Swiftpack.co - Package - thoughtbot/CombineViewModel


An implementation of the Model-View-ViewModel (MVVM) pattern using Combine.


CombineViewModel’s primary goal is to make view updates as easy in UIKit and AppKit as they are in SwiftUI.

In SwiftUI, you write model and view-model classes that conform to Combine’s ObservableObject protocol. SwiftUI:

  1. Observes each model’s objectWillChange publisher via the @ObservedObject property wrapper, and;
  2. Automatically rerenders the appropriate portion of the view hierarchy.

The problem with objectWillChange outside of SwiftUI is that there's no built-in way of achieving (2) — being notified that an object will change is not the same as knowing that it did change and it’s time to update the view.


Consider the following sketch of a view model for displaying a user’s social networking profile:

// ProfileViewModel.swift

import CombineViewModel
import UIKit

class ProfileViewModel: ObservableObject {
  @Published var profileImage: UIImage?
  @Published var topPosts: [Post]

  func refresh() {
    // Request updated profile info from the server.

With CombineViewModel, you can subscribe to did change notifications using the observe(on:) operator:

let profile = ProfileViewModel()

profileSubscription = profile.observe(on: DispatchQueue.main).sink { profile in
  // Called on the main queue when either (or both) of `profileImage`
  // or `topPosts` have changed.


Automatic view updates

Building on ObjectDidChangePublisher is the ViewModelObserver protocol and @ViewModel property wrapper. Instead of manually managing the ObjectDidChangePublisher subscription like above, we can have it managed automatically:

// ProfileViewController.swift

import CombineViewModel
import UIKit

// 1️⃣ Conform your view controller to the ViewModelObserver protocol.
class ProfileViewController: UITableViewController, ViewModelObserver {
  enum Section: Int {
    case topPosts

  @IBOutlet private var profileImageView: UIImageView!
  private var dataSource: UITableViewDiffableDataSource<Section, Post>!

  // 2️⃣ Declare your view model using the `@ViewModel` property wrapper.
  @ViewModel private var profile: ProfileViewModel

  // 3️⃣ Initialize your view model in init().
  required init?(profile: ProfileViewModel, coder: NSCoder) {
    super.init(coder: coder)
    self.profile = profile

  // 4️⃣ The `updateView()` method is automatically called on the main queue
  //     when the view model changes. It is always called after `viewDidLoad()`.
  func updateView() {
    profileImageView.image = profile.profileImage

    var snapshot = NSDiffableDataSourceSnapshot<Section, Post>()

  override func viewWillAppear(_ animated: Bool) {


Further reading

In the Example directory you’ll find a complete iOS sample application that demonstrates how to integrate CombineViewModel into your application.


CombineViewModel is distributed via Swift Package Manager. To add it to your Xcode project, navigate to File > Add Package Dependency…, paste in the repository URL, and follow the prompts.

Screen capture of Xcode on macOS Big Sur, with the Add Package Dependency menu item highlighted


CombineViewModel also provides the complementary Bindings module. It provides two operators — <~, the input binding operator, and ~>, the output binding operator — along with various types and protocols that support it. Note that the concept of a "binding" provided by the Bindings module is different to SwiftUI's Binding type.

Platform-specific binding helpers are also provided:


Have a useful reactive extension in your project? Please consider contributing it back to the community!

For more details, see the CONTRIBUTING document. Thank you, contributors!


CombineViewModel is Copyright © 2019–20 thoughtbot, inc. It is free software, and may be redistributed under the terms specified in the LICENSE file.



CombineViewModel is maintained and funded by thoughtbot, inc. The names and logos for thoughtbot are trademarks of thoughtbot, inc.

We love open source software! See our other projects or hire us to help build your product.


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Buckle Up - 2020-08-25 17:53:00


  • A compiler segfault when building CombineViewModel in Release configuration.

Route Kanal - 2020-08-24 22:30:49


  • Bindings is now distributed as part of CombineViewModel!


  • ViewModelObserver now inherits from Bindings' BindingOwner protocol. As a result, all objects automatically get a subscriptions property automatically synthesized.


  • Now compiles on macOS, tvOS and watchOS.

“A Red Letter Day” - 2020-08-16 20:02:12


  • The Objective-C runtime support is now implemented in Swift. This allows us to remove the private CombineViewModelObjC target, which polluted auto-complete results and was generally a leaked implementation detail.


  • Deleted ViewModelObserver.isReadyForUpdates.

The Best Kind of Correctness - 2020-08-13 18:40:00


  • ObjectDidChangePublisher now passes through completion events from the observed object's objectWillChange publisher.
  • Improve thread-safety of ObjectDidChangePublisher.

Best in Klass - 2020-08-07 18:22:22


  • Improve performance of @ViewModel when intercepting viewDidLoad to register an observer (#2).

Point Insertion - 2020-08-03 19:00:44

The first release of CombineViewModel:

  • View models conform to ObservableObject.
  • View controllers conform to ViewModelObserver.
  • Register for updates using the @ViewModel property wrapper.