mockimp.swift

import UIKit

protocol ViewModelType {
}

protocol Service {
    func request(_ path: String, completion: @escaping ([String]) -> Void)
}

struct MainViewModel: ViewModelType {
    private(set) var service: Service
    
    init(_ service: Service) {
        self.service = service
    }
    
    func getMainArticles(completion: @escaping ([String]) -> Void) {
        service.request("/path/articles", completion: completion)
    }
    
    func articlesLoaded() {
        
    }
}

class MainViewController: UIViewController {
    
    private(set) var viewModel: MainViewModel
    private let indicator = UIActivityIndicatorView(frame: .zero)
    private let tableView = UITableView(frame: .zero)
    
    init(_ viewModel: MainViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }
    
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(indicator)
        view.addSubview(tableView)
    }
    
    // triggered somewhere
    private func callArticles() {
        
        indicator.startAnimating()
        
        viewModel.getMainArticles { [weak self] articles in
            // do something
            // lets assume we're notifiying our view model for updating for it's own logic
            self?.viewModel.articlesLoaded()
            // after loading/parsing complete dissmiss animation
            self?.indicator.stopAnimating()
            self?.tableView.reloadData()
        }
    }
}