What is MVVM?

The Model-View-ViewModel (MVVM) design pattern is a powerful architectural pattern that helps developers create scalable and maintainable iOS applications. By separating concerns and enhancing testability, MVVM has gained popularity among iOS developers. In this article, we'll explore what MVVM is, why it's important, and how to implement it in your iOS projects. In this article we take the UIKIt framework as an example. But MVVM also works really well when we use SwiftUI!

• Model: Represents the data and business logic of the application. It manages the data, rules, and logic.


• View: The user interface of the application. It displays the data and forwards user interactions to the ViewModel.


• ViewModel: Acts as an intermediary between the Model and the View. It retrieves data from the Model, formats it for the View, and handles user input logic.

Why Use MVVM?

MVVM offers several benefits for iOS development:

• **Separation of Concerns:** MVVM separates the application's concerns, making it easier to manage and scale.


• **Testability:** The ViewModel can be tested independently of the View, leading to more reliable code.


• **Data Binding:** In MVVM, data binding facilitates communication between the View and the ViewModel, ensuring that changes in the data are automatically reflected in the UI.

Implementing MVVM in iOS

Let's walk through a simple example of implementing MVVM in an iOS application. We'll create a basic app that displays a list of movies.

Model

The Model contains the data and business logic. Here’s a simple model for a Movie:

struct Movie {
    let title: String
    let director: String
}

ViewModel

The ViewModel handles the presentation logic and interacts with the Model. Here’s a simple ViewModel for managing movies:

import Foundation

class MovieViewModel {
    private var movies: [Movie] = []
    var moviesDidChange: (() -> Void)?
    
    var numberOfMovies: Int {
        return movies.count
    }
    
    func movie(at index: Int) -> Movie {
        return movies[index]
    }
    
    func loadMovies() {
        // Simulate fetching data
        movies = [
            Movie(title: "Inception", director: "Christopher Nolan"),
            Movie(title: "The Matrix", director: "Wachowski Sisters")
        ]
        moviesDidChange?()
    }
}

View

The View displays data to the user. In this case, we will use a UITableView to display the list of movies:

import UIKit

class MovieViewController: UIViewController, UITableViewDataSource {
    @IBOutlet weak var tableView: UITableView!
    
    private var viewModel: MovieViewModel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        viewModel = MovieViewModel()
        viewModel.moviesDidChange = { [weak self] in
            self?.tableView.reloadData()
        }
        tableView.dataSource = self
        viewModel.loadMovies()
    }
    
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return viewModel.numberOfMovies
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "MovieCell", for: indexPath)
        let movie = viewModel.movie(at: indexPath.row)
        cell.textLabel?.text = movie.title
        cell.detailTextLabel?.text = movie.director
        return cell
    }
}

Conclusion

The MVVM design pattern is a powerful tool for iOS developers, offering a clear separation of concerns, enhanced testability, and improved data binding. By adopting MVVM, you can create more maintainable and scalable applications.