How to define a pointer to a class instantiation in a common block

Using Swift
1

I have a common block for constants and variables which all the classes can refer to. How do I declare a variable which will be set by another class? Example: in the common block I have:

var MainView: CMainView

In AppDelegate.swift I need to initialize it:

MainView = CMainView.init(...)

and then other modules need to refer to it. I have tried everything I can think of. What's the answer?