Majid's Blog about Swift development

Understanding Property Wrappers in SwiftUI

Last week we started a new series of posts about SwiftUI framework. Today I want to continue this topic by covering Property Wrappers provided by SwiftUI. SwiftUI gives us @State, @Binding, @ObjectBinding, @EnvironmentObject, and @Environment Property Wrappers. So let’s try to understand the differences between them and when and why which one we have to use.

Property Wrappers

Property Wrappers feature described in SE-0258 proposal. The main goal here is wrapping properties with logic which can be extracted into the separated struct to reuse it across the codebase. This proposal is not accepted yet, but Apple provides Xcode beta with Swift 5.1 snapshot, which has it.

@State

@State is a Property Wrapper which we can use to mark View’s state. SwiftUI will store it in special internal framework memory outside of View struct. Only related View and its children can access it. As soon as the value of @State property change SwiftUI rebuilds View to respect state changes. Here is a simple example.

struct ProductsView: View {
    let products: [Product]

    @State private var showFavorited: Bool = false

    var body: some View {
        List {
            Button(
                action: { self.showFavorited.toggle() },
                label: { Text("Change filter") }
            )

            ForEach(products) { product in
                if !self.showFavorited || product.isFavorited {
                    Text(product.title)
                }
            }
        }
    }
}

In the example above we have a straightforward screen with Button and List of products. As soon as we press the button, it changes the value of state property, and SwiftUI recreates View.

@Binding

@Binding provides reference like access for a value type. Sometimes we need to make the state of our View accessible for its children. But we can’t simply pass that value because it is a value type and Swift will pass the copy of that value. And this is where we can use @Binding Property Wrapper.

struct FilterView: View {
    @Binding var showFavorited: Bool

    var body: some View {
        Toggle(isOn: $showFavorited) {
            return Text("Change filter")
        }
    }
}

struct ProductsView: View {
    let products: [Product]

    @State private var showFavorited: Bool = false

    var body: some View {
        List {
            FilterView(showFavorited: $showFavorited)

            ForEach(products) { product in
                if !self.showFavorited || product.isFavorited {
                    Text(product.title)
                }
            }
        }
    }
}

We use @Binding to mark showFavorited property inside the FilterView. We also use $ to pass a binding reference, because without $ Swift will pass a copy of the property’s value instead of passing bindable reference. FilterView can read and write the value of ProductsView’s showFavorited property, but it can’t observe the changes using this binding. As soon as FilterView changes value of showFavorited property, SwiftUI will recreate the ProductsView and FilterView as its child.

@ObjectBinding

@ObjectBinding work very similarly to @State Property Wrapper, but the main difference is that we can share it between multiple independent Views which can subscribe and observe changes on that object, and as soon as changes appear SwiftUI rebuild all Views bound to this object. Let’s take a look at an example.

final class PodcastPlayer: BindableObject {
    var isPlaying: Bool = false {
        didSet {
            didChange.send(self)
        }
    }

    func play() {
        isPlaying = true
    }

    func pause() {
        isPlaying = false
    }

    var didChange = PassthroughSubject<PodcastPlayer, Never>()
}

Here we have PodcastPlayer class which we share between the screens of our app. Every screen has to show floating pause button in the case when the app is playing a podcast episode. SwiftUI tracks changes on BindableObject with the help of didChange property, which is the single requirement of BindableObject protocol. More detailed information about BindableObject you can find in my previous post.

struct EpisodesView: View {
    @ObjectBinding var player: PodcastPlayer
    let episodes: [Episode]

    var body: some View {
        List {
            Button(
                action: {
                    if self.player.isPlaying {
                        self.player.pause()
                    } else {
                        self.player.play()
                    }
            }, label: {
                    Text(player.isPlaying ? "Pause": "Play")
                }
            )
            ForEach(episodes) { episode in
                Text(episode.title)
            }
        }
    }
}

Here we use @ObjectBinding Property Wrapper to bind our EpisodesView to PodcastPlayer class, and as soon as current View or any other View attached to PodcastPlayer object change it, SwiftUI will rebuild all Views bound to that PodcastPlayer object.

@EnvironmentObject

Instead of passing BindableObject via init method of our View we can implicitly inject it into Environment of our View hierarchy. By doing this, we create the opportunity for all child View of current Environment access to this BindableObject.

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let window = UIWindow(frame: UIScreen.main.bounds)
        let episodes = [
            Episode(id: 1, title: "First episode"),
            Episode(id: 2, title: "Second episode")
        ]

        let player = PodcastPlayer()
        window.rootViewController = UIHostingController(
            rootView: EpisodesView(episodes: episodes)
                .environmentObject(player)
        )
        self.window = window
        window.makeKeyAndVisible()
    }
}

struct EpisodesView: View {
    @EnvironmentObject var player: PodcastPlayer
    let episodes: [Episode]

    var body: some View {
        List {
            Button(
                action: {
                    if self.player.isPlaying {
                        self.player.pause()
                    } else {
                        self.player.play()
                    }
            }, label: {
                    Text(player.isPlaying ? "Pause": "Play")
                }
            )
            ForEach(episodes) { episode in
                Text(episode.title)
            }
        }
    }
}

As you can see, we have to pass PodcastPlayer object via environmentObject method on our View. By doing this, we can easily access to PodcastPlayer by defining it with @EnvironmentObject Property Wrapper. @EnvironmentObject uses dynamic member lookup feature to find PodcastPlayer class instance in the Environment, that’s why you even don’t need to pass it via init method of EpisodesView. The Environment is the right way of Dependency Injection in SwiftUI. It works like magic.

@Environment

As we discussed in the previous chapter, we can pass custom objects into the Environment of a View hierarchy inside SwiftUI. But SwiftUI already has an Environment populated with system-wide settings. We can easily access them with @Environment Property Wrapper.

struct CalendarView: View {
    @Environment(\.calendar) var calendar: Calendar
    @Environment(\.locale) var locale: Locale
    @Environment(\.colorScheme) var colorScheme: ColorScheme

    var body: some View {
        return Text(locale.identifier)
    }
}

By marking our properties with @Environment Property Wrapper, we access and subscribe to changes of system-wide settings. As soon as Locale, Calendar or ColorScheme of the system change, SwiftUI recreates our CalendarView.

Conclusion

Today we talked about Property Wrappers provided by SwiftUI. @State, @Binding, @EnvironmentObject, and @ObjectBinding play huge role in SwiftUI development. Feel free to follow me on Twitter and ask your questions related to this post. Thanks for reading and see you next week!