概述

KVO官网
KVO键值观察是一种机制，它允许对象被通知对其他对象的指定属性的更改.

KVO使用

- (void)viewDidLoad {
    [super viewDidLoad];
    // NSKeyValueObservingOptionNew = 0x01,       //返回新值
    //NSKeyValueObservingOptionOld = 0x02,       //返回旧值
    //NSKeyValueObservingOptionInitial           //注册的时候就会发一次通知，改变的时候也会发通知
   // NSKeyValueObservingOptionPrior         //改变之前发一次，改变之后发一次
    
    QHPerson *person = [[QHPerson alloc] init];
    person.name = @"123";
    [person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    
    _p = person;
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    
    _p.name = @"qinhan";
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"change:%@",change);
}

上面的是自动发送通知，kvc也提供了手动发送通知

//默认是自动
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    return NO;
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [_p willChangeValueForKey:@"name"];
    _p.name = @"qinhan";
    [_p didChangeValueForKey:@"name"];
}

如果我们想见观察多个属性

 QHPerson *person = [[QHPerson alloc] init];
 person.name = @"123";
    //[person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
 [person addObserver:self forKeyPath:@"son" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

+(NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    NSSet *keypaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"son"]) {
        
        NSArray *arr = @[@"_son.nickName",@"_son.age"];
        keypaths = [keypaths setByAddingObjectsFromArray:arr];
    }
     return keypaths;
}

这样我们可以观察person属性的son实例对象

kvo探索

1.我们分别定义了属性、成员变量，然后分别观察

 [person addObserver:self forKeyPath:@"nickName" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
[person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    
    self.p.name =@"qinhan";
    self.p->nickName = @"nickName";
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"change:%@",change);
}

打印结果

2021-06-05 22:22:14.983537+0800 001---KVC基础使用[12792:12309284] change:{
 kind = 1;
new = qinhan;

发送我们只监听到属性、没有监听到成员变量。可见kvo实际上时观察setter方法
2.探索addObserver
在调用addObserver前先打一个断点，

截屏2021-06-05 下午10.26.58.png

截屏2021-06-05 下午10.27.25.png

- (void)printClasses:(Class)cls{

    // 注册类的总数
    int count = objc_getClassList(NULL, 0);
    // 创建一个数组， 其中包含给定对象
    NSMutableArray *mArray = [NSMutableArray arrayWithObject:cls];
    // 获取所有已注册的类
    Class* classes = (Class*)malloc(sizeof(Class)*count);
    objc_getClassList(classes, count);
    for (int i = 0; i<count; i++) {
        if (cls == class_getSuperclass(classes[i])) {
            [mArray addObject:classes[i]];
        }
    }
    free(classes);
    NSLog(@"classes = %@", mArray);
}

结果如下图：

截屏2021-06-05 下午10.41.44.png

模拟KVO实现

注册自定义addobserver方法

  [person qh_addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

自定义一个NSObject 分类

- (void)qh_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    //自定义NSKVONotify_object 子类
    NSString *oldClassName = NSStringFromClass(self.class);
    NSString *newClassName = [@"QHKVO_" stringByAppendingString:oldClassName];
    //创建一个类
    Class myclass = objc_allocateClassPair(self.class, newClassName.UTF8String, 0);
    
    //注册新类
    objc_registerClassPair(myclass);
    
    //动态修改 isa 指向中间类
    object_setClass(self, myclass);
    
    //给中间类重新添加setter方法
    class_addMethod(myclass, @selector(setName:), (IMP)setName, "v@:@");
    
    //将观察者绑定到对象上面
    objc_setAssociatedObject(self, (__bridge  const void *)@"objc", observer,OBJC_ASSOCIATION_ASSIGN );
}
void setName(id self,SEL _cmd ,NSString *newName){
    struct objc_super person1 = {self,class_getSuperclass([self class])};
    //调用父类的方法
    objc_msgSendSuper(&person1, _cmd,newName);
    NSLog(@"newName:%@",newName);
    
    //拿到观察者
  id observer =  objc_getAssociatedObject(self, (__bridge  const void *)@"objc");
    
    //调用observer valueforkey 发送通知
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),@"name",self,@{@"name":newName});
}

总结KVO

• 验证是否存在setter方法
• 动态生成子类，需要重写class、setter方法
• 在子类的setter方法中向父类发消息
• 调用通知方法，把修改后的信息返回回去