一点点扩展：

非懒加载 的类才会走进下面3637行代码，直接跑工程并未进去，我们如何走进去呢 --> 给 MyPerson 类添加 +load() 方法：

load 方法的实现为何就会使 MyPerson 的方法提前加载呢？ --> load 方法是在 load_images 时就会调用的 --> 若类都没有实现如何 load_images 呢 --> so 方法都提前了

realizeClassWithoutSwift (cls, nil ) ;// 在此之前，cls 还只是一个地址和名字，但是 data 数据信息还是macho中，还并未加载读取到cls的内存中。通过 realizeClassWithoutSwift() 实现，下面对此函数源码进行探究。

tip : 我们之前已知，在消息的转发中 lookUpIMPAndForward 流程中，类必须是已经实现的，若没有实现则必须 realizeClassWithoutSwift() 实现。这里就可知了原因，类必须是已经存在完备的，若是类没有实现，我们无法调其中任何方法的，alloc init 自然也是无法操作实例化出来。

那么 懒加载 呢？我们注释掉 +load{} 方法，run，打印堆栈信息：

我们在 main.m 中调用了 MyPerson *objc2 = [MyPerson alloc ] ; // 懒加载 - 敌不动我不动，敌动我动。

懒加载的意义：若没有懒加载，那么我们所有的类的实现全部处理(代码实现、方法排序、临时变量等)有大量的工作要做，这些都要在 main 函数前处理完，那么会造成 main 函数的启动很慢；懒加载使我们不调用的类不先加载，节省了内存，提高了性能。<-- 这里也验证了为何 load 方法能不写就不写的原因。

下面进行进入 realizeClassWithoutSwift() 流程分析。

一、类的加载

realizeClassWithoutSwift() 源码分析：

  1 /***********************************************************************
  2 * realizeClassWithoutSwift
  3 * Performs first-time initialization on class cls, 
  4 * including allocating its read-write data.
  5 * Does not perform any Swift-side initialization.
  6 * Returns the real class structure for the class. 
  7 * Locking: runtimeLock must be write-locked by the caller
  8 **********************************************************************/
  9 static Class realizeClassWithoutSwift(Class cls, Class previously)
 10 {
 11     runtimeLock.assertLocked();
 13     class_rw_t *rw;
 14     Class supercls;
 15     Class metacls;
 17     if (!cls) return nil;
 18     if (cls->isRealized()) return cls;
 19     ASSERT(cls == remapClass(cls));
 21     // fixme verify class is not in an un-dlopened part of the shared cache?
 23     auto ro = (const class_ro_t *)cls->data();
 24     auto isMeta = ro->flags & RO_META;// 元类判断
 25     if (ro->flags & RO_FUTURE) {
 26         // This was a future class. rw data is already allocated.
 27         rw = cls->data();
 28         ro = cls->data()->ro();
 29         ASSERT(!isMeta);
 30         cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
 31     } else {
 32         // Normal class. Allocate writeable class data.
 33         rw = objc::zalloc<class_rw_t>();// 开辟空间
 34         rw->set_ro(ro);// ro 数据赋值给 rw
 35         rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
 36         cls->setData(rw);// cls 重新将 rw 作为其 data 赋值
 37     }
 39 #if FAST_CACHE_META
 40     if (isMeta) cls->cache.setBit(FAST_CACHE_META);
 41 #endif
 43     // Choose an index for this class.
 44     // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
 45     cls->chooseClassArrayIndex();
 47     if (PrintConnecting) {
 48         _objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
 49                      cls->nameForLogging(), isMeta ? " (meta)" : "", 
 50                      (void*)cls, ro, cls->classArrayIndex(),
 51                      cls->isSwiftStable() ? "(swift)" : "",
 52                      cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
 53     }
 55     // Realize superclass and metaclass, if they aren't already.
 56     // This needs to be done after RW_REALIZED is set above, for root classes.
 57     // This needs to be done after class index is chosen, for root metaclasses.
 58     // This assumes that none of those classes have Swift contents,
 59     //   or that Swift's initializers have already been called.
 60     //   fixme that assumption will be wrong if we add support
 61     //   for ObjC subclasses of Swift classes.// cls 的所有的父类 元类 全部实现、确定 --> 继承链确定下来
 62     supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);// 父类 递归
 63     metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);// 元类 递归
 65 #if SUPPORT_NONPOINTER_ISA // isa 的设置
 66     if (isMeta) {
 67         // Metaclasses do not need any features from non pointer ISA
 68         // This allows for a faspath for classes in objc_retain/objc_release.
 69         cls->setInstancesRequireRawIsa();
 70     } else {
 71         // Disable non-pointer isa for some classes and/or platforms.
 72         // Set instancesRequireRawIsa.
 73         bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
 74         bool rawIsaIsInherited = false;
 75         static bool hackedDispatch = false;
 77         if (DisableNonpointerIsa) {
 78             // Non-pointer isa disabled by environment or app SDK version
 79             instancesRequireRawIsa = true;
 80         }
 81         else if (!hackedDispatch  &&  0 == strcmp(ro->name, "OS_object"))
 82         {
 83             // hack for libdispatch et al - isa also acts as vtable pointer
 84             hackedDispatch = true;
 85             instancesRequireRawIsa = true;
 86         }
 87         else if (supercls  &&  supercls->superclass  &&
 88                  supercls->instancesRequireRawIsa())
 89         {
 90             // This is also propagated by addSubclass()
 91             // but nonpointer isa setup needs it earlier.
 92             // Special case: instancesRequireRawIsa does not propagate
 93             // from root class to root metaclass
 94             instancesRequireRawIsa = true;
 95             rawIsaIsInherited = true;
 96         }
 98         if (instancesRequireRawIsa) {
 99             cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
100         }
101     }
102 // SUPPORT_NONPOINTER_ISA
103 #endif
104 　　 // 更新父类和元类
105     // Update superclass and metaclass in case of remapping
106     cls->superclass = supercls;
107     cls->initClassIsa(metacls);
109     // Reconcile instance variable offsets / layout.
110     // This may reallocate class_ro_t, updating our ro variable.
111     if (supercls  &&  !isMeta) reconcileInstanceVariables(cls, supercls, ro);
113     // Set fastInstanceSize if it wasn't set already.
114     cls->setInstanceSize(ro->instanceSize);
116     // Copy some flags from ro to rw
117     if (ro->flags & RO_HAS_CXX_STRUCTORS) {
118         cls->setHasCxxDtor();
119         if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
120             cls->setHasCxxCtor();
121         }
122     }
124     // Propagate the associated objects forbidden flag from ro or from
125     // the superclass.
126     if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
127         (supercls && supercls->forbidsAssociatedObjects()))
128     {
129         rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
130     }
132     // Connect this class to its superclass's subclass lists
133     if (supercls) {
134         addSubclass(supercls, cls);
135     } else {
136         addRootClass(cls);
137     }
139     // Attach categories
140     methodizeClass(cls, previously);
142     return cls;
143 }

