Fix "Accessing StateObject's object without being installed on a View…" warning

Sources/Defaults/SwiftUI.swift

@@ -114,13 +114,20 @@ public struct Default<Value: Defaults.Serializable>: @preconcurrency DynamicProp
 	}
 
 	public var wrappedValue: Value {
-		get { observable.value }
+		get {
+			syncObservableKeyIfNeeded()
+			return observable.value
+		}
 		nonmutating set {
+			syncObservableKeyIfNeeded()
 			observable.value = newValue
 		}
 	}
 
-	public var projectedValue: Binding<Value> { $observable.value }
+	public var projectedValue: Binding<Value> {
+		syncObservableKeyIfNeeded()
+		return $observable.value
+	}
 
 	/**
 	The default value of the key.
@@ -134,10 +141,18 @@ public struct Default<Value: Defaults.Serializable>: @preconcurrency DynamicProp
 
 	@_documentation(visibility: private)
 	public mutating func update() {
-		observable.key = key
 		_observable.update()
 	}
 
+	private func syncObservableKeyIfNeeded() {
+		guard observable.key != key else {
+			return
+		}
+
+		// Avoid touching `@StateObject` in `update()`, which triggers Xcode 16 warnings.
+		observable.key = key
+	}
+
 	/**
 	Reset the key back to its default value.
 

Tests/DefaultsTests/DefaultsSwiftUITests.swift

@@ -1,7 +1,13 @@
 import Foundation
 import SwiftUI
+#if canImport(AppKit)
+import AppKit
+#elseif canImport(UIKit)
+import UIKit
+#endif
 import Testing
 import Defaults
+import Observation
 
 private let suite_ = createSuite()
 
@@ -16,6 +22,8 @@ extension Defaults.Keys {
 	fileprivate static let user = Key<User>("swiftui_user", default: User(username: "Hank", password: "123456"), suite: suite_)
 	fileprivate static let setInt = Key<Set<Int>>("swiftui_setInt", default: Set(1...3), suite: suite_)
 	fileprivate static let color = Key<Color>("swiftui_color", default: .black, suite: suite_)
+	fileprivate static let primarySwitchValue = Key<Bool>("swiftui_primarySwitchValue", default: false, suite: suite_)
+	fileprivate static let secondarySwitchValue = Key<Bool>("swiftui_secondarySwitchValue", default: false, suite: suite_)
 }
 
 struct ContentView: View {
@@ -31,6 +39,39 @@ struct ContentView: View {
 	}
 }
 
+@Observable
+final class DefaultKeySwitchingState {
+	var useSecondaryKey = false
+}
+
+struct KeySwitchingHostView: View {
+	@Bindable var state: DefaultKeySwitchingState
+
+	var body: some View {
+		KeySwitchingDefaultView(
+			key: state.useSecondaryKey ? .secondarySwitchValue : .primarySwitchValue,
+			marker: state.useSecondaryKey
+		)
+	}
+}
+
+struct KeySwitchingDefaultView: View {
+	@Default private var value: Bool
+	private let marker: Bool
+
+	init(key: Defaults.Key<Bool>, marker: Bool) {
+		self.marker = marker
+		_value = Default(key)
+	}
+
+	var body: some View {
+		Color.clear
+			.task(id: marker) {
+				value = marker
+			}
+	}
+}
+
 @Suite(.serialized)
 final class DefaultsSwiftUITests {
 	init() {
@@ -63,4 +104,38 @@ final class DefaultsSwiftUITests {
 		#expect(XColor(view.color) != XColor(Color.black))
 		#expect(XColor(view.color) == XColor(Color(.sRGB, red: 100, green: 100, blue: 100, opacity: 1)))
 	}
+
+	@MainActor
+	@Test
+	func testSwiftUIDefaultUpdatesKeyOnReinit() async throws {
+		Defaults[.primarySwitchValue] = false
+		Defaults[.secondarySwitchValue] = false
+
+		let state = DefaultKeySwitchingState()
+
+		#if os(macOS)
+		let hostingController = NSHostingController(rootView: KeySwitchingHostView(state: state))
+		let window = NSWindow(contentViewController: hostingController)
+		window.makeKeyAndOrderFront(nil)
+		#else
+		let hostingController = UIHostingController(rootView: KeySwitchingHostView(state: state))
+		let window = UIWindow(frame: UIScreen.main.bounds)
+		window.rootViewController = hostingController
+		window.makeKeyAndVisible()
+		#endif
+
+		_ = hostingController.view
+		_ = window
+
+		try await Task.sleep(for: .milliseconds(100))
+
+		#expect(!Defaults[.primarySwitchValue])
+		#expect(!Defaults[.secondarySwitchValue])
+
+		state.useSecondaryKey = true
+		try await Task.sleep(for: .milliseconds(100))
+
+		#expect(!Defaults[.primarySwitchValue])
+		#expect(Defaults[.secondarySwitchValue])
+	}
 }