Kotlin and its cool functions

Kotlin-functions.md

Kotlin and its cool functions

Table of contents

• apply
• also
• let
• run
• with
• use
• Custom one

Official page is here

apply

inline fun <T> T.apply(block: T.() -> Unit): T

The context object is available as a receiver (this). The return value is the object itself.

Example:

class User(var name: String, var age: Int)
val user = User("John", 25) //John, 25

val anotherUser = user.apply {
    age = 32
} //John, 32

also

inline fun <T> T.also(block: (T) -> Unit): T

The context object is available as an argument (it). The return value is the object itself.

Example:

data class User(val name: String, val age: Int)
val user = User("John", 25)

user.also {
    println("I'm ${it.name} and I'm ${it.age} years old")
}

let

inline fun <T, R> T.let(block: (T) -> R): R

The context object is available as an argument (it). The return value is lambda result

Example:

data class User(val name: String, val age: Int)
val user = User("John", 25)

val ageAfter5Years = user.let { it.age + 5 }    // 30

run

The context object is available as a receiver (this). The return value is lambda result.

Example:

data class User(val name: String, val age: Int)
val user = User("John", 25)

val nameUppercase = user.run { name.toUpperCase() } //JOHN

It can also run without context object

val area = run {
    val r = 20.0
    const val PI = 3.1415
    PI*r*r
}

with

Example:

class User(val name: String, val age: Int) {
    fun indroduce() {
        println("I'm ${it.name} and I'm ${it.age} years old")
    }
}
val user = User("John", 25)

with(user) {
    println("Introducing ${name}")
    this.introduce()
}

use

Used with Closeables. Calls close() at the end of the block

Example:

Files.newOutputStream(path).use {
    it.write("Hello")
} //close() is called here

Custom one

To define as this:

class Dog(var name: String) {
    fun bark() { println("Woof woof") }
}

fun Dog.doStuff(block: Dog.() -> Unit) {
    println("Doing dog stuff...")
    block(this)
}

val dog = Dog("Bob")

dog.doStuff {
    println("I'm $name")
    bark()
}

To define as it:

class Cat(var name: String) {
    fun meaw() { println("Meaow!") }
}

fun Cat.doStuff(block: (Cat) -> Unit) {
    println("Doing cat stuff...")
    block(this)
}

val cat = Dog("Kitty")

cat.doStuff {
    println("I'm ${it.name}")
    it.meaow()
}