Kotlin Documentation | Scope functions

Kotlin Documentation 시리즈에 대해

Category: Standard library

문서 링크: https://kotlinlang.org/docs/scope-functions.html

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Function selection

각 함수의 의도:

• Executing a lambda on non-null objects: let

val name: String? = "John"
name?.let { println("Hello, $it") } // Executes the lambda only if name is non-null

• Introducing an expression as a variable in local scope: let

val result = someObject.let { calculateResult(it) } // Introduces the result of the expression as a variable in local scope

• Object configuration: apply

val person = Person().apply {
    name = "John"
    age = 30
    address = "123 Main St"
}

• Object configuration and computing the result: run

val result = someObject.run {
    performCalculation()
    computeResult()
}

• Running statements where an expression is required: non-extension run

val result = run {
    val a = 5
    val b = 10
    a + b // Expression required
}

• Additional effects: also

val name = "John"
val length = name.length.also { println("Length of name: $it") } // Performs additional effect of printing the length

• Grouping function calls on an object: with

val person = Person()
with(person) {
    setName("John")
    setAge(30)
    setAddress("123 Main St")
}

takeIf and takeUnless

predicate가 각각 true, false이면 값을 반환하고, 아닐 경우 null을 반환한다.

val number = Random.nextInt(100)
println("number: $number")
val evenOrNull = number.takeIf { it % 2 == 0 }
val oddOrNull = number.takeUnless { it % 2 == 0 }
println("even: $evenOrNull, odd: $oddOrNull")

number: 3
even: null, odd: 3

number: 2
even: 2, odd: null

scope functions와 결합할 때 유용하다:

input.indexOf(sub).takeIf { it >= 0 }?.let {
    println("The substring $sub is found in $input.")
    println("Its start position is $it.")
}