KVO相关

下边三种方式都可以：

    [self addObserver:self forKeyPath:@"self.now" options:(NSKeyValueObservingOptionNew) context:nil];
    [self addObserver:self forKeyPath:@"now" options:(NSKeyValueObservingOptionNew) context:nil];
    [self addObserver:self forKeyPath:@"_now" options:(NSKeyValueObservingOptionNew) context:nil];

触发监听

self.now = [NSDate date];// 正常自动触发监听
_now = [NSDate date];// 不能触发监听

/// 手动触发KVO    
[self willChangeValueForKey:@"_now"];
[self didChangeValueForKey:@"_now"];

重写set方法事，不需要调用willChangeValueForKey和didChangeValueForKey两个方法，如果同时加了，会重复调响应observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context方法；

KVO机制，重写了set方法，必须有self.now(setNow)的操作，不然不能触发监听；

    //这三种方式都可以触发
    [self setValue:now forKey:@"now"];
    [self setNow:[NSDate date]];
    self.now = [NSDate date];
    //这两种不能触发
    [self setValue:now forKey:@"_now"];
    _now = [NSDate date];