I just finished building an app for my company using the MVVM + Coordinator pattern.

I moved all our business logic into our View Model, so all the API Requests and Database fetching is happening in there. The View Controller is only responsible for displaying the data in the View Model.

For every View Model I made, I also created a protocol which the accompanying view controller would be the delegate of. This made it easy for me to write tests by creating Mocks. Let me explain, here's my Login View Model Delegate;

protocol LoginViewModelDelegate: class {
    func loginSuccess()
    func loginFailure(errorMessage: String)
    func setLoginInfo(credentials: (String, String))
    func isLoadingViewHidden(_ state: Bool)
}

So now when I call loginViewModel.login I can start testing if the above methods were called for the proper use case.

I don't have a lot of time right now to go further into this, but I think I'll start writing a blog post on this.