Development iOS

Application Architecture with SwiftUI

June 15, 2022
Application Architecture with SwiftUI

What is Soft­ware Archi­tec­ture? #

Soft¬≠ware archi¬≠tec¬≠ture is one of those things that can feel over¬≠bear¬≠ing and ‚Äč‚Äúboil¬≠er-plate rid¬≠den‚ÄĚ when done wrong, espe¬≠cial¬≠ly at the begin¬≠ning of a project. It is not until your project grows sig¬≠nif¬≠i¬≠cant¬≠ly that a well-thought out struc¬≠ture bears fruit, and once your project grows to such a size it can be dif¬≠fi¬≠cult to adapt a bur¬≠geon¬≠ing code¬≠base to a con¬≠sis¬≠tent pat¬≠tern. It pays to think through a few things up front.

At the core of soft¬≠ware archi¬≠tec¬≠ture are the questions 

Where do I put this? Where should this busi¬≠ness log¬≠ic go? Where should I store this data? How do I get that data from its loca¬≠tion to a view?

We then must fol­low up with anoth­er question.

What does that mean for maintainability?

Soft¬≠ware projects are nev¬≠er real¬≠ly done. Plat¬≠forms change, users give feed¬≠back, stake¬≠hold¬≠ers make new pri¬≠or¬≠i¬≠ties and we are con¬≠stant¬≠ly mea¬≠sur¬≠ing user engage¬≠ment and try¬≠ing to improve the user expe¬≠ri¬≠ence. If we can set some con¬≠sis¬≠tent guide¬≠lines about where to put the func¬≠tion¬≠al ‚Äč‚Äúlev¬≠els‚ÄĚ of our app we can more eas¬≠i¬≠ly grow and adapt our projects over time.

Goals #

We think a good archi¬≠tec¬≠ture should improve our allowance to for¬≠get things. We want to max¬≠i¬≠mize the inde¬≠pen¬≠dence of each part of our app, even down to mak¬≠ing sure the View does not know where its data comes from. We think this helps with the main¬≠tain¬≠abil¬≠i¬≠ty of a project, if you have clear¬≠ly defined lines between your Views, your data, and how that data is manip¬≠u¬≠lat¬≠ed for dis¬≠play future devel¬≠op¬≠ers should be able to pick up just the pieces they need for a giv¬≠en task and not have to hold the entire app in their mind before mak¬≠ing a change or adding a feature.

We also want our archi¬≠tec¬≠ture to help facil¬≠i¬≠tate the dis¬≠cus¬≠sion around the ques¬≠tion ‚Äč‚Äúwhere should I put this func¬≠tion¬≠al¬≠i¬≠ty?‚ÄĚ Giv¬≠ing archi¬≠tec¬≠tur¬≠al guide¬≠posts for where things go can speed up devel¬≠op¬≠ment in the longterm, pay¬≠ing off the invest¬≠ment in set¬≠up we make at the start of a project. 

A few words on acronyms #

His¬≠tor¬≠i¬≠cal¬≠ly UIK¬≠it based iOS apps have their roots in an MVC set¬≠up (Mod¬≠el‚ÄČ‚ÄĒ‚ÄČView‚ÄČ‚ÄĒ‚ÄČCon¬≠troller). There much nuance and opin¬≠ion about how much log¬≠ic should go in a UIV¬≠iew¬≠Con¬≠troller, whether to sub¬≠class UIV¬≠iew, how to com¬≠pose views togeth¬≠er, and how to get from screen to screen. Over time we have set¬≠tled into a pat¬≠tern for our UIK¬≠it apps that uses some fla¬≠vor of MVC or MVVM but uses a sep¬≠a¬≠rate Coor¬≠di¬≠na¬≠tor con¬≠cept to man¬≠age get¬≠ting from screen to screen.

Swif¬≠tUI, the hot new declar¬≠a¬≠tive UI frame¬≠work for iOS, push¬≠es iOS archi¬≠tec¬≠ture toward an MVVM (Mod¬≠el ‚ÄĎView‚ÄČ‚ÄĒ‚ÄČView Mod¬≠el) set¬≠up and away from UIKit‚Äôs MVC roots. SwiftUI‚Äôs struct-based Views are restrict¬≠ed in how much state they can rea¬≠son¬≠ably man¬≠age on their own and can¬≠not main¬≠tain and manip¬≠u¬≠late their par¬≠ent or child views in the same way we can with UIK¬≠it. As we con¬≠tin¬≠ue to devel¬≠op more and more using Swif¬≠tUI, it is a good time to re-eval¬≠u¬≠ate how we struc¬≠ture our iOS projects.

