Kotlin apply scope function in action

The apply keyword is one of the keywords used to create a scope function in Kotlin.

A scope function is a block of code that’s executed under the context of an object where you call the function.

With the apply function keyword, you’ll be able to modify or call the methods of your object in one scoped block.

Let’s see the apply function in action. Suppose you have a User class with the following definitions:

class User(var username: String, var isActive: Boolean) {
    fun hello() {
        println("Hello! I'm $username")
    }
}

Next, you create an instance of the class with the following code:

val user = User("Nathan", true)

You can change the object properties and call its hello() method as follows:

val user = User("Nathan", true)

user.username = "Jane"
user.isActive = false
user.hello()

Alternatively, you can use the apply keyword and create a lambda expression that operates on your object.

Take a look at the following code:

user.apply {
    username = "Jane" // or this.username = "Jane"
    isActive = false
    hello()
}

Inside the apply lambda expression body scope, you can access the properties and methods of the context object using the this keyword.

But Kotlin is also smart enough that you can omit the this keyword when accessing the members of the object, as shown above.

The apply method returns the context object as its return value. While you can call the methods defined in your class from inside it, any value returned by your method will be ignored.

For example, let’s add a function named getNum() that returns a number value in the User class:

class User(var username: String, var isActive: Boolean) {
    fun hello() {
        println("Hello! I'm $username")
    }

    fun getNum(): Int {
        return 7
    }
}

Next, call it inside the apply block as follows:

val user = User("Nathan", true)

val num = user.apply {
    getNum()
}

println(num) // User@51016012

Instead of returning the number 7 to the num variable, the apply method returns the object itself.

The apply method can be read as an instruction to “apply the following configuration changes to the object”, so calling methods from inside the method is not recommended.

Because the object is the return value, you can add a function call chain immediately after the apply block like this:

user.apply {
    username = "Jane"
    isActive = false
}.hello() // Hello! I'm Jane

If you need a different return value from the scoped function, you need to use the let function instead of the apply function.

Learn more: Kotlin let function keyword explained

Now you’ve learned how the apply function works in Kotlin. Good work! 👍