我们仍然是通过自己的类 MyPerson 进行分析(系统类毕竟看不见其具体操作，自己写的类更易分析)。

1、源码分析

如下图 2500行 (上面源码23行) 读取 cls 对应的地址中的 data 信息，并对其进行格式强转， ro 中具体数据信息见下图：

源码 23行起，所做操作： 取 macho 格式中的 data 数据： ro (原始，只读的干净内存 ) 进行 class_ro_t 格式强转 --> 开辟 rw(可读写的脏内存) 空间 --> rw 赋值为 ro --> cls 重新赋值 rw 为 data .

问题：1、为何要在开辟出一份 rw 呢? --> 因为 iOS 的运行时，内存会被不断插入添加删除操作，防止对我们的原始数据的修改，所以 copy 一份干净内存 ro 到 rw 里面；

2、为何有了一个 rw了还要有一个 rwe 呢？--> 因为并非每个类都会进行动态的操作，为避免内存的大量操作，我们只对进行了动态操作的类进行 rwe 。rw 是从 ro 读取拷贝的。

关于干净/脏内存可以看下 WWDC 的视频介绍: Advancements in the Objective-C runtime 。

get_ro 代码:

1）36行 setData(rw) ：

2）我们继续向下执行 (上面源码的62行) ：

realizeClassWithoutSwift() 方法递归：cls 的所有父类元类全部实现确定下来 --> 继承链的确定(这里是 MyPerson 的继承链)

