@Nullable
    protected T latestValue() {
        // TODO(ikhoon): Make this method abstract in Armeria 2.0 to avoid the mistake of not defining
        //               the latest value in subclasses.
        return null;
    }
Returns the latest value notified before.
{@code null} if the value has not been initialized yet or the implementation of this class cannot
cache it.
core/src/main/java/com/linecorp/armeria/common/util/AbstractListenable.java