type SystemData = (
        WriteStorage<'s, Ball>,
        WriteStorage<'s, Transform>,
        Read<'s, EventChannel<ScoreEvent>>,
        Read<'s, InputHandler<StringBindings>>,
        Read<'s, Time>,
    );

    fn setup(&mut self, res: &mut Resources) {
        Self::SystemData::setup(res);
        self.waiting_for_input = true;
        self.reader = Some(res.fetch_mut::<EventChannel<ScoreEvent>>().register_reader());
    }

    fn run(&mut self, (
        mut balls,
        mut transforms,
        score_channel,
        input,
        time
    ): Self::SystemData) {
        // If all balls are gone, create a new one.
        for event in score_channel.read(self.reader.as_mut().unwrap()) {
            if balls.count() == 0 {
                self.waiting_for_input = true;

            }
        }