一.原理
KVO是系統(tǒng)用運行時機(jī)制給需要觀察的對象增加一個子類的方法實現(xiàn)的，如果一個對象被觀察，系統(tǒng)會給他增加以各子類，重寫它的set方法，從而實現(xiàn)觀察對象屬性變化的功能，為了使外部看不出來，系統(tǒng)還重寫了被觀察對象的-class，-isEqual:等方法。
二.自定義KVO(僅實現(xiàn)key,未實現(xiàn)keyPath)
1.這個方式是給要觀察的類添加一個子類

- (Class)achiveNewSubClass:(NSString *)className
          withCurrentClass:(Class)currentClass
{
  Class newClass = objc_allocateClassPair([self class],
                                          className.UTF8String,
                                          0);
  
  // 可以在這里給類添加成員變量等
  objc_registerClassPair(newClass);
  return newClass;
}

2.這個是重寫被觀察對象的set方法

void kvo_setter(id self, SEL selecter, id newValue) {
  
  KHHandle handle = objc_getAssociatedObject(self,
                                             @"123".UTF8String);
  NSString * key = achiveKeyPath(selecter);
  
  
  struct objc_super superclazz = {
    .receiver = self,
    .super_class = class_getSuperclass(object_getClass(self))
  };
  
    id oldValue = [self valueForKey:key];

  void (* objc_msgSendToSuper)(struct objc_super * ,SEL , id) = (void *)objc_msgSendSuper;
  // 在這里調(diào)取運行時創(chuàng)建的子類的父類的set方法賦值
  objc_msgSendToSuper(&superclazz, selecter, newValue);
    
  NSDictionary * dict = @{KHKeyValueNew : newValue ?:  @"",
                          KHKeyValueOld : oldValue ?: @"",
                          };
  handle(dict);
}

3.這個是提供給外部調(diào)用的方法

- (void)KH_addObserverWithKeyPath:(NSString *)keyPath
                        withBlock:(KHHandle)handle
{
  Class currentClass = [self class];
  NSString * className = NSStringFromClass(currentClass);
  if ([className hasPrefix:CLASS_PREFIX]) return;
  className = [CLASS_PREFIX stringByAppendingString:className];
  Class subClass =NSClassFromString(className);
  if (subClass) return;
  objc_setAssociatedObject(self,
                           @"123".UTF8String,
                           handle,
                           OBJC_ASSOCIATION_COPY_NONATOMIC);
  subClass = [self achiveNewSubClass:className
                    withCurrentClass:currentClass];
  
  object_setClass(self, subClass);
  
  SEL selecter = NSSelectorFromString(achiveMethodName(keyPath));
  
  Method me = class_getInstanceMethod(currentClass, selecter);
  
  const char * types =  method_getTypeEncoding(me);
  
 BOOL isSuccess = class_addMethod(subClass, selecter, (IMP)kvo_setter, types);
  
  
  NSLog(@"%d",isSuccess);
}