LoginViewModel - An example of how a LoginViewModel might look like for a login module, including: validation, talking to a repository, coordinator and view (via delegation).

LoginViewModel.swift

import Foundation

protocol LoginViewModelInterface {
    var viewDelegate: LoginViewModelDelegate? {get set}
    func handleLogin(username: String, password: String)
}

/// You're coordinator implements this
protocol LoginViewModelCoordinatorDelegate: class {
    func goToLoginSuccessful()
    func goToSignup()
}

/// You're view module implements this
protocol LoginViewModelDelegate: class {
    func showAlert(title: String?, message: String?)
}

enum LoginEntryValidation: Error {
    case UsernameEmpty
    case UsernameTooShort
    
    case PasswordEmpty
    case PasswordTooShort
    
    var localizedDescription: String {
        switch self {
        case .UsernameEmpty:
            return "Please enter a username"
        case .UsernameTooShort:
            return "Your username must be greater than 5 characters"
        case .PasswordEmpty:
            return "Please enter a password"
        case .PasswordTooShort:
            return "Your password must be greater than 5 characters"
        }
    }
}

class LoginViewModel: LoginViewModelInterface {
    
    weak var coordinatorDelegate: LoginViewModelCoordinatorDelegate?
    weak var viewDelegate: LoginViewModelDelegate?
    
    private let repository: LoginRepositoryInterface!
    
    init(repository: LoginRepositoryInterface = RemoteLoginRepository()) {
        self.repository = repository
    }
    
    func handleLogin(username: String, password: String) {
        guard isUserCredentialsValid(username: username, password: password) else { return }
        
        repository.authenticate(username: username, password: password) { [weak self] (result) in
            guard let `self` = self else { return }
            
            switch result {
            case .success(let authToken):
                self.repository.saveAuthToken(authToken)
            case .failure(let error):
                self.viewDelegate?.showAlert(title: "Login Error", message: error.localizedDescription)
            }
        }
    }
}

private extension LoginViewModel {
    func isUserCredentialsValid(username: String, password: String) -> Bool {
        do {
            try validateUsername(username)
            try validatePassword(password)
            
            return true
        } catch (let error as LoginEntryValidation) {
            viewDelegate?.showAlert(title: "Login Error", message: error.localizedDescription)
        } catch {
            fatalError("Unknown error occurred")
        }
        
        return false
    }
    
    func validateUsername(_ username: String) throws {
        guard username != "" else {
            throw LoginEntryValidation.UsernameEmpty
        }
        
        guard username.count >= 5 else {
            throw LoginEntryValidation.UsernameTooShort
        }
    }
    
    func validatePassword(_ password: String) throws {
        guard password != "" else {
            throw LoginEntryValidation.PasswordEmpty
        }
        
        guard password.count >= 5 else {
            throw LoginEntryValidation.PasswordTooShort
        }
    }
}