (!cls->isRealized()) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}

主类和分类懒加载分析

上面只提到load方法造成了懒加载和非懒加载的区别，其实实际当中会有4种情况

• 1. 懒加载类 + 非懒加载分类
     主类在发送消息的时候才有 — 但是分类提前了 — 需要加载 —-read_image —addUnattacheCategoryForClass —但是没有实现类 ——prepare_load_methods —提前实现了主类realizeClassWithoutSwift

• 2.非懒加载类 + 非懒加载分类
  readImages — realizeClassWithoutSwift —— methodLizeClass — addUnattacheCategoryForClass —判断主类是否实现了，但是这时候主类已经实现了，if(cls —> isRealize){ remethodizeClass
  }
  加载分类进来
  cls —-> demangleName () 用这个，不能用Name
  addUnattachedCategoryForClass(cat, cls, hi);
  const char *cname = ro->name;
  const char *oname = "LGTeacher";//LGTeacher
  if (cname && (strcmp(cname, oname) == 0)) {
  // printf("_read_images - _getObjc2ClassList 类名 :%s - %p\n",cname,cls);
  }
  if(cls —> isRealize)
  {
  remethodizeClass(cls);
  classExists == YES;
  }
  分类没有实现load —— 编译处理 — data() —> ro

• 3.非懒加载类 + 懒加载分类
  readImages — realizeClassWithoutSwift —— methodLizeClass — 不需要添加表 ——直接 data() ——> ro

• 4.懒加载类 + 懒加载分类
  消息发送的时候 ——lookupImpOrForward —— realizeClassWithoutSwift —— methodLizeClass — addUnattacheCategoryForClass 不进来 ——直接走data()

该部分参考链接 https://juejin.im/post/5e1c36fcf265da3df47adc1f

面试题

1.runtime底层api使用？

#import 
#import "LGTeacher.h"

void lgSetter(NSString *value){
    printf("%s/n",__func__);
}

NSString *lgName(){
    printf("%s/n",__func__);
    return @"master NB";
}

void lg_class_addProperty(Class targetClass , const char *propertyName){
    
    objc_property_attribute_t type = { "T", [[NSString stringWithFormat:@"@\"%@\"",NSStringFromClass([NSString class])] UTF8String] }; //type
    objc_property_attribute_t ownership0 = { "C", "" }; // C = copy
    objc_property_attribute_t ownership = { "N", "" }; //N = nonatomic
    objc_property_attribute_t backingivar  = { "V", [NSString stringWithFormat:@"_%@",[NSString stringWithCString:propertyName encoding:NSUTF8StringEncoding]].UTF8String };  //variable name
    objc_property_attribute_t attrs[] = {type, ownership0, ownership,backingivar};

    class_addProperty(targetClass, propertyName, attrs, 4);

}

void lg_printerProperty(Class targetClass){
    unsigned int outCount, i;
    objc_property_t *properties = class_copyPropertyList(targetClass, &outCount);
    for (i = 0; i < outCount; i++) {
        objc_property_t property = properties[i];
        fprintf(stdout, "%s %s\n", property_getName(property), property_getAttributes(property));
    }
}


// class_data_bits_t

int main(int argc, const char * argv[]) {
    @autoreleasepool {


    // ro - rw  - 有没有面试题
        
        // 1: 动态创建类
        Class LGPerson = objc_allocateClassPair([NSObject class], "LGPerson", 0);
        // 2: 添加成员变量 1<ISA()->changeInfo(RW_CONSTRUCTED, RW_CONSTRUCTING | RW_REALIZING);
        // cls->changeInfo(RW_CONSTRUCTED, RW_CONSTRUCTING | RW_REALIZING);
       
        // 3.1 添加property - rw
        lg_class_addProperty(LGPerson, "subject");
        lg_printerProperty([LGTeacher class]);
        lg_printerProperty(LGPerson);

        // 3.2 添加setter  +  getter 方法
        class_addMethod(LGPerson, @selector(setSubject:), (IMP)lgSetter, "v@:@");
        class_addMethod(LGPerson, @selector(subject), (IMP)lgName, "@@:");

        // 开始使用
        id person = [LGPerson alloc];
        [person setValue:@"KC" forKey:@"lgName"];
        NSLog(@"%@",[person valueForKey:@"lgName"]);
        
        id teacher = [LGTeacher alloc];
        [teacher setValue:@"iOS" forKey:@"subject"];
        NSLog(@"%@",[teacher valueForKey:@"subject"]);
        
        [person setValue:@"master" forKey:@"subject"];
        NSLog(@"%@",[person valueForKey:@"subject"]);
    }
    return 0;
}


