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Apple Developer Notes | Data Essentials in SwiftUI

Instantiate a Model Object in a View

Connect data to show in views with data defined in Model.

Model class:

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class Book: ObservableObject {
    /// @Published: Once title changed, Views related with Book refreshed.
    /// @Published works when this class is an ObservableObject
    @Published var title = "Great Expectations" 
}

SwiftUI codes:

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struct LibraryView: View {
    @StateObject var book = Book() // Book is a class conforming to ObservableObject. Only instantiating uses `@StateObject`

    var body: some View {
        BookView(book: book)
    }
}

struct BookView: View {
    @ObservedObject var book: Book // Here book is not an instance of Model. So yse @ObservedObject

    // ...
}

Share an Object Throughout the App

Save your time to pass arguments again and again if there are plenty of layers.

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@main
struct BookReader: App {
    @StateObject var library = Library() // initiate ObservableObject with `@StateObject`

    var body: some Scene {
        WindowGroup {
            LibraryView()
                .environmentObject(library) // Here pass `library` to all sub-views of LibraryView().
        }
    }
}
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struct LibraryView: View {
    @EnvironmentObject var library: Library // Get that EnvironmentObject

    // ...
}

Manage Mutable Values

〇 当层View中需要更改的值定义为@State var(因为要保存这个值，而不是每次refresh UI的时候值都回到初始值)

〇 当层View使用的不可更改的值，定义该值为let

〇 当层View需要给下层View传值、且下层View只是使用而不改变该值，下层View中定义该值为var

〇 当层View需要给下层View传值、且下层View会改变该值，下层View中定义该值为@Binding var，确保下层View改的是当层View值的引用（改的是同一个）；当层View传递时写$foo

〇 某值只有当层和subView用，加private，上层View无法访问到该量

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struct PlayerView: View {
    let episode: Episode
    @State private var isPlaying: Bool = false

    var body: some View {
        VStack {
            Text(episode.title)
            Text(episode.showTitle)
            PlayButton(isPlaying: $isPlaying) // Pass a binding.
        }
    }
}
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struct PlayButton: View {
    @Binding var isPlaying: Bool

    var body: some View {
        Button(action: {
            self.isPlaying.toggle() // change isPlaying in sub-view. So use @Binding
        }) {
            Image(systemName: isPlaying ? "pause.circle" : "play.circle")
        }
    }
}

References

〇 Main

Apple Developer Documentation | Managing Model Data in Your App

Apple Developer Documentation | Managing User Interface State

WWDC20 | Data Essentials in SwiftUI

〇 Secondary

Apple Developer Documentation | State and Data Flow