kotlin standard functions - let, with, apply, run, also, takeIf, takeUnless, use, repeat

1. Kotlin StandardLib
let, with, apply, run, also…
LazySoul
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2. Standard
• let
• with
• apply
• run
• also
• takeIf, takeUnless
• use
• repeat

3. Standard
data class Person(var name: String = "myeongin",
var age: Int = 18)
val person = Person()

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

5. let
val name = person.let { it.name }
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

6. let
val name = person.let { it.name }
println(name) // myeongin
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

7. let
val person: Person? = Person()
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

8. let
val person: Person? = Person()
val name = person?.let { it.name }
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

9. let
val person: Person? = Person()
val name = person?.let { it.name }
println(name) // myeongin
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

10. let
val person: Person? = null
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

11. let
val person: Person? = null
val name = person.let { it.name } // Error
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

12. let
val person: Person? = null
val name = person?.let { it.name }
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

13. let
val person: Person? = null
val name = person?.let { it.name }
println(name) // null
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

14. let
val person: Person? = null
val name = person?.let { it.name } ?: "Lezhin"
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

15. let
val person: Person? = null
val name = person?.let { it.name } ?: "Lezhin"
println(name) // Lezhin
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

16. let
fun sendEmail(email: String) {
TODO()
}
var email: String? = null
email?.let { sendEmail(it) }
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

17. let
• type T(자신)을 block으로 감쌉니다.
• block 안에서 자기자신에게 접근 할때는 it을 사용합니다.
• 표현식으로 사용한다면 반환 값 R은 괄호 제일 마지막 라인의 값이 됩니다.
• ?.을 사용해 null check를 할 수 있습니다.
• null일 경우에 ?. 을 사용하면 null 을 반환합니다.
inline fun <T, R> T.let(block: (T) -> R)
: R = block(this)

18. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()

19. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()
with(person) {
name = "ari"
}

20. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()
with(person) {
name = "ari"
}
println(person) // Person(name=ari, age=18)

21. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()
recyclerView.apply {
layoutManager = LinearLayoutManager(context, LinearLayoutManager
adapter = this@Activity.adapter
addItemDecoration(Adapter.ContentDecoration(context))
}

22. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()
val ari = with(person) {
name = "ari"
}
println(ari) // Unit

23. with
inline fun <T, R> with(receiver: T, block: T.() -> R)
: R = receiver.block()
• T를 with의 인자로 전달합니다.
• block 안에서는 마치 object 내부 처럼 property를 자유롭게 접근합니다.
• 표현식으로 사용한다면 반환 값 R은 괄호 제일 마지막 라인의 값이 됩니다.
• 특정 instance의 내부 값들을 수정 하거나 조합할때 사용하면 좋습니다.

24. apply
inline fun <T> T.apply(f: T.() -> Unit)
: T { f(); return this }

25. apply
inline fun <T> T.apply(f: T.() -> Unit)
: T { f(); return this }
val person = Person().apply {
name = "ari"
age = 24
}

26. apply
inline fun <T> T.apply(f: T.() -> Unit)
: T { f(); return this }
val person = Person().apply {
name = "ari"
age = 24
}
println(person) //Person(name=ari, age=24)

27. apply
inline fun <T> T.apply(f: T.() -> Unit)
: T { f(); return this }
AlertDialog.Builder(this).apply {
setTitle(getString(R.string.title))
setMessage(getString(R.string.message))
setPositiveButton(getString(R.string.ok),
{ dlg, _ -> dlg.dismiss() })
create()
show()
}

28. apply
inline fun <T> T.apply(f: T.() -> Unit)
: T { f(); return this }
• type.apply { .. } 을 사용합니다.
• type T를 입력받아 { .. } 안을 수행하고 자기 자신을 반환합니다.
• 생성과 동시에 추가적인 작업을 해줘야 할 때 유용합니다.

29. run
inline fun <R> run(block: () -> R): R = block()
inline fun <T, R> T.run(block: T.() -> R)
: R = block()

30. run
val name = person.run { name }
inline fun <R> run(block: () -> R): R = block()
inline fun <T, R> T.run(block: T.() -> R)
: R = block()

31. run
val name = person.run { name }
println(name) // myeongin
inline fun <R> run(block: () -> R): R = block()
inline fun <T, R> T.run(block: T.() -> R)
: R = block()

32. run
run {
sendEmail(name)
}
inline fun <R> run(block: () -> R): R = block()
inline fun <T, R> T.run(block: T.() -> R)
: R = block()

