Introduction

MVP (Model-View-Presenter) is an architectural pattern derived from MVC, which is frequently used in the creation of mobile applications. This pattern allows you to separate presentation logic from business logic. This separation helps make the code more maintainable, scalable, and testable.

Components

The MVP has three components:

• Model. Represents data and business logic.

• View. It’s the user interface. Also contains de Controller (UIVIewController).

• Presenter. It acts as a bridge between the Model and the View. On the one hand, it receives the events of the View and passes them to the Model and, on the other hand, it updates the View when the data changes in the Model.

The Presenter receives user input from the View, processes it, and updates the Model. In turn, the Model notifies the View of any changes to the data.

Example code

Now we are going to see a simple example of applying the MVP pattern.

To do this, let’s create a small app with a single screen that will have a TextField, a Button and a Label. When we add a username in the TextField and click on the button, the label will show “My name is: " and the name that we have entered.

Model

The Model is very simple, it is a struct with a single parameter: the name of the user.

struct User {
    var name: String
}

Presenter

The Presenter has a reference to the View but, to avoid a strong reference cycle, is a weak reference. A strong reference cycle occurs when two objects have strong references to each other, resulting in an unbreakable retain cycle. Because the objects cannot be deallocated by the system, this can result in a memory leak.

class UserPresenter {
    
    weak var userView: UserViewDelegate?
    var user = User(name: "")

    init(userView: UserViewDelegate) {
        self.userView = userView
    }

    func updateUserName(name: String) {
        user.name = name
        userView?.showNameForUser(user: user)
    }
}

By holding a weak reference to the View, the Presenter can only communicate with it as long as it exists. That is, when the View is destroyed, the weak reference becomes null and the Presenter can no longer access said View. This helps ensure that memory is managed correctly in the application.

View and UIViewController

The UIViewController is in charge of instantiating both the View and the Presenter. Once both are instantiated, the View is passed to the Presenter and the Presenter is also assigned to the view to establish communication.

Finally, the View’s setupView method is called to create its components and is assigned to the UIViewController’s view.

class UserViewController: UIViewController {
    
    var userView = UserView()
    
    init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func loadView() {
        super.loadView()
        setupView()
    }
    
    func setupView() {
        let presenter = UserPresenter(userView: userView)
        userView.presenter = presenter
        userView.setupView()
        self.view = userView
    }
}

In the View, we can see the reference to the Presenter (which we assign from the UIViewContrioller) and how this reference is used to call the Presenter’s updateUserName(name:) method to update the model. Once updated, the Prensenter (as we have seen) calls the View’s showNameForUser(user: User) method thanks to the delegate we have created.

protocol UserViewDelegate: class {
    func showNameForUser(user: User)
}

class UserView: UIView, UserViewDelegate {
    
    let nameTextField = UITextField()
    let updateNameButton = UIButton()
    let userNameLabel = UILabel()
    
    var presenter: UserPresenter!
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setupView()
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func setupView() {
        backgroundColor = .white
        
        nameTextField.frame = CGRect(x: 20, y: 100, width: 200, height: 44)
        nameTextField.borderStyle = .roundedRect
        addSubview(nameTextField)
        
        updateNameButton.frame = CGRect(x: 20, y: 150, width: 200, height: 44)
        updateNameButton.setTitle("Update Name", for: .normal)
        updateNameButton.backgroundColor = .blue
        updateNameButton.addTarget(self, action: #selector(updateNameButtonTapped), for: .touchUpInside)
        addSubview(updateNameButton)
        
        userNameLabel.frame = CGRect(x: 20, y: 200, width: 200, height: 44)
        userNameLabel.backgroundColor = .lightGray
        addSubview(userNameLabel)
    }
    
    @objc func updateNameButtonTapped() {
        presenter.updateUserName(name: nameTextField.text ?? "")
    }
    
    func showNameForUser(user: User) {
        userNameLabel.text = "My name is: \(user.name)"
    }
}

Advantages and Disadvantages of MVP

Advantages

• Compared to the MVC pattern, it offers a greater separation of duties.
• Although it is a bit more complex than MVC, it is derived from it, so we can quickly get used to using it.
• Business logic tests could be improved.

Disadvantages

• Because it’s more complex than MVC, it’s typically not recommended for use in small, simple applications.
• The Presenter in MVP has the same potential to become an important component as the Controller in MVC.
• Even though we have further modularized the design, there are still some issues, such as continuous controller control over screen switching.

Conclusion

MVP (Model-View-Presenter) is an architectural pattern that separates the application’s presentation layer from its business logic. This allows for a clear separation of responsibilities and reduces component coupling. The View in MVP serves as a passive interface that displays data and forwards user actions to the Presenter, while the Presenter serves as a mediator between the View and the Model, updating the View based on Model changes and forwarding user actions to the Model. The Model represents the application’s data and business logic.