EmberJS + EmberFire：如何根据登录到网站的用户数显示/隐藏模态

I am currently using EmberJS 2.6 and EmberFire to build a real time game. I want to display a modal that says 'Waiting for Opponents' when there is only one user in the website and hidden when there are 2.I am currently using EmberJS 2.6 and EmberFire