A blog exploring functional programming and Swift.

SwiftUI and State Management Corrections

Tuesday Jul 30, 2019

This week we completed our 3-part introduction (part 1, part 2, part 3) to SwiftUI and the problems of state management and application architecture. All 3 episodes are completely free for everyone to watch and they aim to show what tools SwiftUI gives us for managing state in a moderately complex application.

Unfortunately, SwiftUI is still very much in beta and it seems that every 2 weeks there are substantial changes. These changes range from APIs being renamed to foundational changes to how SwiftUI behaves. Due to some of these changes our last 3 episodes now contain some misinformation, so we’d like to take an opportunity to correct these!

BindableObject versus ObservableObject

The main mechanism for turning your models into objects that can be bound in views was a protocol named BindableObject. Conforming to this protocol required you to provide a didChange publisher that you would ping just after you made any changes to your model. For example:

import SwiftUI

class AppState: BindableObject {
  let didChange = PassthroughSubject<Void, Never>()

  var count = 0 {
    didSet { self.didChange.send() }
  }

  var favoritePrimes: [Int] = [] {
    didSet { self.didChange.send() }
  }
}

However, as of Xcode 11 beta 5 this protocol has been renamed to ObservableObject (and moved from the SwiftUI framework to the Combine framework), and you are now required to notify a publisher when your model is about to change rather than after it is changed. This means the above code snippet should now look like this:

import Combine

class AppState: ObservableObject {
  let objectWillChange = ObservableObjectPublisher()

  var count = 0 {
    willSet { self.objectWillChange.send() }
  }

  var favoritePrimes: [Int] = [] {
    willSet { self.objectWillChange.send() }
  }
}

This now satisfies the compiler and is the correct way to implement this protocol.

The name of the property wrapper that connects our model to a view was also renamed in beta 5, from ObjectBinding to ObservedObject. To use it you can simply do:

struct ContentView: View {
  @ObservedObject var state: AppState

  var body: some View { ... }
}

ObservableObject boilerplate

There is also a change in Xcode 11 beta 5 that greatly simplifies how one creates observable objects. The amount of boilerplate required to implement the ObservableObject protocol was pretty significant. Just look at what happens to our AppState if we add two more properties:

import Combine

class AppState: ObservableObject {
  let objectWillChange = ObservableObjectPublisher()

  var count = 0 {
    willSet { self.objectWillChange.send() }
  }

  var favoritePrimes: [Int] = [] {
    willSet { self.objectWillChange.send() }
  }

  var activityFeed: [Activity] = [] {
    willSet { self.objectWillChange.send() }
  }

  var loggedInUser: User? = nil {
    willSet { self.objectWillChange.send() }
  }
}

This was one of the problems that we discussed in part 3 of our series, and luckily Xcode 11 beta 5 provides a solution. It is now possible for SwiftUI to automatically synthesize the objectWillChange for you, and by using the @Published property wrapper you can automatically have the publisher pinged when any of your fields change:

import Combine

class AppState: ObservableObject {
  @Published var count = 0
  @Published var favoritePrimes: [Int] = []
  @Published var activityFeed: [Activity] = []
  @Published var loggedInUser: User? = nil
}

Much nicer!

Derived bindings

Another problem we discussed in part 3 of our series is the idea that SwiftUI state didn’t appear to be “composable”. After much testing and playing around with SwiftUI we weren’t able to figure out how to easily derive bindings of sub-state from our main AppState.

For example, say we had an ActivityView screen that only needs access to the activityFeed field of AppState. We would love if there was a way to project out that single field into a binding, and hand it over to the activity view. We approached this by creating a view that had a binding to the activity feed:

struct ActivityView: View {
  @Binding var activityFeed: [Activity]

  var body: some View { ... }
}

Then, to create a Binding<Activity> to pass along to this view we could derive a binding from the AppState observable object:

struct ContentView {
  @ObservedObject var state: AppState

  var body: some View {
    NavigationView {
      List {
        ...
        NavigationLink(destination: ActivityView(activityFeed: self.$state.activityFeed)) {
          Text("Activity feed")
        }
        ...
      }
    }
  }
}

The most important part of this snippet is the following:

ActivityView(activityFeed: self.$state.activityFeed)

This first accesses the state as the underlying Binding<AppState> that comes from the property wrapper, and this is done via self.$state. Then we derive a Binding<[Activity]> from the Binding<AppState> by using dot-syntax, which is possible thanks to the Key Path Member Lookup feature of Swift 5.1

This is how we hoped derived bindings would work in SwiftUI, but unfortunately up until Xcode 11 beta 5 this did not work. There was a bug that prevented the ActivityView from re-rendering when the app state changed. This has now been fixed, and it’s a great way for creating views that operate on only a small subset of data instead of passing around the full AppState to all views!

Identifiable

And lastly, the Identifiable protocol has been moved out of the SwiftUI framework and into the standard library. This means you no longer need to import SwiftUI in order to get access to this protocol. However, in this change there appears to be a bug in which Int no longer conforms to Identifiable. We’re not sure if this was done on purpose, or if it’s a temporary bug, but it means that our code to show an alert from a binding no longer compiles:

struct CounterView: View {
  @State var alertNthPrime: Int?
  ...

  var body: some View {
    ...
      .alert(item: self.$alertNthPrime) { prime in
        ...
      }
  }
}

Since Int is no longer Identifiable, we cannot use a Binding<Int?> value in the alert(item:content:) method. The workaround is easy enough, just wrap the integer in a struct that you can make identifiable:

struct PrimeAlert: Identifiable {
  let prime: Int
  var id: Int { self.prime }
}

struct CounterView: View {
  @State var alertNthPrime: PrimeAlert?
  ...

  var body: some View {
    ...
      .alert(item: self.$alertNthPrime) { prime in
        ...
      }
  }
}

Now this works as it did before. Hopefully the conformance of integers to Identifiable is added back before Xcode 11 ships!

Move fast, correct things

The past few Xcode 11 betas have had some really nice improvements to SwiftUI, and we are really happy to see the changes. Some of the fixes have directly addressed some of the concerns we brought up in this week’s episode, and make SwiftUI much more appropriate for large, complex applications.

However, we still feel there is room for improvement in a number of key areas in application development. Next week we begin developing an architecture to directly address these problems using functional programming as our North Star.

Until next time!


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