// 相关api 的注释

/**
 * 创建类对
 *superClass: 父类，传Nil会创建一个新的根类
 *name: 类名
 *extraBytes: 0
 *return:返回新类，创建失败返回Nil，如果类名已经存在，则创建失败
  objc_allocateClassPair(<#Class  _Nullable __unsafe_unretained superclass#>, <#const char * _Nonnull name#>, <#size_t extraBytes#>)
 */


/**
 *添加成员变量
 *
 *cls 往哪个类添加
 *name 添加的名字
 *size 大小
 *alignment 对齐处理方式
 *types 签名
 *
 *这个函数只能在objc_allocateClassPair和objc_registerClassPair之前调用。不支持向现有类添加一个实例变量。
 *这个类不能是元类。不支持在元类中添加一个实例变量。
 *实例变量的最小对齐为1 << align。实例变量的最小对齐依赖于ivar的类型和机器架构。对于任何指针类型的变量，请通过log2(sizeof(pointer_type))。
  class_addIvar(<#Class  _Nullable __unsafe_unretained cls#>, <#const char * _Nonnull name#>, <#size_t size#>, <#uint8_t alignment#>, <#const char * _Nullable types#>)
 */

/**
 *往内存注册类
 *
 * cls 要注册的类
 *
 * objc_registerClassPair(<#Class  _Nonnull __unsafe_unretained cls#>)
 */


/**
 *往类里面添加方法
 *
 *cls 要添加方法的类
 *sel 方法编号
 *imp 函数实现指针
 *types 签名
 *
 *class_addMethod(<#Class  _Nullable __unsafe_unretained cls#>, <#SEL  _Nonnull name#>, <#IMP  _Nonnull imp#>, <#const char * _Nullable types#>)
 */



/**
 *往类里面添加属性
 *
 *cls 要添加属性的类
 *name 属性名字
 *attributes 属性的属性数组。
 *attriCount 属性中属性的数量。
 *
 *class_addProperty(<#Class  _Nullable __unsafe_unretained cls#>, <#const char * _Nonnull name#>, <#const objc_property_attribute_t * _Nullable attributes#>, <#unsigned int attributeCount#>)
 */

2.类拓展&分类

类拓展：匿名分类，增加属性和方法；
在上一章我们知道分类是在read_images中加载到内存段的，而类拓展会在编译时作为类的一部分编译到内存段中，所以可以直接读取ro

!!! 如果类拓展没有被import到.m文件中时，是不会被编译到当前类的内存段的

分类如何增加属性？
1.通过关联对象

2.系统会管理一张关联对象的大的HashMap，然后每个类(拥有关联对象的类)又会有一张小的HashMap管理关联属性；以对象的指针为key找到小表然后将value、“name”、关联策略存起来,取值过程刚好相反

截屏2020-02-1912.16.33.png

3.当对象delloc的时候，又会去系统中查找当前对象有没有关联对象，有的话就先移除，之后再delloc

3.load方法加载顺序

先看源码：
load_images()

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        mutex_locker_t lock2(runtimeLock);
        //准备非懒加载类load方法，将其放在一个数组中
        prepare_load_methods((const headerType *)mh);
    }
   //遍历加载load方法
    call_load_methods();
}

call_load_methods();

void call_load_methods(void)
{
    static bool loading = NO;
    bool more_categories;

    loadMethodLock.assertLocked();

    // Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

    void *pool = objc_autoreleasePoolPush();

    do {
        // 1.先调用主类的load方法
        while (loadable_classes_used > 0) {
            call_class_loads();
        }

        // 2.再调用分类的load方法 Call category +loads ONCE
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    objc_autoreleasePoolPop(pool);

    loading = NO;
}

从上面的源码可以得到load方法：

• 主类的load方法会先于分类load方法
• 主类和分类拥有相同方法时，分类会被调用，因为每个分类的方法会通过attachlist方法以数组的形式插入到二维数组的前面，所以会造成分类覆盖主类的假象
• 如何修改分类load方法顺序 - target - Bulid Phases - Complies Sources，shuoebi 就是调整编译顺序

initialize在什么时候调用

IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
    IMP imp = nil;
    bool triedResolver = NO;

    ...
    ...

    if (initialize && !cls->isInitialized()) {
        cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
        // runtimeLock may have been dropped but is now locked again

        // If sel == initialize, class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }
    ...
    ...
    return imp;
}

可以看到initialize方法在lookUpImpOrForward在这个方法后调用；所以我们总结下

• 晚于load方法调用
• +initialize方法
  通过消息机制调用(objc_msgSend)，当子类没有实现+initialize方法时，会调用父类的initialize方法
• 先调用父类的+initialize方法，后调用子类的+initialize方法
• 如果一个类有分类，那么会调用最后编译的分类实现的