Struc­ture #

Let‚Äôs zoom in on MVVM for a moment. The View is our structs that imple¬≠ment SwiftUI‚Äôs View pro¬≠to¬≠col. Large¬≠ly our Mod¬≠el lay¬≠er is the same as it has always been, some tables in a data¬≠base, pref¬≠er¬≠ences in UserDe¬≠faults, etc. The View Mod¬≠el sits between the View and Mod¬≠el, act¬≠ing as the con¬≠nec¬≠tive tis¬≠sue that pre¬≠pares your app‚Äôs data for dis¬≠play. The Mod¬≠el is the where and how your app stores and retrieves its data. The Model‚Äôs imple¬≠men¬≠ta¬≠tion is not impor¬≠tant for the pur¬≠pose of the dis¬≠cus¬≠sion in this post. We will use some lan¬≠guage around Core Data as an exam¬≠ple, but none of the tech¬≠niques dis¬≠cussed hinge upon its use. Instead we will focus on the rela¬≠tion¬≠ship between View and View Mod¬≠el spe¬≠cif¬≠ic to how we use them with SwiftUI.

V is for View #

The View, a screen, the application‚Äôs UI‚ÄČ‚ÄĒ‚ÄČat this lay¬≠er we are only con¬≠cerned with orga¬≠niz¬≠ing our infor¬≠ma¬≠tion for dis¬≠play and respond¬≠ing to the user‚Äôs inputs. Our views are not con¬≠cerned with where the data came from or how changes the user makes are prop¬≠a¬≠gat¬≠ed and saved, that respon¬≠si¬≠bil¬≠i¬≠ty falls to the View Model.

In gen¬≠er¬≠al a View should have exact¬≠ly one View Mod¬≠el that dri¬≠ves it, and that View Mod¬≠el should be the View‚Äôs only depen¬≠den¬≠cy. We also define a View‚Äôs require¬≠ments via a pro¬≠to¬≠col, which we call a View¬≠Con¬≠tract. A View can be dri¬≠ven by any type that imple¬≠ments its con¬≠tract, and we can have more than one fla¬≠vor of View Mod¬≠el dri¬≠ve the same UI. As a result, a View will nev¬≠er know the actu¬≠al type of its View Mod¬≠el, it is only con¬≠cerned with the ele¬≠ments of its contract.

struct ItemListView<ViewModel: ItemListViewContract>: View {
  @ObservedObject var viewModel: ViewModel

  var body: some View {
    ScrollView {
      VStack {
        TextField("Search", binding: self.viewModel.searchTerm)
        ForEach(self.viewModel.listOfItems, id: \.self) { item in
          ItemDetailView(viewModel: item)
        Button(action: viewModel.buttonClick) {

Abstract¬≠ing the View¬≠Mod¬≠el behind a View Con¬≠tract Pro¬≠to¬≠col #

SwiftUI‚Äôs Pre¬≠view sys¬≠tem is a pow¬≠er¬≠ful tool for build¬≠ing and main¬≠tain¬≠ing UI. Pre¬≠views allow you to see what your View will look like under dif¬≠fer¬≠ent envi¬≠ron¬≠ment con¬≠di¬≠tions and with dif¬≠fer¬≠ent sets of data. This works just fine for sim¬≠ple Views, but when your View is dri¬≠ven by a View Mod¬≠el that loads its data from an end¬≠point we need to start think¬≠ing about ways to make Pre¬≠views use¬≠ful with¬≠out the need for set¬≠ting up or mock¬≠ing out all of our app‚Äôs services.

To get the most out of SwiftUI‚Äôs pre¬≠view sys¬≠tem we insert a pro¬≠to¬≠col (inter¬≠face for any non-Swift folks out there) that describes the data require¬≠ments for a View. For our pur¬≠pos¬≠es, we will call this a View¬≠Con¬≠tract. Check out Using View Mod¬≠el Pro¬≠to¬≠cols to man¬≠age com¬≠plex Swif¬≠tUI Views for an intro¬≠duc¬≠tion to this idea.

