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final class UserData: ObservableObject { @Published var showFavoritesOnly = false @Published var landmarks = landmarkData } ͜Ε

4& IUUQTHJUIVCDPNBQQMFTXJGUFWPMVUJPOCMPCNBTUFS QSPQPTBMTQSPQFSUZXSBQQFSTNE There are property implementation patterns that come up repeatedly. Rather than hardcode a fixed set of patterns into the compiler (as we have done for lazy and @NSCopying), we should provide a general "property wrapper" mechanism to allow these patterns to be defined as libraries.

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ྫ ʮ஗ԆॳظԽʯ

struct Object { @LateInitialized var text: String } @propertyWrapper struct LateInitialized { private var storage: T? var wrappedValue: T { get { guard let value = storage else { fatalError() } return value } set { storage = newValue } } }

var obj1 = Object() print("\(obj1.text)") // -> fatal error var obj2 = Object() obj2.text = "hello" print("\(obj2.text)") // -> "hello"

ྫ ʮ6TFS%FGBVMUTʯ

@propertyWrapper struct UserDefault { let key: String let defaultValue: T var wrappedValue: T { get { return (UserDefaults.standard.object(forKey: key) as? T) ?? defaultValue } set { UserDefaults.standard.set(newValue, forKey: key) } } }

@UserDefault(key: "initialized", defaultValue: false) var initialized: Bool @UserDefault(key: "username", defaultValue: "") var username: String obj.username = "mobile" print("\(UserDefaults.standard.string(forKey: "username")!)") // -> "mobile" UserDefaults.standard.set("act", forKey: "username") print("\(obj.username)") // -> "act"

΋͏গ͠ৄ͘͠

ॻ͖׵͑ϧʔϧ @propertyWrapper struct WrapperA {} @propertyWrapper struct WrapperB {} // ॳظ஋ @WrapperA var obj: String = "test" ɹ↓ private var _obj: WrapperA(wrappedValue: "test") // ࿈݁Մೳ @WrapperA @WrapperB var obj: String ɹ↓ private var _obj: WrapperA>

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