WrappedValue and projectedValue in Swift

Introduction

WrappedValue and projectedValue help you control how a property behaves and what extra information it can provide.

When you want to add extra behavior to a property, like validation or logging.

When you want to expose additional info about a property, like whether it changed.

When you want to customize how a property's value is stored or accessed.

When you want to create reusable property wrappers to simplify your code.

Syntax

Swift

@propertyWrapper
struct WrapperName {
    var wrappedValue: Type
    var projectedValue: ExtraType

    init(wrappedValue: Type) {
        self.wrappedValue = wrappedValue
        self.projectedValue = ... // set extra info
    }
}

wrappedValue is the main value the property holds.

projectedValue is extra info you can access with a $ prefix.

Examples

This wrapper capitalizes the string whenever it is set.

Swift

@propertyWrapper
struct Capitalized {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.capitalized }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

This wrapper prints a message every time the value changes.

Swift

@propertyWrapper
struct Logged {
    var wrappedValue: Int {
        didSet { print("Value changed to \(wrappedValue)") }
    }

    init(wrappedValue: Int) {
        self.wrappedValue = wrappedValue
    }
}

This wrapper provides a string description of the boolean value via projectedValue.

Swift

@propertyWrapper
struct Toggle {
    var wrappedValue: Bool
    var projectedValue: String {
        wrappedValue ? "On" : "Off"
    }

    init(wrappedValue: Bool) {
        self.wrappedValue = wrappedValue
    }
}

Sample Program

This program uses a property wrapper Trimmed that removes spaces around a string. The projectedValue gives the length of the trimmed string. We create a user with spaces around the name, then print the trimmed name and its length.

Swift

import Foundation

@propertyWrapper
struct Trimmed {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.trimmingCharacters(in: .whitespacesAndNewlines) }
    }
    var projectedValue: Int {
        value.count
    }

    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

struct User {
    @Trimmed var name: String
}

var user = User(name: "  Alice  ")
print("Name: '\(user.name)'" )
print("Length: \(user.$name)")

Important Notes

You access wrappedValue normally, but projectedValue with a $ before the property name.

Property wrappers make your code cleaner by reusing common property logic.

Not all property wrappers need a projectedValue, but it is useful for extra info.

Summary

wrappedValue is the main value stored or controlled by the property wrapper.

projectedValue is extra information or behavior you can access with $property.

Use these to add helpful features to your properties without repeating code.