给控件增加监听事件后出现错误

An instance 0xca90200 of class UILabel was deallocated while key value observers were still registered with it

一个错误，大意是UILabel被内存释放后，它得值依然在被观察

原因是给UILabel增加了个监听事件

[self.valueLabel addObserver:self
                  forKeyPath:@"text"
                     options:NSKeyValueObservingOptionNew
                     context:nil];

然后在UIView被pop后，监听时间没有停止，解决办法为：
- (void)dealloc
{
[_valueLabel removeObserver:self forKeyPath:@"text"];
}