Runtime之理解Instance、Class、MetaCla

原文链接:https://www.jianshu.com/p/6b1192f6d671 转载请注明出处, 谢谢
相信很多人都已经知道了他们三者的关系了, 下面借用一张图

来源: http://www.sealiesoftware.com/blog/class%20diagram.pdf
我们先来动态创建一个类

//动态创建一个新类
- (void)createNewClass{
    Class newClass = [self createNewClass:"SubError"];
    //创建这个类的实例对象
    id instance = [[newClass alloc] initWithDomain:@"some domain" code:0 userInfo:nil];
    //调用方法
    SEL sel = NSSelectorFromString(@"TestMetaClass"); // 获取定义的方法
    //这里报错, Build Setting -> apple clang preprocessing -> Enable Strict Checking of objc_msgSend Calls 设置成NO
    id result = objc_msgSend(instance,sel); //调用该方法
}

//创建的这个类继承于NSError
- (Class)createNewClass:(char *)className{
    //创建一个NSError的子类 SubError
    Class newClass = objc_allocateClassPair([NSError class], className, 0);
    //添加实例对象方法 IMP函数指针
    SEL sel = sel_registerName("TestMetaClass"); //注册一个方法选择器 不能重复注册同一个选择器
    class_addMethod(newClass, sel, (IMP)TestMetaClass, "v@:");
    //注册这个类
    objc_registerClassPair(newClass);
    return newClass;
}

//实例对象、类、元类等的关系
void TestMetaClass(id self, SEL _cmd) {
    
    [self setValue:@"zhang" forKey:@"name"];
    NSLog(@"This objcet is %p", self); // TestClass类的实例对象
    
    //获取类的名称
    const char *className1 = class_getName([self class]);
    const char *className2 = object_getClassName([self class]);
    NSString *className3 = NSStringFromClass([self class]);
    NSLog(@"ClassName is %s, %s, %@", className1, className2, className3);
    //获取类
    Class class1 = [self class];//调用实例对象的方法
    Class class2 = object_getClass(self);//根据实例对象获取类
    Class class3 = objc_getClass(className1);//根据类名获取类
    NSLog(@"Class is %p, %p, %p", class1, class2, class3);
    //父类
    Class superClass1 = class_getSuperclass(class1);
    Class superClass2 = [self superclass];
    NSLog(@"SupperClass is %p, %p", superClass1, superClass2);
    
    //获取元类
    Class metaClass = objc_getMetaClass(className1);
    
    //判断是否为元类
    BOOL isMetaClass = class_isMetaClass(metaClass);
    
    if (isMetaClass) {
        NSLog(@"%@ is meta class", metaClass);
    }
    NSLog(@"NSError's metaClass is %@", objc_getMetaClass(object_getClassName([NSError class])));
    NSLog(@"NSError's metaClass is %p", objc_getMetaClass(object_getClassName([NSError class])));
    NSLog(@"Class is %@, super class is %@, meta class is %@, metaclass's superclass is %@", [self class], [self superclass], metaClass, class_getSuperclass(metaClass));
    NSLog(@"Class is %p, super class is %p, meta class is %p, metaclass's superclass is %p", [self class], [self superclass], metaClass, class_getSuperclass(metaClass));
    //获取实例的大小
    size_t size = class_getInstanceSize(class1);
    NSLog(@"%zu",size);    
}

