Property Wrapper in Swift: Brief Introduction with Code Examples

Q.1 What is Property Wrapper?

Ans. In Swift, PropertyWrapper is a feature that allows you to add extra behavior to properties by defining a custom type and using it to wrap the property’s value.

It helps you manage repetitive logic, such as validation, default values, or transformation, in a clean and reusable way.

To make it a property wrapper, mark it with the @propertyWrapper attribute, which must have a property named wrappedValue.

This wrappedValue property is where the actual value, wrapped by the property wrapper, is stored.

Property wrappers were introduced in Swift 5.1

Q.2 How to define a Property Wrapper?

Ans. We have to take care of two requirements for a property wrapper type

1- We define the property wrapper using the @propertyWrapper attribute.
2- It must have a wrappedValue property which is a computed property.

Example-1 Write a wrapper for the capitalized first character.

Let’s implement Property Wrapper in five easy steps–

Step-1 We create a new struct called Capital

struct Capital {
    var value: String
}

Step-2 We annotate the struct with @propertyWrapper

@propertyWrapper
struct Capital {
    var value: String
}

Step-3 We implement wrappedValue with our business logic

@propertyWrapper
struct Capital {
    var value: String
    var wrappedValue: String {
        get { value.capitalized }
        set { value = newValue }
    }
}

Step- 4 We implement an initializer

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

Step -5 we will use our property wrapper Capital

struct User {
    @Capital var firstName: String
    @Capital var lastName: String
     var address: String
}

let user = User(firstName: "shivi", lastName: "tiwari", address: "varanasi")
print(user.firstName)
print(user.lastName)
print(user.address)

Output:-
Shivi
Tiwari
varanasi

Explanation:-

In the code, we have defined a property wrapper Capital that capitalizes the value it wraps.

We have also defined a User struct with properties firstName, lastName, and address. The firstName and lastName properties use the @Capital property wrapper to automatically capitalize the values assigned to them.

As we can see, when we create a User instance and assign values to firstName and lastName, the @Capital property wrapper automatically capitalizes the values, ensuring that the first letter of each word is in uppercase. The address property is not using the @Capital property wrapper, so its value remains unchanged.

Read More:

Property Wrapper Official Doc
Top iOS Interview Questions and Answers