[SwiftDemos] PropertyWrapper

SwiftDemos/SwiftDemos/ContentView.swift

@@ -8,14 +8,16 @@
 import SwiftUI
 
 struct ContentView: View {
-    private var demos = ["COW"]
+    private var demos = ["COW", "PropertyWrapper"]
 
     var body: some View {
         NavigationStack {
             List {
                 ForEach(demos.enumerated(), id: \.element) { idx, item in
                     if idx == 0 {
                         NavigationLink(item, destination: COWView())
+                    } else if idx == 1 {
+                        NavigationLink(item, destination: PropertyWrapperView())
                     }
                 }
             }

SwiftDemos/SwiftDemos/Features/TypedThrows.swift

@@ -20,7 +20,7 @@ enum FileError: Error {
 
 // throws：声明函数可能抛出错误
 func fetchData(from urlString: String) throws -> Data {
-    guard let url = URL(string: urlString) else {
+    guard let _ = URL(string: urlString) else {
         throw NetworkError.invalidURL
     }
     // ...
@@ -55,7 +55,7 @@ func fetch() throws(NetworkError) -> Data {
 func test2() {
     // 调用方 catch 分支无需类型转换
     do {
-        let data = try fetch()
+        let _ = try fetch()
     } catch {
         // error 的类型是 NetworkError（不是 any Error）
         switch error {

SwiftDemos/SwiftDemos/PropertyWrapper/PropertyWrapper.swift

@@ -0,0 +1,57 @@
+//
+//  PropertyWrapper.swift
+//  SwiftDemos
+//
+//  Created by Jinchao Lin on 2026/2/25.
+//
+
+import Foundation
+import Combine
+
+@propertyWrapper
+struct UserDefault<T> {
+    let key: String
+    let defaultValue: T
+
+    var wrappedValue: T {
+        get {
+            UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
+        }
+        nonmutating set {
+            UserDefaults.standard.set(newValue, forKey: key)
+        }
+    }
+
+    // 通过 $ 访问
+    var projectedValue: AnyPublisher<T, Never> {
+        /// publisher(for:) 是 Combine 的 KVO Publisher，\.self 表示观察 UserDefaults 对象本身。
+        /// 但 UserDefaults.standard 是单例，对象引用永远不变，KVO 只在订阅时触发一次初始值，之后不再触发
+//        UserDefaults.standard
+//            .publisher(for: \.self)          // 监听整个 UserDefaults
+//            .compactMap { $0.object(forKey: self.key) as? T }
+//            .eraseToAnyPublisher()
+
+        NotificationCenter.default
+            .publisher(for: UserDefaults.didChangeNotification)
+            .map { _ in UserDefaults.standard.object(forKey: key) as? T ?? defaultValue }
+            .prepend(wrappedValue)  // 补一个初始值，行为与之前一致
+            .eraseToAnyPublisher()
+    }
+}
+
+/*
+ // 源码
+ struct Foo {
+ @UserDefault(key: "x", defaultValue: 0)
+ var x: Int
+ }
+
+ // 编译器生成（等价代码）
+ struct Foo {
+ private var _x = UserDefault<Int>(key: "x", defaultValue: 0)
+ var x: Int {
+ get { _x.wrappedValue }
+ set { _x.wrappedValue = newValue }
+ }
+ }
+ */

SwiftDemos/SwiftDemos/PropertyWrapper/PropertyWrapperView.swift

@@ -0,0 +1,35 @@
+//
+//  PropertyWrapperView.swift
+//  SwiftDemos
+//
+//  Created by Jinchao Lin on 2026/2/26.
+//
+
+import SwiftUI
+
+struct PropertyWrapperView: View {
+    @UserDefault(key: "isDarkMode", defaultValue: false)
+    var isDarkMode: Bool    // _isDarkMode.wrappedValue
+
+    var body: some View {
+        VStack {
+            Button("Switch to Dark") {
+                isDarkMode = true
+            }
+            .padding()
+
+            Button("Switch to White") {
+                isDarkMode = false
+            }
+            .padding()
+        }
+        .navigationTitle("Property Wrapper")
+        .onReceive($isDarkMode) { newValue in
+            debugPrint("isDarkMode changed to \(newValue)")
+        }
+    }
+}
+
+#Preview {
+    PropertyWrapperView()
+}