Wonder Shine Blog

Kotlin Scope Functions (run, apply, let, also, with)

run

inline fun <T, R> T.run(block: T.() -> R): R {
    return block()
}

// usage
val item : R = T().run {
   val myDao = getMyDao();
   return@run myDao.insert(Row(this.name));
}
  • to calculate some value
  • to limit multiple local variables’ scope

apply

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

// usage
val item : T = T().apply { 
     this.age = 16
     this.name = "name"
}
  • to change property without using its function

let

inline fun <T, R> T.let(block: (T) -> R): R {
    return block(this)
}

// usage
val item : R = T()?.let {
    dao.insert(Row(it.name));
    return@let it.toR();
}
  • to run code when object is not null
  • changing nullable object to another nullable object
  • limit local variable scope

also

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

// usage
val item : T = T().also {
    requireNotNull(it.age)
}
  • takes effect not using “this” function or changing “this” property.
  • “this” property sanity check, logging, etc

with

inline fun <T, R> with(receiver: T, block: T.() -> R): R {
    return receiver.block()
}

// usage
val item : R = with(T()) { 
    print(this.name)
    print(this.age)
}
  • when object is not nullable
  • when result of block is not necessary

References

https://medium.com/@limgyumin/%EC%BD%94%ED%8B%80%EB%A6%B0-%EC%9D%98-apply-with-let-also-run-%EC%9D%80-%EC%96%B8%EC%A0%9C-%EC%82%AC%EC%9A%A9%ED%95%98%EB%8A%94%EA%B0%80-4a517292df29 https://kotlinlang.org/docs/reference/scope-functions.html