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.