This View Con¬≠tract pro¬≠to¬≠col defines exact¬≠ly what is need¬≠ed to dri¬≠ve a piece of UI‚ÄČ‚ÄĒ‚ÄČits func¬≠tions, vari¬≠ables, states, etc. This sep¬≠a¬≠rates the require¬≠ments of the UI from their under¬≠ly¬≠ing Mod¬≠el-lay¬≠er com¬≠po¬≠nents. We can then imple¬≠ment that pro¬≠to¬≠col in mul¬≠ti¬≠ple con¬≠crete view mod¬≠el class¬≠es and use each to dri¬≠ve a par¬≠tic¬≠u¬≠lar view in dif¬≠fer¬≠ent sce¬≠nar¬≠ios. The list¬≠ing below is an exam¬≠ple of what a pro¬≠to¬≠col might look like for a screen that dis¬≠plays a list of items. 

protocol ItemListViewContract: ObservableObject {
  associatedtype ItemDetailViewModelType: ItemDetailViewContract

  var searchTerm: String { get set }
  var listOfItems: [ItemDetailViewModelType] { get }

  func buttonClick()

We extend ObservableObject so that Swif¬≠tUI can use the @ObservedObject or @StateObject prop¬≠er¬≠ty wrap¬≠pers. The prop¬≠er¬≠ties we define in the Con¬≠tract should have fair¬≠ly sim¬≠ple types (String, Int, etc.) or have their types hid¬≠den behind asso¬≠ci¬≠at¬≠ed type with pro¬≠to¬≠col require¬≠ments (e.g. ItemViewModelType). The pur¬≠pose is to hide the actu¬≠al source of this infor¬≠ma¬≠tion from the UI so we can‚Äôt go about return¬≠ing Core¬≠Da¬≠ta objects direct¬≠ly here.

This is also where you would define a hier¬≠ar¬≠chy of View Mod¬≠els, for exam¬≠ple if a list view needs to pro¬≠vide an array of View Mod¬≠els for a detail screen. In the list¬≠ing above, our con¬≠tract requires an array of ItemViewModelType which can be any type that imple¬≠ments the con¬≠tract for the item detail view (ItemContentViewContract).

The View Con¬≠tract pro¬≠to¬≠col approach allows us to define mul¬≠ti¬≠ple vari¬≠a¬≠tions on our View Mod¬≠els with¬≠out sub¬≠class¬≠ing and with¬≠out tying our Views to any spe¬≠cif¬≠ic data source. We reap div¬≠i¬≠dends from this set¬≠up when it comes to mak¬≠ing the most of Swif¬≠tUI Pre¬≠views. As we will see in the next sec¬≠tion, we can define a con¬≠crete View Mod¬≠el that uses all hard cod¬≠ed data for Swif¬≠tUI Pre¬≠views, or reads from a JSON file, whilst also let¬≠ting our actu¬≠al View Mod¬≠els talk to resources that are not avail¬≠able or dif¬≠fi¬≠cult to use in Pre¬≠views (like Core¬≠Da¬≠ta, an API, UserDe¬≠faults, etc.).

struct ItemListView_Previews: PreviewProvider {
  static var previews: some View {
    NavigationView {
      ItemListView(viewModel: PreviewItemListViewModel())
    NavigationView {
      ItemListView(viewModel: PreviewItemListViewModel())

Cre­at­ing Con­crete View Mod­els #

Now that we have our require¬≠ments defined for our View we can work on imple¬≠ment¬≠ing a pair of View Mod¬≠els that we can use to dri¬≠ve it‚ÄČ‚ÄĒ‚ÄČone for pre¬≠views and anoth¬≠er for our app‚Äôs actu¬≠al data.

Pre­view View Mod­el Class #

Per¬≠haps it is eas¬≠i¬≠est to start by defin¬≠ing a View Mod¬≠el we can use for pre¬≠views. We find it help¬≠ful to do this upfront, in tan¬≠dem with build¬≠ing our actu¬≠al UI. We can use hard-cod¬≠ed data or pass in val¬≠ues to cre¬≠ate dif¬≠fer¬≠ent scenarios.

final class PreviewItemListViewModel: ItemListViewContract {
  var searchTerm: String = "Preview"
  var listOfItems: [PreviewItemDetailViewModel] = []

  func buttonClick() {}

  init() {}

The list¬≠ing above hard codes the data that we will show in pre¬≠views and pro¬≠vides a sim¬≠ple imple¬≠men¬≠ta¬≠tion of the actions required by our View Con¬≠tract. Addi¬≠tion¬≠al¬≠ly, we need to cre¬≠ate anoth¬≠er class, PreviewItemContentViewModel, that will imple¬≠ment the con¬≠tract for our detail view (ItemContentViewContract).

Now that we have our UI ele¬≠ments fig¬≠ured out we can write anoth¬≠er View Mod¬≠el imple¬≠men¬≠ta¬≠tion that pulls from our app‚Äôs actu¬≠al data.

Core­Da­ta Backed View­Mod­el Class #

Core­Da­ta is by no means the only place you can store your app’s data. It is worth reit­er­at­ing that this approach is inten­tion­al­ly agnos­tic of where your data comes from; the UI can only see what is defined in its contract.

