CombineViewModel

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

• Introduction
• Installation
• Bindings
• Contributing
• About

Introduction

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.

ObjectDidChangePublisher

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.
}

profile.refresh()

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>()
    snapshot.appendSections([.topPosts])
    snapshot.appendItems(profile.topPosts)
    dataSource.apply(snapshot)
  }

  override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    profile.refresh()
  }
}

Further reading

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

Installation

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.

Bindings

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:

• UIKitBindings

Contributing

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!

License

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

About

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

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