iOS 源码分析 Class 本质，objc_class，class_rw_t，class_ro_t 分析
我们先来看下源码内部对clas的定义
typedef struct objc_class *Class;
可以看出来，他就是一个 objc_class 指针
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() {
return bits.data();
}
我们摘出来重要的部分，可以看出来，我们的 objc_class 是继承 objc_object 结构体
struct objc_object {
private:
isa_t isa;
我们的 objc_object 有 isa 指针，所以我们的 class 也有 isa 指针，这就是为什么 我们可以通过我们的类对象找到我们的元类的原因，因为我们的额class 有 isa 指针，如果没有isa指针是不行的，然后还有个 superclass class 指针，只用来指向当前类的父类的，苹果有一张图，画了父类和元类的关系，指向，class 的这个结构体，完全可以诠释那张图，cache_t 我之前的文章也讲过，是加快查找效率的，可以看看我那篇文章，然后，下一个很重要的东西就是 bits
struct class_data_bits_t {
// Values are the FAST_ flags above.
uintptr_t bits;
bits 可以理解为一个指针，里面存放着 class_rw_t 和 class_ro_t 的地址，之前文章也有说过，苹果为了节省空间，让一个指针里面保存更多的信息，到时候通过按位与运算，取出不同的值就行。
比如当我们取出来我们class的data的时候,实际上就是取出来这个类的一些信息
class_rw_t *data() {
return bits.data();
}
可以看到，返回的是一个 class_rw_t 结构体，然后调用 bits.data();，我们看下实现
class_rw_t* data() {
return (class_rw_t *)(bits & FAST_DATA_MASK);
}
这段代码什么意思呢？就是就是找到 bits 这个指针中 class_rw_t 这个结构体对应的那个data的值，通过按位与运算，#define FAST_DATA_MASK 0x00007ffffffffff8UL 其实就是取出来，bits中第3-64位，然后就拿到了我们想要的值，这个设计是不是很牛，然后我们看下 class_rw_t 这个结构体
struct class_rw_t {
// Be warned that Symbolication knows the layout of this structure.
uint32_t flags;
uint32_t version;
const class_ro_t *ro;
method_array_t methods;
property_array_t properties;
protocol_array_t protocols;
Class firstSubclass;
Class nextSiblingClass;
从结构体重可以看到，累的 方法，属性，协议，都保存在这里，然后还有个 const class_ro_t *ro; 这个是什么呢？
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize;
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name;
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars;
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
1. baseMethodList 方法列表
2. baseProtocols 协议列表
3. ivars 成员变量列表
4. baseProperties 属性列表
5. weakIvarLayout weak 成员变量内存布局
6. ivarLayout 成员变量 ivar 内存布局，是放在我们的 io 里面的，并且是 const 不允许修改的，也就是说明，我们的 成员变量布局，在编译阶段就确定了，内存布局已经确定了，在运行时是不可以修改了，这就说明了，为什么运行时不能往类中动态添加成员变量。
class_ro_t 的意思是 readonly 的，在编译阶段就已经确定了，不可以修改。
class_ro_t 是只读的，是再编译的时候，将累的属性，方法，协议和成员变量，添加到我们的 class_ro_t 中，然后运行的时候，会动态的创建 class_rw_t 然后将 class_ro_t 和分类中的属性，协议方法存储到 class_rw_t 中，并进行排序，分类中的存储在数组的前部，原始类信息，存储在数组的后面，class_ro_t 是只能的，在运行时是不可以添加进去的
class_rw_t 是运行时可以添加的，比如分类中的方法会在运行时，添加到 class_rw_t 的 method_array_t methods; 中去，可以看到，我们的 class_rw_t 中没有成员变量的信息，成员变量的信息是以编译就确定添加到 class_ro_t 中去，并且只读
接着跟着源码分析下
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);
// 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);
return cls;
}
我们在 alloc 的时候，会调用上面的方法，然后紧接着有个判断
// 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);
}
判断我们的rw是否创建过了，如果没有，我们会创建一个rw的结构体，然后将ro赋值给rw中的ro，然后将rw赋值给class，
supercls = realizeClassWithoutSwift(remapClass(cls->superclass));
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()));
然后递归遍历他的父类和元类，同样的方式分配。最后
// Attach categories
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
}
可以看到
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);
}
是将ro里面改的方法列表啊，属性列表还有协议列表，添加到rw里面,然后紧接着
// Attach categories.