33. run
• let과 거의 유사합니다. 하지만 let은 T를 인자로 받지만 (T) run은 T.() T의 자신을 블럭 으로 지
정합니다.(this)
• 별도로 함수를 만들지 않고 block 을 통해 함수 구문을 만들고 실행 할수 있습니다.
• 역시나 마지막줄의 값을 반환합니다.
inline fun <R> run(block: () -> R): R = block()
inline fun <T, R> T.run(block: T.() -> R)
: R = block()

34. also
inline fun <T> T.also(block: (T) -> Unit)
: T { block(this); return this }

35. also
inline fun <T> T.also(block: (T) -> Unit)
: T { block(this); return this }
person.also {
it.name = "ari"
}
println(person) //Person(name=ari, age=18)

36. also
inline fun <T> T.also(block: (T) -> Unit)
: T { block(this); return this }
• let과 거의 유사합니다. let은 R을 반환 하는 반면 also는 자기자신 T를 반환합니다.
• 마지막줄의 값과 상관없이 자기 자신을 반환합니다.

37. let, run, with, also, apply

38. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null

39. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }

40. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }
println(ari) // null

41. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }
println(ari) // null
val myeongin = person.takeIf { it.name == "myeongin" }

42. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }
println(ari) // null
val myeongin = person.takeIf { it.name == "myeongin" }
println(myeongin) // Person(name=myeongin, age=18)

43. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }
println(ari) // null
val myeongin = person.takeIf { it.name == "myeongin" }
println(myeongin) // Person(name=myeongin, age=18)
val default = person.takeIf { it.name == "ari" } ?: Person("ari", 24)

44. takeIf
inline fun <T> T.takeIf(predicate: (T) -> Boolean)
: T? = if (predicate(this)) this else null
val ari = person.takeIf { it.name == "ari" }
println(ari) // null
val myeongin = person.takeIf { it.name == "myeongin" }
println(myeongin) // Person(name=myeongin, age=18)
val default = person.takeIf { it.name == "ari" } ?: Person("ari", 24)
println(default) // Person(name=ari, age=24)

45. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null

46. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }

47. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }
println(ari) // Person(name=myeongin, age=18)

48. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }
println(ari) // Person(name=myeongin, age=18)
val myeongin = person.takeUnless {
it.name == "myeongin" }

49. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }
println(ari) // Person(name=myeongin, age=18)
val myeongin = person.takeUnless {
it.name == "myeongin" }
println(myeongin) // null

50. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }
println(ari) // Person(name=myeongin, age=18)
val myeongin = person.takeUnless {
it.name == "myeongin" }
println(myeongin) // null
val default = person.takeUnless { it.name == "ari" }
?: Person("ari", 24)

51. takeUnless
inline fun <T> T.takeUnless(predicate: (T) -> Boolean)
: T? = if (!predicate(this)) this else null
val ari = person.takeUnless { it.name == "ari" }
println(ari) // Person(name=myeongin, age=18)
val myeongin = person.takeUnless {
it.name == "myeongin" }
println(myeongin) // null
val default = person.takeUnless { it.name == "ari" }
?: Person("ari", 24)
println(default) // Person(name=myeongin, age=18)

52. use
// Java 1.6
Properties property = new Properties();
FileInputStream stream = new FileInputStream("config.properties");
try {
property.load(steam);
} finally {
stream.close();
}
// Java 1.7
Properties property = new Properties();
try (FileInputStream stream = new FileInputStream("config.properties")) {
property.load(stream);
} // FileInputStream는 자동으로 close됨

53. use
public inline fun <T : Closeable?, R> T.use(block: (T) -> R): R {
var closed = false
try {
return block(this)
} catch (e: Exception) {
closed = true
try {
this?.close()
} catch (closeException: Exception) {
}
throw e
} finally {
if (!closed) {
this?.close()
}
}
}

54. use
public inline fun <T : Closeable?, R> T.use(block: (T) -> R): R {
var closed = false
try {
return block(this)
} catch (e: Exception) {
closed = true
try {
this?.close()
} catch (closeException: Exception) {
}
throw e
} finally {
if (!closed) {
this?.close()
}
}
}
val property = Properties()
FileInputStream("config.properties").use {
property.load(it)
} // FileInputStream이 자동으로 close됨.

55. repeat
inline fun repeat(times: Int, action: (Int) -> Unit) {
for (index in 0..times - 1) {
action(index)
}
}

56. repeat
inline fun repeat(times: Int, action: (Int) -> Unit) {
for (index in 0..times - 1) {
action(index)
}
}
repeat(4, { println("Hello, World") })

57. repeat
inline fun repeat(times: Int, action: (Int) -> Unit) {
for (index in 0..times - 1) {
action(index)
}
}
repeat(4, { println("Hello, World") })
// Hello, World
// Hello, World
// Hello, World
// Hello, World