KVO的本质

1.生成子类

系统会通过runtime动态生成一个名为NSKVONotifying_xxx(被监听类名)继承自被监听类的子类
例如 YHObject  -> NSKVONotifying_YHObject
调用方法 objc_allocateClassPair([YHObject class], "YHObject", 0);
 
 @interface YHObject : NSObject
 
 @property (nonatomic, assign) int age;
 
 @end
 
 @implementation YHObject
 
 @end
 
 
 @interface NSKVONotifying_YHObject : YHObject
 
 @end
 
 @implementation NSKVONotifying_YHObject
 
 - (void)setAge:(int)age
 {
     _NSSetIntValueAndNotify();
 }
 
 // 伪代码
 void _NSSetIntValueAndNotify()
 {
     [self willChangeValueForKey:@"age"];
     [super setAge:age];
     [self didChangeValueForKey:@"age"];
 }
 
 - (void)didChangeValueForKey:(NSString *)key
 {
     // 通知监听器，某某属性值发生了改变
     [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
 }
 
 // 屏幕内部实现，隐藏了NSKVONotifying_MJPerson类的存在
 - (Class)class
 {
     return [YHObject class];
 }
 
 - (void)dealloc
 {
 // 收尾工作
 }
 
 - (BOOL)_isKVOA
 {
 return YES;
 }

 @end

2.重写父类的set方法

  子类重写被监听属性的set方法,在set方法里面调用  _NSSetIntValueAndNotify();
  
  _NSSetIntValueAndNotify的大概实现是这样的
  [self willChangeValueForKey:@"age"];  // 将会改变
  [super setAge:age];                   // 调用父类的set方法给成员变量赋值
  [self didChangeValueForKey:@"age"];   // 改变

  didChangeValueForKey
  通知监听器某某属性发生了改变既调用公共的方法
  [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];

3子类重写class方法

// 返回原来的类来迷惑开发者,隐藏实现原理
  - (Class)class
  {
      return [YHObject class];
  }