Standard.kt一覽

TODO

@kotlin.internal.InlineOnly
public inline fun TODO(): Nothing = throw NotImplementedError()

/**
 * Always throws [NotImplementedError] stating that operation is not implemented.
 *
 * @param reason a string explaining why the implementation is missing.
 */
@kotlin.internal.InlineOnly
public inline fun TODO(reason: String): Nothing = throw NotImplementedError("An operation is not implemented: $reason")
           

與java的//TODO很像，最大的差別是這個TODO會抛出異常

例：

fun main(args: Array<String>) {
    TODO()
}
           

Exception in thread “main” kotlin.NotImplementedError: An operation is not implemented.

或者：

fun main(args: Array<String>) {
    TODO("qfxl")
}
           

Exception in thread “main” kotlin.NotImplementedError: An operation is not implemented: qfxl

run

/**
 * Calls the specified function [block] and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <R> run(block: () -> R): R = block()
           

調用

block

并且傳回

block

的結果

例：

val qfxl = run {
        println("qfxl")
        "qfxl"
    }
    println(qfxl)
           

或者：

val qfxl = "qfxl".run {
        println(this)
        this.substring(,)//this可以省略
    }
    println(qfxl)
           

輸出

qfxl

qf

在run中，用this代表目前引用對象，并且調用其方法時，this可省略。

傳回值是語句塊的最後一行，若最後一行語句無傳回值，則整個run語句塊也無傳回值

with

/**
 * Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
           

執行接收類型為

T

的

block

并且傳回

block

的結果

例：

val qfxl = with("qfxl") {
        println(this)
        substring(, )
    }
    println(qfxl)
           

輸出

qfxl

qf

also

@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
           

執行參數為目前對象的

block

并且傳回目前對象

例：

val qfxl = "qfxl".also {
        println(it)
        it.substring(, )
    }
    println(qfxl)//傳回值依然是"qfxl"
           

輸出

qfxl

qfxl

let

/**
 * Calls the specified function [block] with `this` value as its argument and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
           

執行參數為目前對象的

block

并且

block

的結果

例：

val qfxl = "qfxl".let {
        println(it)
        it.substring(, )
    }
    println(qfxl)
           

輸出

qfxl

qf

takeIf

/**
 * Returns `this` value if it satisfies the given [predicate] or `null`, if it doesn't.
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
           

條件成立傳回該對象，否則傳回null

例：

val qfxl = "qfxl".takeIf {
        it.length > 
    }
    println(qfxl)
           

輸出

null

或者:

val qfxl = "qfxl".takeIf {
        it.length > 
    }
    println(qfxl)
           

輸出

qfxl

takeUnless

/**
 * Returns `this` value if it _does not_ satisfy the given [predicate] or `null`, if it does.
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
           

*與takeIf相反，條件成立傳回null，否則傳回該對象*

例：

val qfxl = "qfxl".takeUnless {
        it.length > 
    }
    println(qfxl)
           

輸出

null

或者:

val qfxl = "qfxl".takeUnless {
        it.length > 
    }
    println(qfxl)
           

輸出

qfxl

repeat

/**
 * Executes the given function [action] specified number of [times].
 *
 * A zero-based index of current iteration is passed as a parameter to [action].
 */
@kotlin.internal.InlineOnly
public inline fun repeat(times: Int, action: (Int) -> Unit) {
    for (index in .times - ) {
        action(index)
    }
}
           

源碼可知

repeat(, {
        println("qfxl")
    })
           

輸出

qfxl

qfxl

qfxl

總結

傳回目前對象的操作符

apply、also

傳回block結果的操作符

run、with、let