代碼自己實現，深入探究KVO的內部實現

原創

2020-02-22 11:08

#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{

    /*
     自定義子類
     重寫方法
     修改isa指針
     */


    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。相當於重寫！！
    /*
     子類沒有這個方法就會去父類找這個方法，但是並不代表子類擁有了這個方法。
     */

    class_addMethod(Mycalss, NSSelectorFromString(@"setName"), (IMP)setName, "v@:@");

    //註冊這個類
    objc_registerClassPair(Mycalss);

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

    //將觀察者保存到當前對象
    objc_setAssociatedObject(self, (__bridge const void *)@"objc", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

}


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

    //保存當前類型
    id class =  [self class];
    //改變isa指針
    object_setClass(self, class_getSuperclass(class));
    //調用父類set方法
    objc_msgSend(self, @selector(setName:),newName);



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

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

    //改回子類類型
    object_setClass(self, class);



    /*
     將XCode升級到6後，報Too many arguments to function call, expected 0, have *，在XCode5.1裏能編譯通過的，到xcode6就報錯

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

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

     問了下度娘，

     選中項目 - Project - Build Settings - ENABLE_STRICT_OBJC_MSGSEND 將其設置爲 NO 即可
     */


}


@end