@StateObject

╱🚧 under construction

SwiftUI ⟩ view ⟩ state ⟩ object ⟩ @StateObject

Use @StateObject to declare and initialize a state object in a view that observes the object's data.

// ⭐️ 1. declare a type that conforms to `ObservableObject`
class Zoo : ObservableObject {

    // ⭐️ 2. declare published value(s) in this type
    @Published var creatures = [
        Creature(name: "Gorilla", emoji: "🦍"),
        Creature(name: "Peacock", emoji: "🦚"),
    ]
    
}

// content view
struct ContentView: View {

    // ⭐️ 3. declare & initialize a state object
    @StateObject var data = Zoo()
    
    var body: some View {
        // 4. access the state object's data in subviews
    }
}

💈範例

📗 參考：Swift Playgrounds (Keep Going with Apps) - Sharing Data Between Views.

📁 CreatureZoo

// ⭐️ 1. declare an observable object type
class CreatureZoo : ObservableObject {
    // ⭐️ 2. declare published properties
    @Published var creatures = [
        Creature(name: "Gorilla", emoji: "🦍"),
        Creature(name: "Peacock", emoji: "🦚"),
    ]
}

📁 MyApp

@main
struct MyApp: App {
    
    // ⭐️ 3. intialize an observable object
    @StateObject var data = CreatureZoo()
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                // ⭐️ 4. put data in environment
                .environmentObject(data)
        }
    }
}

📁 ContentView

struct ContentView: View {
    
    // ⭐️ 5. inject data from environment
    @EnvironmentObject var data : CreatureZoo

    var body: some View {
        // ⭐️ 6. access data in subviews
    }
}

🔴 主題

• Binding from StateObject

📗 參考

• How to initialize @StateObject with parameters in SwiftUI
• Sarun ⟩ Should we manually call @StateObject initializer

🔸 定義

StateObject 是一個 property wrapper，它內含的資料一定要遵循 ObservableObject 協定。

@frozen @propertyWrapper 
struct StateObject<ObjectType> where ObjectType : ObservableObject

📘 手冊

• SwiftUI ⟩

  • View ⟩ State Modifiers ⟩

    • environmentObject(_:)
  • State and Data Flow ⟩

    • StateObject (struct)
    • ObservedObject (struct)
    • EnvironmentObject (struct)
• Combine ⟩

  • ObservableObject (protocol)

👥 相關

• ObservableObject：declare an observable object type.
• @Published：declare a published value in an observable object type.
• @StateObject：initialize an observable object.
• is property wrapper.

• wraps an object that conforms to ObservableObject.

• ＠Binding can be created.
• data model：the source of truth of your app.