kotlin let、run、with、also、apply函数的区别

一、let

当前对象：it
返回值：闭包

@kotlin.internal.InlineOnly
public inline fun  T.let(block: (T) -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block(this)
}

二、run

当前对象：this或省略
返回值：闭包

@kotlin.internal.InlineOnly
public inline fun  T.run(block: T.() -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block()
}

三、also

当前对象：it
返回值：this

@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun  T.also(block: (T) -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block(this)
    return this
}

四、apply

当前对象：this和省略
返回值：this

@kotlin.internal.InlineOnly
public inline fun  T.apply(block: T.() -> Unit): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block()
    return this
}

五、with

当前对象：this或省略
返回对象：闭包

@kotlin.internal.InlineOnly
public inline fun  with(receiver: T, block: T.() -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return receiver.block()
}