Interactor

An Interactor is responsible for data logic. It is the layer that retrieves the data from any source it needs to. For instance, it can communicate with a Service or a local data store such as CoreData.

It has outlets only to the Presenter of the VIPER stack, something like this:

weak var presenter: InteractorToPresenterInterface!

Communicating with a Service

Lets take a look at a typical flow. Lets say your user wants to login to the application, so the presenter has been told the user wants to login with their username and password:

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

//Interactor.swift
lazy var loginService: LoginService = LoginService()
func login(withUserName username: String, andPassword password: String) {
    loginService.login(withUsername: username, andPassword: password,
        success: { (user: User) in
             self.loggedIn(withUser: user)
        },
        failure: { (error: Error) in
             self.failedLogin(withError: error)
        })
}

Here, the Interactor is told to login with the username and password by the Presenter. The Interactor knows that it needs to make a call to the loginService to login the user and get the user object from the web service. It can then implement the success and failure completion blocks as it needs to.

Communicating with the Presenter

So the loginService succeeded, and the success block of the Service is ran, and now this information needs to be conveyed to the Presenter.

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

Communicating with a DataStore

So what if instead of calling a Service, you instead want to get information from some sort of data manager like Realm? Lets say we are making a jogging application that records the user's jogging sessions.

//Presenter.swift
func beganPresenting() {
    interactor.fetchJogs()
}

//Interactor.swift
func fetchJogs() {
    let realm = try! Realm()
    let allJogs = realm.objects(Jog.self)
    presenter.fetchedJogs(Array(allJogs))
}

Notice the interface to the Interactor from the Presenter is the same as if the Interactor was going to call a Service. The Presenter has no idea how the Interactor fetches jogs (or logs in the user). The Interactor is responsible for this interaction.