显示登录Viewcontroler并解除它

So I have a simple app running flawlessly, now I want to add login screen, in other words when the app opens it check a boolean property (userIsAuthorisedBool) if yes continue running app normally, if no, switch user to Sign in screen (SigninViewController). The problem is checking and switching user to login screen. as well as dismissing it. here's my simple storyboard. So I have a simple app running flawlessly, now