WithBindable

ObservableState.swift

// Copyright (C) 2025 Gwendal Roué
//
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// copy of this software and associated documentation files (the
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import SwiftUI

/// A property wrapper that instantiates an observable object.
///
/// It's like `@State`, except that just like `@StateObject` its
/// initializer accepts an autoclosure so that a single instance of the
/// observable object is created for the whole lifetime of the view.
///
/// For example:
///
/// ```swift
/// @Observable Model {
///     init() { ... }
/// }
///
/// struct MyView: View {
///     @ObservableState var model = Model()
///
///     var body: some View { ... }
/// }
/// ```
@propertyWrapper @MainActor public struct ObservableState<Value: AnyObject & Observable>: DynamicProperty {
    @StateObject private var container = ValueContainer()
    let makeValue: () -> Value
    
    public init(wrappedValue: @autoclosure @escaping () -> Value) {
        self.makeValue = wrappedValue
    }
    
    public var wrappedValue: Value {
        container.value ?? makeValue()
    }
    
    public var projectedValue: Bindable<Value> {
        Bindable(wrappedValue)
    }
    
    public nonisolated func update() {
        MainActor.assumeIsolated {
            if container.value == nil {
                container.value = makeValue()
            }
        }
    }
    
    private final class ValueContainer: ObservableObject {
        var value: Value?
    }
}

/// Supplies an observable object to a view’s hierarchy.
///
/// The purpose of `WithBindable` is to make it possible to instantiate
/// observable objects from environment values, while keeping the object
/// alive as long as the view is rendered.
///
/// For example:
///
/// ```swift
/// @Observable Model {
///     var name = ""
///
///     init(service: Service) { ... }
/// }
///
/// struct MyView: View {
///     @Environment(\.service) private var service
///
///     var body: some View {
///         WithBindable {
///             Model(service: service)
///         } content: { $model in
///             TextField("Name", text: $model.name)
///         }
///     }
/// }
/// ```
///
/// `WithBindable` makes sure a single instance of the observable object
/// is created for the whole lifetime of the view. It is thus possible to
/// perform work from the object initializer.
public struct WithBindable<Value: AnyObject & Observable, Content: View>: View {
    @ObservableState var state: Value
    private let content: (Bindable<Value>) -> Content
    
    /// Supplies a `Bindable` object to a view’s hierarchy.
    ///
    /// For example:
    ///
    /// ```swift
    /// WithBindable {
    ///     Model(...)
    /// } content: { $model in
    ///     TextField($model.name)
    /// }
    /// ```
    public init(
        _ makeValue: @escaping () -> Value,
        @ViewBuilder content: @escaping (Bindable<Value>) -> Content
    ) {
        self._state = ObservableState(wrappedValue: makeValue())
        self.content = content
    }
    
    public var body: some View {
        content($state)
    }
}

// MARK: - Preview

#if DEBUG
@MainActor @Observable private final class DemoModel {
    static var instanceCount = 0
    var text: String
    
    init() {
        // Preview crashes if DemoModel is instantiated more than expected.
        // This helps asserting the desired behavior of the preview, which
        // is that model instances are not created as long as the view
        // is rendered.
        precondition(DemoModel.instanceCount < 2)
        DemoModel.instanceCount += 1
        self.text = ""
    }
}

#Preview {
    struct ContainerView: View {
        @State var counter: Int = 1
        
        var body: some View {
            Form {
                Section {
                    Text(verbatim: "Container body rendered \(counter) times")
                        .contentTransition(.numericText())
                        .animation(.default, value: counter)
                        .monospacedDigit()
                } header: {
                    Text(verbatim: "Container View")
                }
                
                // ObservableState demo
                Section {
                    DemoView()
                } header: {
                    Text(verbatim: "@ObservableState demo")
                }
                
                // WithBindable demo
                Section {
                    WithBindable {
                        DemoModel()
                    } content: { $model in
                        TextField("", text: $model.text, prompt: Text(verbatim: "Enter text here"))
                        Text(verbatim: "Text is '\(model.text)'")
                    }
                } header: {
                    Text(verbatim: "WithBindable demo")
                }
                
                Button {
                    counter += 1
                } label: {
                    Text(verbatim: "Force container body re-evaluation")
                }
            }
            .headerProminence(.increased)
        }
    }
    
    struct DemoView: View {
        @ObservableState var model = DemoModel()
        
        var body: some View {
            TextField("", text: $model.text, prompt: Text(verbatim: "Enter text here"))
            Text(verbatim: "Text is '\(model.text)'")
        }
    }
    
    return ContainerView()
}
#endif

Thank you for this snippet, it has helped us tremendously while migrating to @observable.
Could you explain how can we be sure that function below will always be called from MainActor - or to put it differently that it will in fact always be isolated on MainActor even though it's nonisolated?

nonisolated func update() {
        MainActor.assumeIsolated {
            if container.value == nil {
                container.value = makeValue()
            }
        }
    }

Hi @paskowski.

Could you explain how can we be sure that function below will always be called from MainActor - or to put it differently that it will in fact always be isolated on MainActor even though it's nonisolated?

DynamicProperty.update is not marked @MainActor, so there is no way to be sure. Hence the use of MainActor.assumeIsolated, that crashes whenever this assumption turns wrong at runtime. I've never seen SwiftUI call it outside of the main actor, though.