A 主题或 Observable 没有当前值。当一个值被发出时，它被传递给订阅者并且 Observable 就完成了。

A Subject or Observable doesn't have a current value. When a value is emitted, it is passed to subscribers and the Observable is done with it.

如果你想要要获得当前值，请使用 BehaviorSubject ，这是为了这个目的而设计的。 BehaviorSubject 保留最后一次发出的值，并立即将其发送给新订阅者。

If you want to have a current value, use BehaviorSubject which is designed for exactly that purpose. BehaviorSubject keeps the last emitted value and emits it immediately to new subscribers.

它还有一个方法 getValue（）获取当前值。

It also has a method getValue() to get the current value.