Swift Academy · Lesson

Creating a Custom Property Wrapper

Implementing @propertyWrapper with wrappedValue to encapsulate logic.

Lesson 1 of 412 steps

Creating a Custom Property Wrapper is a free Swift Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Welcome

Property wrappers encapsulate property storage logic, letting you reuse boilerplate like validation, transformation, or persistence.

Basic Syntax

```swift @propertyWrapper struct Clamped { var wrappedValue: Int { didSet { wrappedValue = min(max(0,wrappedValue),100) } } init(wrappedValue: Int) { self.wrappedValue = min(max(0,wrappedValue),100) } } struct Gauge { @Clamped var level = 50 } var g = Gauge() g.level = 150 // clamped to 100 ```

The wrappedValue Requirement

Every property wrapper must define `wrappedValue`. This is what the user of the wrapper reads and writes.

Generic Property Wrappers

```swift @propertyWrapper struct Logged { private var value: T var wrappedValue: T { get { print("get"); return value } set { print("set to \(newValue)"); value = newValue } } init(wrappedValue: T) { value = wrappedValue } } ```

Using the Wrapper

```swift struct Model { @Logged var name: String = "Default" } var m = Model() let n = m.name // prints "get" m.name = "Alice" // prints "set to Alice" ```

Custom Initialiser Arguments

```swift @propertyWrapper struct Default { var wrappedValue: T init(wrappedValue: T) { self.wrappedValue = wrappedValue } init(_ default: T) { wrappedValue = default } } @Default(42) var count: Int ```

Wrapper on a Stored vs Computed

Property wrappers add hidden storage (_wrappedValue). The annotated property is syntactic sugar: ```swift @Clamped var level = 50 // Desugars to: var _level = Clamped(wrappedValue: 50) var level: Int { get { _level.wrappedValue } set { _level.wrappedValue = newValue } } ```

UserDefaults Wrapper

```swift @propertyWrapper struct UserDefault { let key: String; let defaultValue: T var wrappedValue: T { get { UserDefaults.standard.object(forKey: key) as? T ?? defaultValue } set { UserDefaults.standard.set(newValue, forKey: key) } } } @UserDefault(key:"theme",defaultValue:"light") var theme: String ```

Thread-Safe Wrapper

```swift @propertyWrapper final class Atomic { private var value: T; private let lock = NSLock() var wrappedValue: T { get { lock.lock(); defer { lock.unlock() }; return value } set { lock.lock(); defer { lock.unlock() }; value = newValue } } init(wrappedValue: T) { value = wrappedValue } } ```

Combining Wrappers in a Type

```swift struct Settings { @Clamped var volume = 50 @UserDefault(key:"username",defaultValue:"") var username: String @Logged var debugMode = false } ```

Quick Check

What is the required property in every `@propertyWrapper` type?

Answer it in the interactive lesson

Recap

Key takeaways: • `@propertyWrapper` + `wrappedValue` = minimal property wrapper • The wrapper adds hidden `_name` storage • Generic wrappers work with any type T • Common uses: clamping, logging, UserDefaults, thread-safety • Custom init args allow configuration Next: projected values.

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Frequently Asked Questions

Is the “Creating a Custom Property Wrapper” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Creating a Custom Property Wrapper”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Creating a Custom Property Wrapper”?

Implementing @propertyWrapper with wrappedValue to encapsulate logic. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Creating a Custom Property Wrapper” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

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