ObservedClass {

    TimeObserver getTimeObserver() {
        return TimeObserver{[&] { return getTimeRunning(); }, [&] { return getTimeRemaining(); }};
    }

    Duration getTimeRunning();
    Duration getTimeRemaining();
}

TimeObserver {
public:
    using DurationFunction = std::function<Duration()>;

    TimeObserver(DurationFunction timeRunning, DurationFunction timeRemaining);

    Duration getTimeRunning() {
        return _timeRunning();
    }

    Duration getTimeRemaining() {
        return _timeRemaining();
    }

private:
    DurationFunction _timeRunning;
    DurationFunction _timeRemaining;
}