Property Wrappers in Swift: How to simplify your codebase and add functionality

What are property wrappers?

Property wrappers are a feature introduced in Swift 5.1 that allows developers to add behaviors or functionality to properties in a reusable and composable way. These are structures that ‘wrap’ a property and provide a custom getter and setter for it.

Thus, for example, we can use property containers to add functionality such as: thread safety, data validation, or observation of key-value pairs of properties.

The @ syntax. In Swift, the @ symbol is used to indicate that what follows is an annotation, attribute, or modifier (as in @propertyWrapper).

Example of a property wrapper

For example, suppose we have a property to which we want to add a property wrapper that allows us to know when the property was last accessed:

@propertyWrapper
struct TimeStamp<Value> {
    private var value: Value
    private(set) var lastAccessed: Date

    var wrappedValue: Value {
        get {
            lastAccessed = Date()
            return value
        }
        set {
            value = newValue
        }
    }

    init(wrappedValue: Value) {
        self.value = wrappedValue
        self.lastAccessed = Date()
    }
}

struct User {
    @TimeStamp var name: String
}

In this example, the TimeStamp wrapper property is defined with a generic value type, which represents the type of property it will wrap. TimeStamp has a private stored property, value, that stores the actual wrapped value and a private computed property, lastAccessed, that will store the timestamp of the last access to the wrapped value. The wrapValue computed property is the core of a property wrapper, as it is used to provide the custom getter and setter for the wrapped property.

Let’s se how to use this new property wrapper:

let user = User(name: "John")
print(user.name)  // "John"
print(user.$name.lastAccessed)  // the time when the name was accessed

Useful applications of property wrappers

In applications, one of the most common ways to save information is using UserDefaults, but when using this system we find all the code to retrieve and save data. Let’s see if we can simplify it by using property wrappers.

@propertyWrapper
struct UserDefault<T> {
    let key: String
    let defaultValue: T

    init(_ key: String, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }

    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key)
        }
    }
}

In this example, the wrapper structure for the UserDefault property takes a key and a defaultValue as its initialization parameters. The wrapValue computed property provides the custom getter and setter for the wrapped property:

• The getter uses the object(forKey:) method of UserDefaults to retrieve the value of the specified key.

• The setter uses the set(_:forKey:) method to store the value of the specified key.

struct Settings {
    @UserDefault("username", defaultValue: "")
    var username: String
}

let settings = Settings()
settings.username = "John"
print(settings.username)  // "John"

As we can see, the username property is wrapped with the UserDefault property wrapper (with the key ‘username’ and an empty string as a default value). When the username property is set to ‘John’, the UserDefault property wrapper’s setter is called, ant it stores the value ‘John’ in UserDefaults with the key username. When the username property is accessed, the getter of the UserDefault property wrapper is called, which returns the value ‘John’.

With this we have achieved to abstract away the details of reading and writing to UserDefaults, making it easy to add or change the persistence layer in the future, without affecting the rest of the codebase.

Pros and cons of property wrappers

Pros

• They make it simple to encapsulate complex property logic and behavior in a single and reusable struct.
• They make it simple to add thread-safety, data validation, and key-value observing to properties.
• They enable developers to add additional behavior or functionality to properties in a modular fashion.

Cons

• They can make the codebase more complex and difficult to understand, especially if used excessively or in unexpected ways.
• They can make the codebase more verbose by increasing the amount of boilerplate code required to implement a feature.
• Because of the additional indirection introduced by the property wrapper, they can add a small amount of overhead to the app’s performance.

Conclusion

Finally, property wrappers are a feature in Swift 5.1 that allows developers to attach extra behavior or functionality to properties in a reusable, composable manner.