Splitting the View Models of TableFlip, a Short Aftermath

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I executed my plan from earlier this week to split TableFlip’s monolithic view model into sub-view models. The process wasn’t too complicated. The highlights: The current hierarchy of controllers in the view layer is as follows: The window doesn’t look too complicated and there truly are not that many view components involved, but still it’s quite some work to keep things well coordinated. The TableViewController file clocks in at about 400 lines of code although it mostly delegates stuff to other objects. There’s still room for improvement, but I have to see patterns, first. I even extracted NSTableViewaDataSource and NSTableViewDelegate into different objects months ago. Today I doubt this was a good idea in the first place. We’ll see.

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I'll Split Up the Monolithic View Model in TableFlip. Here's What I Expect to Happen

The upcoming changes in TableFlip’s user experience made me ponder how I structured the app’s modules. The bottleneck I created in my app is simple: there’s one presenter which transforms the model into a view model; the presenter defines a protocol for the view it expects and passes the view model to the object that satisfies this contract.

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5 Heuristics for "I have a complicated nested view controller setup. How do I handle passing data?"

That’s a recent question from the comments put in my own words. A view model that encapsulates the display state sounds promising at first. But when you don’t have a simple view that displays one thing, how do you model that in code? How do you model a sequence of view controllers, for example a more complex checkout process?

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How to Couple UITableView Cell Identifiers and View Models

Rui Peres proposes to make UITableViewCell view models the topmost model data. Traditionally, Cocoa developers stored something in arrays that corresponded to the indexPath of a cell. In principle, this qualifies as “model” data already, but it’s not yet a view model. In practice, it can even be something different than a view model entirely – and make your view controllers slimmer!

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Separate Read Model from Write Model to Support Complex Forms

One thing that repeatedly messes up the conceptual purity of a view model is figuring out which entity should be mutated upon user interaction. Part of this purity stems from the fact that a view model is best served as view data to stress that it doesn’t contain much (business) logic. Making the data mutable introduces a lot of problems figuring out what that view model really is.

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iOS View Architectures and VIPER Explained

There’s an excellent overview of MVC, MVP, MVVM, and VIPER in iOS with sample codes by Bohdan Orlov of Badoo I recommend you read.

There are two main takeaways:

  1. One of the commenters nailed it: any MV* variant focuses on architecting view components. Only VIPER takes the application as a whole into account. MVC by itself is an architectural pattern, but not an approach to app architecture in itself.
  2. A UIViewController belongs into the user interface layer and is, in fact, a composite view itself. That’s confusing at first because of the name. This insight will liberate you from thinking that a view controller is sufficient to glue data to UIKit components. There’s room for a whole application between these two.

The VIPER example is exceptionally good. It takes VIPER’s heritage of Clean Architecture and Hexagonal into account and defines the Interactor through an output port. In that way Bohdan’s sample code is more east-oriented and cleaner than what you’d usually find on the topic:

protocol GreetingProvider {
    func provideGreetingData()

protocol GreetingOutput: class {
    func receiveGreetingData(greetingData: GreetingData)

class GreetingInteractor : GreetingProvider {
    weak var output: GreetingOutput!

    func provideGreetingData() {
        let person = Person(firstName: "David", lastName: "Blaine") // usually comes from data access layer
        let subject = person.firstName + " " + person.lastName
        let greeting = GreetingData(greeting: "Hello", subject: subject)

Usually, you’d model provideGreetingData() as a function that returns the data to the caller. This will cause trouble in async processes of course.

You see in the full example that the amount of types seem to explode. Don’t be afraid of that as long as you can assign each type a specific responsibility. Then it won’t turn into the mess everyone seems to be afraid of.

Having used VIPER in apps myself, I see a problem with the names, though. XYZInteractor and XYZPresenter aren’t much better than XYZController in terms of expressiveness. On top of that, a concrete Presenter is always modelled to act as event handler, too. Don’t let this fool you into thinking you absolutely have to do this yourself – there’s always room to separate writing from reading operations, or event handling from view population.

Model-View-View Model in Swift

Srdan Rasic wrote a good post on creating wrappers around value objects (like Strings, say) and custom-glue them to an application’s interface. It’s a technique I have used in my book on Mac architecture at one point but which seems to be tremendously useful for virtually anything view-related once you get to a high enough level of abstraction.

It’s weird that articles related to programming are so hard to quote. Instead of a quotation, let the resulting code illustrates what he has to say:

class ArticleViewController {
  var bodyTextView: UITextView
  var titleLabel: UILabel
  var dateLabel: UILabel
  var thumbnailImageView: UIImageView
  var viewModel: ArticleViewViewModel {
    didSet {
      viewModel.title.bindAndFire {
        [unowned self] in
        self.titleLabel.text = $0
      viewModel.body.bindAndFire {
        [unowned self] in
        self.bodyTextView.text = $0
      viewModel.date.bindAndFire {
        [unowned self] in
        self.dateLabel.text = $0
      viewModel.thumbnail.bindAndFire {
        [unowned self] in
        self.thumbnailImageView.image = $0

So now when viewModel’s properties change, the view controller will update the corresponding interface component. Sweet.

Exploring Mac App Development Strategies – Patterns & Best Practices for Clean Software Architecture on the Mac. Don’t worry about big software design decisions anymore. Take on Mac app development with solid principles, guided by extensive examples and explanation. Buy the book now!