Presenter

The Presenter is where business logic lives. It is what drives all the other layers, making decisions based on events that happen in the other layers. Think of the Presenter like a manager, it knows what needs to happen to get certain task done, it knows who is best to do the job, and it tells those in its employ to do them, but doesn`t do any of the heavy lifting itself.

It has outlets to the other components of the VIPER stack, something like this:

weak var delegate: Delegate?
weak var interactor: PresenterToInteractorInterface!
weak var view: PresenterToViewInterface!
weak var wireframe: PresenterToWireframeInterface!
var moduleWireframe: Login {
     get {
         return self.wireframe as! Login
     }
}

Communicating with the Interactor

Lets take a look at a typical flow. Lets say your user wants to login to the application, so they have entered their username and password in the View, and now they tap the Login button.

//View.swift
@IBAction func loginTapped(sender: AnyObject) {
    let username = usernameTextField.text
    let password = passwordTextField.text
    presenter.userTappedLogin(withUsername: username, andPassword: password)
}

//Presenter.swift
func userTappedLogin(withUsername username: String, andPassword password: String) {
     interactor.login(withUsername: username, andPassword: password)
}

The View will tell the Presenter of the user event, and pass the related information. When the Presenter gets it, it will tell the Interactor that it needs to call a service to login the user with the username and password the user entered.

Communicating with the Delegate

Lets say the call to the login service succeeded, and the module now needs to tell the Delegate the user has been logged in.

//Interactor.swift
func loggedIn(withUser user: User) {
    presenter.loginSucceeded()
}

//Presenter.swift
func loginSucceeded() {
    delegate?.loggedIn(login: moduleWireframe)
}

The Interactor will tell the Presenter of the success, and the presenter decides to tell the Delegate that login succeeded.

Communicating with the View

What if the login failed? Maybe the username doesn't exist, or the password was incorrect.

//Interactor.swift
func failedLogin(withError error: Error) {
    presenter.loginFailed(withError: error)
}

//Presenter.swift
func loginFailed(withError error: Error) {
    view.displayLoginError(withDescription: error.description)
}

The Interactor will tell the Presenter that login failed, and pass the error along. The presenter decides to tell the View to display a login error with the description received from backend. The View can then decide how it displays said error, maybe with an alert, or just a label, what ever it wants to do.

Communicating with the Wireframe

Maybe the user forgot their password and the reset password module needs to be presented.

//View.swift
@IBAction func resetPasswordTapped(sender: AnyObject) {
    presenter.userTappedResetPassword()
}

//Presenter.swift
func userTappedResetPassword() {
    wireframe.presentResetPassword()
}

Here, the user event is reported from the View to the Presenter, since there is navigation away from the login stack, to the reset password stack, the Wireframe needs to be notified. The Presenter tells the Wireframe to present that module, however it needs to.