iOS实例、类、元类

从源码看本质

源代码：

@interface XKPerson ()

@property (copy, nonatomic) NSString *name;

@end

@implementation XKPerson

+ (instancetype)personWithName:(NSString *)name {
    XKPerson *p = [XKPerson new];
    p.name = name;
    return p;
}

转化成cpp，整理相关代码：

// 对象信息的实际存储者
struct _class_ro_t {
    unsigned int flags;
    unsigned int instanceStart;
    unsigned int instanceSize;
    const unsigned char *ivarLayout;
    const char *name;
    const struct _method_list_t *baseMethods;
    const struct _objc_protocol_list *baseProtocols;
    const struct _ivar_list_t *ivars;
    const unsigned char *weakIvarLayout;
    const struct _prop_list_t *properties;
};

// 定义了XKPerson的元类
static struct _class_ro_t _OBJC_METACLASS_RO_$_XKPerson = {
    1, sizeof(struct _class_t), sizeof(struct _class_t), 
    0, 
    "XKPerson",
    (const struct _method_list_t *)&_OBJC_$_CLASS_METHODS_XKPerson,
    0, 
    0, 
    0, 
    0, 
};

// 定义了XKPerson的类对象
static struct _class_ro_t _OBJC_CLASS_RO_$_XKPerson = {
    0, __OFFSETOFIVAR__(struct XKPerson, _name), sizeof(struct XKPerson_IMPL), 
    0, 
    "XKPerson",
    (const struct _method_list_t *)&_OBJC_$_INSTANCE_METHODS_XKPerson,
    0, 
    (const struct _ivar_list_t *)&_OBJC_$_INSTANCE_VARIABLES_XKPerson,
    0, 
    0, 
};

// 对象的管理者
struct _class_t {
    // 传说中的isa
    struct _class_t *isa;
    // 传说中的superclass
    struct _class_t *superclass;
    // 缓存相关
    void *cache;
    // 暂不使用
    void *vtable;
    // 对象信息的实际保存者
    struct _class_ro_t *ro;
};

struct _class_t OBJC_METACLASS_$_NSObject;

// 是_class_t，用于管理_XKPerson元类
struct _class_t OBJC_METACLASS_$_XKPerson = {
    0, // &OBJC_METACLASS_$_NSObject,
    0, // &OBJC_METACLASS_$_NSObject,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_METACLASS_RO_$_XKPerson,
};

struct _class_t OBJC_CLASS_$_NSObject;

// 是_class_t，管理XKPerson的类对象
struct _class_t OBJC_CLASS_$_XKPerson __attribute__ ((used, section ("__DATA,__objc_data"))) = {
    0, // &OBJC_METACLASS_$_XKPerson,
    0, // &OBJC_CLASS_$_NSObject,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_CLASS_RO_$_XKPerson,
};

// 真正使用的是_class_t，需要对齐进行初始化
static void OBJC_CLASS_SETUP_$_XKPerson(void ) {
  // 所有元类的isa指向根源类，也就是metaNSObject
  // metaNSObject的isa指向跟类对象，也就是classNSObject
    OBJC_METACLASS_$_XKPerson.isa = &OBJC_METACLASS_$_NSObject;
  // 元类的superclass指向父元类
    OBJC_METACLASS_$_XKPerson.superclass = &OBJC_METACLASS_$_NSObject;
    OBJC_METACLASS_$_XKPerson.cache = &_objc_empty_cache;

  // 类对象的isa指向元类
    OBJC_CLASS_$_XKPerson.isa = &OBJC_METACLASS_$_XKPerson;
  // 类对象的superclass指向父[类对象]
  // 跟类对象(classNSObject)指向为nil，触发消息转发机制
    OBJC_CLASS_$_XKPerson.superclass = &OBJC_CLASS_$_NSObject;
    OBJC_CLASS_$_XKPerson.cache = &_objc_empty_cache;
}

实例-类对象-元类

为什么 [[obj class] class] 方法获取不到元类

实例对象、类对象、元类的获取代码如下：

        NSObject *obj1 = [[NSObject alloc] init];
        NSObject *obj2 = [[NSObject alloc] init];
        NSObject *obj3 = [[NSObject alloc] init];
        
        NSLog(@"instance address:%p %p %p",obj1, obj2, obj3);
        NSLog(@"class address: %p %p %p",[obj1 class], [obj2 class], [obj3 class]);
        NSLog(@"meta class address:%p %p %p",object_getClass([obj1 class]), object_getClass([obj2 class]), object_getClass([obj3 class]));
        NSLog(@"error meta class address:%p %p %p",[[obj1 class] class], [[obj2 class] class], [[obj3 class] class]);

打印结果：

2020-11-04 09:05:26.264248+0800 XKObjTest[82779:33963605] instance address:0x10285f9a0 0x10285ffb0 0x10285e3d0
2020-11-04 09:05:26.265428+0800 XKObjTest[82779:33963605] class address: 0x7fff9b7d5140 0x7fff9b7d5140 0x7fff9b7d5140
2020-11-04 09:05:26.265520+0800 XKObjTest[82779:33963605] meta class address:0x7fff9b7d50f0 0x7fff9b7d50f0 0x7fff9b7d50f0
2020-11-04 09:05:26.265600+0800 XKObjTest[82779:33963605] error meta class address:0x7fff9b7d5140 0x7fff9b7d5140 0x7fff9b7d5140
Program ended with exit code: 0

我们都知道[[instance class] class]这样是无法获取到元类的，那原因是什么呢？查找 objc 源码得到 class 方法的源码：

+ (Class)class {
    return self;
}

- (Class)class {
    return object_getClass(self);
}

如上图所示，虽然 class 方法本质是调用 object_getClass，但是类方法 class 确是直接返回自己，也就是类对象，所以类对象无论调用多少次 class 方法，返回的都是类对象自己。

而 object_getClass 的源码如下：

Class object_getClass(id obj)
{
    if (obj) return obj->getIsa();
    else return Nil;
}

这个方法直接返回的是 isa 指针的指向，所以可以获取到元类；