This is where the data actu¬≠al¬≠ly is accessed. It defines every¬≠thing estab¬≠lished in the pro¬≠to¬≠col it imple¬≠ments. Here we also pro¬≠vide an imple¬≠men¬≠ta¬≠tion of our buttonClick func¬≠tion that reloads the data.

final class ItemListViewModel: ItemListViewContract {
  @Published var searchTerm: String = "CoreData ūüėé"
  @Published var listOfItems: [ItemDetailViewModel] = []

  private let context: NSManagedObjectContext

  init(context: NSManagedObjectContext) {
    self.context = context
  private func fetch() {
    // Load the list of items from CoreData and convert
    // them into our detail view models
    self.listOfItems = self.context
      .fetch(MyCoreDataModel.searchRequest(term: self.searchTerm))

  func buttonClick() {

If Core¬≠Da¬≠ta is not your back¬≠ing store of choice, hope¬≠ful¬≠ly you can see how you might cre¬≠ate a View Mod¬≠el class that imple¬≠ments our View Con¬≠tract but gets its data from SQLite, Realm, an API, or sim¬≠i¬≠lar. The end result is a hier¬≠ar¬≠chy that looks some¬≠thing like the image below.

Archi­tec­ture ben­e­fits #

We want the imple¬≠men¬≠ta¬≠tion of our View Mod¬≠els to be entire¬≠ly invis¬≠i¬≠ble to our Views. This strong iso¬≠la¬≠tion between View and Mod¬≠el allows us to devel¬≠op in par¬≠al¬≠lel bet¬≠ter. When start¬≠ing work on a screen we can define the screen‚Äôs require¬≠ments based on designs and the inter¬≠ac¬≠tions the user needs to take and cod¬≠i¬≠fy it into a View Con¬≠tract. Mul¬≠ti¬≠ple devel¬≠op¬≠ers can then work on imple¬≠ment¬≠ing the each side of the con¬≠tract independently.

This approach can also ease main¬≠te¬≠nance of both the UI and an app‚Äôs busi¬≠ness log¬≠ic. As long as the view receives the data in the cor¬≠rect for¬≠mat, accord¬≠ing to its con¬≠tract, the source of it can change and the page will func¬≠tion the same. Addi¬≠tion¬≠al¬≠ly, we can redesign or rewrite a screen and know that it will not affect the under¬≠ly¬≠ing busi¬≠ness log¬≠ic con¬≠tained in our Mod¬≠els and View Mod¬≠els. We also can rely on the com¬≠pil¬≠er when we need to change the View Con¬≠tract to make sure noth¬≠ing slips through the cracks.

This is all in addi¬≠tion to the real¬≠i¬≠ties of devel¬≠op¬≠ment where often the UI design is estab¬≠lished before all of the exter¬≠nal resources and infra¬≠struc¬≠ture are ready to use. By estab¬≠lish¬≠ing this abstrac¬≠tion between our Views and the rest of our app, we can eas¬≠i¬≠ly write a View Mod¬≠el imple¬≠men¬≠ta¬≠tion that works for demo¬≠ing the UI and lat¬≠er fol¬≠low up with a View Mod¬≠el that pro¬≠vides real data. We have con¬≠fi¬≠dence to do this with min¬≠i¬≠mal rework because of our require¬≠ments defined in the View Contract.

It is worth not¬≠ing that not all Views war¬≠rant the cre¬≠at¬≠ing of a View Mod¬≠el and View Con¬≠tract. Views can be sim¬≠ple, for exam¬≠ple just tak¬≠ing a few strings and dis¬≠play¬≠ing them in a stack. These types of Views are use¬≠ful to keep your UI ele¬≠ments con¬≠sis¬≠tent. Once your View needs to respond to changes, manip¬≠u¬≠late state, or per¬≠form oth¬≠er inte¬≠gra¬≠tions with sys¬≠tem APIs it might be time to reach for a View Model.

Shar­ing func­tion­al­i­ty across screens #

Above we stat¬≠ed that in gen¬≠er¬≠al a View should only have one depen¬≠den¬≠cy, its View Mod¬≠el, but often times in larg¬≠er appli¬≠ca¬≠tions we need to manip¬≠u¬≠late appli¬≠ca¬≠tion state in the same way across dif¬≠fer¬≠ent screens and in dif¬≠fer¬≠ent view mod¬≠els. For this we intro¬≠duce a new lay¬≠er into the mix between our View Mod¬≠els and our data¬≠base, the Use Case .