3）继续读代码(上面源码65行起)，isa 的设置，继续执行，cls 如下(地址:MyPerson 的元类信息)：

此时 bits 仍是无值的，继续执行。

4） setInstanceSize() 执行后 bits 开始有值了，然后继续对 Cxx 信息处理，见下图：

读取一下地址中的数据，各信息都是存在的(见下图)。而我们的内存 bits 目前是 0x00000034。

放开内部断点直接运行到 main 函数中：

可看到 bits 的内存地址是 0x000280340000003 除了 ‘34’ 其他的值是何时赋的呢？后面再进行探究。

5）我们继续源码的流程，在 Cxx 的处理之后走到 methodizeClass( cls, previously ) ，cls 的方法的处理。

2、 methodizeClass() 方法的处理

 1 源码2
 2 /***********************************************************************
 3 * methodizeClass
 4 * Fixes up cls's method list, protocol list, and property list.
 5 * Attaches any outstanding categories.
 6 * Locking: runtimeLock must be held by the caller
 7 **********************************************************************/
 8 static void methodizeClass(Class cls, Class previously)
10     runtimeLock.assertLocked();
12     bool isMeta = cls->isMetaClass();
13     auto rw = cls->data();
14     auto ro = rw->ro();
15     auto rwe = rw->ext();
17     // Methodizing for the first time
18     if (PrintConnecting) {
19         _objc_inform("CLASS: methodizing class '%s' %s", 
20                      cls->nameForLogging(), isMeta ? "(meta)" : "");
21     }
23     // Install methods and properties that the class implements itself.
24     method_list_t *list = ro->baseMethods();
25     if (list) {// 方法 list
26         prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
27         if (rwe) rwe->methods.attachLists(&list, 1);
28     }
30     property_list_t *proplist = ro->baseProperties;
31     if (rwe && proplist) {// 属性 list
32         rwe->properties.attachLists(&proplist, 1);
33     }
35     protocol_list_t *protolist = ro->baseProtocols;
36     if (rwe && protolist) {// 协议 list
37         rwe->protocols.attachLists(&protolist, 1);
38     }
40     // Root classes get bonus method implementations if they don't have 
41     // them already. These apply before category replacements.
42     if (cls->isRootMetaclass()) {
43         // root metaclass
44         addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
45     }
47     // Attach categories. 附着关联分类
48     if (previously) {
49         if (isMeta) {
50             objc::unattachedCategories.attachToClass(cls, previously,
51                                                      ATTACH_METACLASS);
52         } else {
53             // When a class relocates, categories with class methods
54             // may be registered on the class itself rather than on
55             // the metaclass. Tell attachToClass to look for those.
56             objc::unattachedCategories.attachToClass(cls, previously,
57                                                      ATTACH_CLASS_AND_METACLASS);
58         }
59     }
60     objc::unattachedCategories.attachToClass(cls, cls,
61                                              isMeta ? ATTACH_METACLASS : ATTACH_CLASS);
63 #if DEBUG
64     // Debug: sanity-check all SELs; log method list contents
65     for (const auto& meth : rw->methods()) {
66         if (PrintConnecting) {
67             _objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-', 
68                          cls->nameForLogging(), sel_getName(meth.name));
69         }
70         ASSERT(sel_registerName(sel_getName(meth.name)) == meth.name); 
71     }
72 #endif
方法列表的准备，prepareMethodLists(); 。
 1 static void 
 2 prepareMethodLists(Class cls, method_list_t **addedLists, int addedCount,
 3                    bool baseMethods, bool methodsFromBundle)
 5     runtimeLock.assertLocked();
 7     if (addedCount == 0) return;
 9     // There exist RR/AWZ/Core special cases for some class's base methods.
10     // But this code should never need to scan base methods for RR/AWZ/Core:
11     // default RR/AWZ/Core cannot be set before setInitialized().
12     // Therefore we need not handle any special cases here.
13     if (baseMethods) {
14         ASSERT(cls->hasCustomAWZ() && cls->hasCustomRR() && cls->hasCustomCore());
15     }
17     // Add method lists to array.
18     // Reallocate un-fixed method lists.
19     // The new methods are PREPENDED to the method list array.
21     for (int i = 0; i < addedCount; i++) {
22         method_list_t *mlist = addedLists[i];
23         ASSERT(mlist);
25         // Fixup selectors if necessary
26         if (!mlist->isFixedUp()) {
27             // 方法排序
28             fixupMethodList(mlist, methodsFromBundle, true/*sort*/);
29         }
30     }
32     // If the class is initialized, then scan for method implementations
33     // tracked by the class's flags. If it's not initialized yet,
34     // then objc_class::setInitialized() will take care of it.
35     if (cls->isInitialized()) {
36         objc::AWZScanner::scanAddedMethodLists(cls, addedLists, addedCount);
37         objc::RRScanner::scanAddedMethodLists(cls, addedLists, addedCount);
38         objc::CoreScanner::scanAddedMethodLists(cls, addedLists, addedCount);
39     }
方法排序 fixupMethodList() :
 1 static void 
 2 fixupMethodList(method_list_t *mlist, bool bundleCopy, bool sort)
 4     runtimeLock.assertLocked();
 5     ASSERT(!mlist->isFixedUp());
 7     // fixme lock less in attachMethodLists ?
 8     // dyld3 may have already uniqued, but not sorted, the list
 9     if (!mlist->isUniqued()) {
10         mutex_locker_t lock(selLock);
12         // Unique selectors in list.
13         for (auto& meth : *mlist) {
14             const char *name = sel_cname(meth.name);
15             meth.name = sel_registerNameNoLock(name, bundleCopy);
16         }
17     }
19     // Sort by selector address.
20     // 根据 方法的地址 进行排序
21     /**
22      若方法重名则根据 sel 排序：
23      1、sel 混乱则进行 fixedUp  2、sel 没有混乱则根据 imp 排序
24      */
26     if (sort) {
27         method_t::SortBySELAddress sorter;
28        /**
29         struct method_t {
30             SEL name;
31             const char *types;
32             MethodListIMP imp;
34             struct SortBySELAddress :
35                 public std::binary_function<const method_t&,
36                                             const method_t&, bool>
37             {
38                 bool operator() (const method_t& lhs,
39                                  const method_t& rhs)
40                 { return lhs.name < rhs.name; }// 根据名字进行排序
41             };
42         };
43         */
44         std::stable_sort(mlist->begin(), mlist->end(), sorter);
45     }
47     // Mark method list as uniqued and sorted
48     mlist->setFixedUp();
方法排序前 ro->baseMethods() 的list：
方法排序后 fixupMethodList() 的 list：
类加载流程的执行总结：
以上我们探究的是本类的加载，我们给本类添加分类后是如何加载的呢？
OC 底层探索 15 继续探究。