忽略其他的日志输出

2019-05-22 09:40:38.779045+0800 RuntimeDemo[996:17329] This objcet is 0x6000015de730
2019-05-22 09:40:38.779215+0800 RuntimeDemo[996:17329] ClassName is SubError, SubError, SubError
2019-05-22 09:40:38.779313+0800 RuntimeDemo[996:17329] Class is 0x6000015de580, 0x6000015de580, 0x6000015de580
2019-05-22 09:40:38.779391+0800 RuntimeDemo[996:17329] SupperClass is 0x10072c208, 0x10072c208
2019-05-22 09:40:38.779467+0800 RuntimeDemo[996:17329] SubError is meta class
2019-05-22 09:40:38.779542+0800 RuntimeDemo[996:17329] NSError's metaClass is NSError
2019-05-22 09:40:38.779608+0800 RuntimeDemo[996:17329] NSError's metaClass is 0x10072c258
2019-05-22 09:40:38.779692+0800 RuntimeDemo[996:17329] Class is SubError, super class is NSError, meta class is SubError, metaclass's superclass is NSError
2019-05-22 09:40:38.779800+0800 RuntimeDemo[996:17329] Class is 0x6000015de580, super class is 0x10072c208, meta class is 0x6000015de1c0, metaclass's superclass is 0x10072c258
2019-05-22 09:40:38.779875+0800 RuntimeDemo[996:17329] 48

这个instance是SubError类的实例对象, SubError类的父类是NSError, SubError类的元类是SubError, NSError类的元类是NSError, SubError元类的父类是NSError元类,具体的你要理解isa指针, 下面我们再来看NSObject
我们先来一组错误的案例

NSLog(@"NSObject's class is %p", [NSObject class]);
NSLog(@"NSObject's meta class is %p", objc_getClass((__bridge void *)[NSObject class]));
NSLog(@"NSObject's class is %p", objc_getClass("NSObject"));//object_getClassName([NSObject class])
NSLog(@"NSObject's meta class is %p", objc_getMetaClass("NSObject"));

2019-05-22 09:52:42.129790+0800 RuntimeDemo[1114:23535] NSObject's class is 0x10f389ec8
2019-05-22 09:52:42.129895+0800 RuntimeDemo[1114:23535] NSObject's meta class is 0x0
2019-05-22 09:52:42.129971+0800 RuntimeDemo[1114:23535] NSObject's class is 0x10f389ec8
2019-05-22 09:52:42.130053+0800 RuntimeDemo[1114:23535] NSObject's meta class is 0x10f389e78

网上有好多博客是这么说明NSObject自身和元类的关系的, 但是你仔细看输出结果的第二句和第四句, 打印的地址不一致, 而且他们认为objc_getClass这个方法返回的是元类, 是不是元类可以用系统的方法来判断, 下面来看一下我的做法

NSObject *obj = [[NSObject alloc] init];
NSLog(@"instance is %p", obj);
Class objClass = [obj class];
const char *className = class_getName([obj class]);
Class objSuperClass = class_getSuperclass(objClass);
Class metaClass = objc_getMetaClass(className);
if (class_isMetaClass(metaClass)) {
    NSLog(@"%@ is meta class, %p", metaClass, metaClass);
}
NSLog(@"MetaClass name is %s", class_getName(metaClass));
NSLog(@"Class is %p, super class is %p, meta class is %p, metaclass's superclass is %p", objClass, objSuperClass, metaClass, class_getSuperclass(metaClass));

2019-05-22 10:10:23.803262+0800 RuntimeDemo[1313:32937] instance is 0x600003384370
2019-05-22 10:10:23.803389+0800 RuntimeDemo[1313:32937] NSObject is meta class, 0x1095dde78
2019-05-22 10:10:23.803447+0800 RuntimeDemo[1313:32937] MetaClass name is NSObject
2019-05-22 10:10:23.803519+0800 RuntimeDemo[1313:32937] Class is 0x1095ddec8, super class is 0x0, meta class is 0x1095dde78, metaclass's superclass is 0x1095ddec8

从打印输出的结果可以看出, 使用class_getMetaClass方法可以可以获取到元类, NSObject的父类是nil, 元类是NSObject元类, NSObject元类的父类是NSObject自身, 这样就符合上图的指向逻辑了, 最后形成了一个闭环
这里我们可以猜测(__bridge void *)的作用并不是像别人说的(类型转换 转换成id), 而是转换成了(void *)空指针类型也就是nil

参考文献:
http://blog.devtang.com/2013/10/15/objective-c-object-model/
http://southpeak.github.io/2014/10/25/objective-c-runtime-1/