KVO

镇楼专用

类的单个属性观察

观察值的4中模式

/**
 *  NSKeyValueObservingOptionNew     新值
 *  NSKeyValueObservingOptionOld     旧值
 *  NSKeyValueObservingOptionInitial 注册发送通知，改变时发送通知
 *  NSKeyValueObservingOptionPrior   改变之前发送通知，改变后发送通知
 */

针对Person类里面的name属性进行观察

[person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

查看观察

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

打印效果

2019-03-22 09:20:16.875033+0800 KVO[1844:81256] {
    kind = 1;
    new = 0;
}

类的里面嵌套其他的属性观察

观察Person里面Dog类的age，通过点语法直接监听

[person addObserver:self forKeyPath:@"dog.age" options:NSKeyValueObservingOptionNew context:nil];

输入结果

2019-03-22 09:23:25.854859+0800 KVO[1876:84141] {
    kind = 1;
    new = 11;
}

一次性观察多个值

例如观察Person里面的Dog的age和level，则在Person类中重新+ (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key

+ (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"dog"]) {
        NSArray *array = @[@"_dog.name", @"_dog.level"];
        keyPaths = [keyPaths setByAddingObjectsFromArray:array];
    }
    return keyPaths;
}

直接观察Dog类

[person addObserver:self forKeyPath:@"dog" options:NSKeyValueObservingOptionNew context:nil];

打印结果

2019-03-22 09:27:11.485240+0800 KVO[1929:87568] {
    kind = 1;
    new = "<Dog: 0x6000027c0b80>";
}

手动模式

在Person类中重写+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
    // 修改为手动模式
    return NO;
}

手动触发

[self.person willChangeValueForKey:@"name"];
[self.person didChangeValueForKey:@"name"];

观察容器类

需要利用KVC
例如观察person中的array

 [[self.person mutableArrayValueForKey:@"array"] addObject:@"obj"];

输出结果

2019-03-22 11:02:13.996095+0800 KVO[5521:170395] {
    indexes = "<_NSCachedIndexSet: 0x600002216160>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 2;
    new =     (
        obj
    );
}

---

KVO的底层实现

实质：观察值的setter方法
通过runtime，新建一个子类继承于观察类（NSKVONotifing_类名），动态的修改了观察类的类型为子类的类型,在子类里面重写set方法，调用

- (void)setName:(NSString *)name {
    [self willChangeValueForKey:@"name"];
    [super setName:name];
    [self didChangeValueForKey:@"name"];   
}