Model controllers

The next step is to extract validation and networking logic into their own controllers. That way, we can easily extend their features, while maintaining a simple API surface:

class PostMessageNetworking {
    func post(message: String,
              callback: @escaping (Bool, Error?) ->  Void) {
        /* original call */
    }
}

class MessageValidator {
    enum ValidationError: Error {
    case emptyMessage
    }
    
    func validate(message: String) -> ValidationError? {
        if message.count == 0 {
            return .emptyMessage
        }
        return nil
    }
}

We extracted both features, validation and networking, into two independent controllers for the model layer. As you can see, none of them interact with the view layer. Their interfaces are simple: one has a callback, the other returns an optional Error.

All of the logic is done, and we can test every component independently and easily:

• The view controller can be subjected to UI testing, in order to ensure that the button tap calls the delegate properly, and so on.
• The network controller can be tested against serialization.
• The validator can be unit tested.

The last step is to implement the controller that will tie all those objects together.