category_list *cats = unattachedCategoriesForClass(cls, true /*realizing*/);
attachCategories(cls, cats, false /*don't flush caches*/);
static void
attachCategories(Class cls, category_list *cats, bool flush_caches)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
bool isMeta = cls->isMetaClass();
// fixme rearrange to remove these intermediate allocations
method_list_t **mlists = (method_list_t **)
malloc(cats->count * sizeof(*mlists));
property_list_t **proplists = (property_list_t **)
malloc(cats->count * sizeof(*proplists));
protocol_list_t **protolists = (protocol_list_t **)
malloc(cats->count * sizeof(*protolists));
// Count backwards through cats to get newest categories first
int mcount = 0;
int propcount = 0;
int protocount = 0;
int i = cats->count;
bool fromBundle = NO;
while (i--) {
auto& entry = cats->list[i];
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
mlists[mcount++] = mlist;
fromBundle |= entry.hi->isBundle();
}
property_list_t *proplist =
entry.cat->propertiesForMeta(isMeta, entry.hi);
if (proplist) {
proplists[propcount++] = proplist;
}
protocol_list_t *protolist = entry.cat->protocols;
if (protolist) {
protolists[protocount++] = protolist;
}
}
auto rw = cls->data();
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
rw->methods.attachLists(mlists, mcount);
free(mlists);
if (flush_caches && mcount > 0) flushCaches(cls);
rw->properties.attachLists(proplists, propcount);
free(proplists);
rw->protocols.attachLists(protolists, protocount);
free(protolists);
}
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
rw->methods.attachLists(mlists, mcount);
free(mlists);
if (flush_caches && mcount > 0) flushCaches(cls);
rw->properties.attachLists(proplists, propcount);
free(proplists);
rw->protocols.attachLists(protolists, protocount);
free(protolists);
将分类中的发昂发，属性，协议，添加到rw中
总结
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize;
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name;
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars;
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
最后看一眼我们的 class_ro_t ，有三个属性是不允许我们修改的
const uint8_t * ivarLayout;
const char * name;
const ivar_list_t * ivars;
当我们初始化一个类的时候
- 在编译的时候已经确定我们的类的原始信息，并将它存储在 class_ro_t 结构体中，并且运行时不能改变
- 递归初始化他的父类和元类
- 将ro中的方法协议属性等，添加到rw对应的数组中
- 将分类中的属性方法协议添加到rw中
- 在运行时，不能动态的想类中添加成员变量，还有弱引用成员变量，和修改类名
- 因为运行时，我们的rw对ro进行了引用，ro的方法列表协议列表添加到了我们的rw对用的数组中，所以就给我们在运行时对方法等做动态修改提供了可能。
很多人可能会有疑问，runtime 不是提供了 动态添加成员变量的方法 class_addIvar() ，但是苹果的官方文档已经有明确的说明
This function may only be called after objc_allocateClassPair and before objc_registerClassPair. Adding an instance variable to an existing class is not supported.
必须在alloc 和 register 之间调用，之前说过，程序编译的时候就生成了成员变量布局，程序启动后就没有机会再添加成员变量了，
因为我们的类实例是需要一块内存空间的，他有isa指针指向，如果我们在运行时允许动态修改成员变量的布局，那么创建出来的类实例就属于无效的了，能够被所以修改，但是属性和方法是我们 objc_class 可以管理的，增删改都不影响我们实例内存布局。
