代码自己实现，深入探究KVO的内部实现

#import "NSObject+KVO.h"
#import <objc/message.h>

@implementation NSObject_KVO


- (void)WK_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context{

    /*
     自定义子类
     重写方法
     修改isa指针
     */


    NSString *oldClassName =   NSStringFromClass([self class]);
    NSString *newClassName =  [ @"WKKVO_" stringByAppendingString:oldClassName];

    const char *newName = [newClassName UTF8String];

    Class Mycalss =  objc_allocateClassPair([self class], newName, 0);


    //添加set方法。相当于重写！！
    /*
     子类没有这个方法就会去父类找这个方法，但是并不代表子类拥有了这个方法。
     */

    class_addMethod(Mycalss, NSSelectorFromString(@"setName"), (IMP)setName, "[e-mail]:@");

    //注册这个类
    objc_registerClassPair(Mycalss);

    //修改self的isa指针
    object_setClass(self, Mycalss);

    //将观察者保存到当前对象
    objc_setAssociatedObject(self, (__bridge const void *)@"objc", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

}


void  setName (id self , SEL _cmd ,NSString *newName){

    //保存当前类型
    id class =  [self class];
    //改变isa指针
    object_setClass(self, class_getSuperclass(class));
    //调用父类set方法
    objc_msgSend(self, @selector(setName:),newName);



    //拿出观察者
   id objc =  objc_getAssociatedObject(self, (__bridge const void *)@"objc");

   //通知观察者
    objc_msgSend(objc, @selector(observeValueForKeyPath:ofObject:change:context:), @"name",self,nil,nil);

    //改回子类类型
    object_setClass(self, class);



    /*
     将XCode升级到6后，报Too many arguments to function call, expected 0, have *，在XCode5.1里能编译通过的，到xcode6就报错

     objc_msgSend(objc, @selector(observeValueForKeyPath:ofObject:change:context:), @"name",self,nil,nil);

     Too many arguments to function call, expected 0, have *

     问了下度娘，

     选中项目 - Project - Build Settings - ENABLE_STRICT_OBJC_MSGSEND 将其设置为 NO 即可
     */


}


@end