How to Create a Custom Property Wrapper in Swift

Property wrappers are a powerful feature in Swift that allow you to define a piece of reusable code that can modify the behavior of a property. In this post, we will create a custom property wrapper that converts a string to lowercase.

Step 1: Define the Property Wrapper

To define our custom property wrapper, we need to use the @propertyWrapper attribute. The @propertyWrapper attribute indicates that we are defining a new property wrapper.

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1@propertyWrapper
2struct Lowercased {
3    private var value: String
4    
5    init(wrappedValue: String) {
6        self.value = wrappedValue.lowercased()
7    }
8    
9    var wrappedValue: String {
10        get { value }
11        set { value = newValue.lowercased() }
12    }
13}

In this example, we defined a new property wrapper called Lowercased. This property wrapper has a single property called value, which is a string. We also defined an initializer, which takes a string and sets the value property to the lowercase version of the input string. Finally, we defined a computed property called wrappedValue, which gets and sets the value of the value property.

Step 2: Use the Property Wrapper

Now that we have defined our custom property wrapper, we can use it on any property with a string type. Here's an example:

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1struct Person {
2    @Lowercased var firstName: String
3    @Lowercased var lastName: String
4}
5
6let person = Person(firstName: "John", lastName: "SMITH")
7print(person.firstName) // prints "john"
8print(person.lastName) // prints "smith"
9
10person.firstName = "JANE"
11print(person.firstName) // prints "jane"

In this example, we defined a Person struct with two properties: firstName and lastName. Both properties are decorated with the @Lowercased property wrapper. When we instantiate a new Person object, the input strings are automatically converted to lowercase.

Conclusion

In conclusion, we learned how to create a custom property wrapper in Swift that converts a string to lowercase. Property wrappers are a powerful tool that can help you write more concise and reusable code. I hope this post was helpful in understanding how to create your own custom property wrappers.