Use Cas¬≠es may be famil¬≠iar to our Android friends, but for the unfa¬≠mil¬≠iar they are state¬≠less objects that manip¬≠u¬≠late your appli¬≠ca¬≠tion state accord¬≠ing to busi¬≠ness require¬≠ments‚ÄČ‚ÄĒ‚ÄČessen¬≠tial¬≠ly a cod¬≠i¬≠fied, reusable slice of busi¬≠ness log¬≠ic. This means that you can pass around a shared Use Case, or cre¬≠ate mul¬≠ti¬≠ple instances and share them to each View Mod¬≠el as need¬≠ed, using what¬≠ev¬≠er depen¬≠den¬≠cy injec¬≠tion mech¬≠a¬≠nism you prefer.

Below is an exam¬≠ple of a Use Case you may write to pull togeth¬≠er an API and some local¬≠ly stored data to pro¬≠vide a con¬≠sis¬≠tent way to access a user‚Äôs login state in your app. Addi¬≠tion¬≠al¬≠ly we show two View¬≠Mod¬≠els that might make use of this shared functionality.

/// A Use Case that encapsulates the Login state of the application, allowing the
/// View Models a consistent mechanism to alter and respond to change in the state
final public class LoginStateUseCase: NSObject {
  var loginState: AnyPublisher<LoginState, Never>
  func login(credentials: Credentials) {
    // ... Do login
  func logout() {
    // ... Do logout

// Create a shared instance so all consumers can get the same state. This can
// also be done through more nuanced DI setups
extension LoginStateUseCase {
  public static let shared = ExampleUseCase()

/// View Model for the Login Screen that manipulates the login state by attempting
/// to login with the user's entered credentials
final class LoginScreenViewModel: LoginScreenViewContract {
  @Published var usernameField: String = ""
  @Published var passwordField: String = ""
  let useCase: LoginStateUseCase
  init(login: LoginStateUseCase = .shared) {
    self.useCase = login
  func login() {
      with: Credentials(
        username: self.usernameField,
        password: self.passwordField

/// View Model for the Root Screen that serves as the app's top level router, 
/// showing the main content or the login screen depending on the login state
final class RootScreenViewModel: RootScreenViewContract {
  @Published var isLoggedIn: Bool = false

  let useCase: LoginStateUseCase
  init(login: LoginStateUseCase = .shared) {
    self.useCase = login
      .map { state in
        return state == .loggedIn
      .assign(to: &$isLoggedIn)

Use Cas¬≠es can also depend on oth¬≠er Use Cas¬≠es allow¬≠ing you to com¬≠pose your app‚Äôs busi¬≠ness log¬≠ic from mul¬≠ti¬≠ple, inde¬≠pen¬≠dent parts. This is a pow¬≠er¬≠ful way to com¬≠pose func¬≠tion¬≠al¬≠i¬≠ty togeth¬≠er rather than build¬≠ing one giant ser¬≠vice that gets passed about through your entire app. The dia¬≠gram below shows where in the hier¬≠ar¬≠chy a Use Case fits.

Mov­ing for­ward #

There is a ben¬≠e¬≠fit to think¬≠ing through the ques¬≠tions of where things go and how to main¬≠tain a soft¬≠ware project regard¬≠less of whether the spe¬≠cif¬≠ic approach described in this post works for you. Some apps will war¬≠rant addi¬≠tion¬≠al lay¬≠ers and abstrac¬≠tions like Repos¬≠i¬≠to¬≠ries and oth¬≠er Ser¬≠vice types. Some will not ben¬≠e¬≠fit from the addi¬≠tion of Use Cas¬≠es. It all depends on a project‚Äôs com¬≠plex¬≠i¬≠ty and what the main¬≠te¬≠nance life¬≠time will be. The approach above works for us as a sol¬≠id mid¬≠dle ground that pro¬≠vides enough struc¬≠ture to ease some deci¬≠sion mak¬≠ing and main¬≠te¬≠nance whilst not being so heavy hand¬≠ed as to become bur¬≠den¬≠some and a bar¬≠ri¬≠er to devel¬≠op¬≠ment. Any archi¬≠tec¬≠ture can work for you, just putting in a lit¬≠tle thought and plan¬≠ning up front can go a long way.

Addi­tion­al resources #

The App Archi¬≠tec¬≠ture book from objc‚Äč.io is a great intro¬≠duc¬≠tion to mul¬≠ti¬≠ple dif¬≠fer¬≠ent archi¬≠tec¬≠ture pat¬≠terns. Check it out if you are inter¬≠est¬≠ed in learn¬≠ing more.

Jeff Kloosterman
Jeff Kloosterman
Head of Client Services
Sarah Hendriksen
Sarah Hendriksen
Software Developer

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