Kotlin：return@是什么意思

In Kotlin, the return@label syntax is used for specifying which function among several nested ones this statement returns from.

It works with function literals (lambdas) and local functions. Non-labeled return statements return from the nearest (i.e. innermost) enclosing fun (ignoring lambdas). Consider this function:

fun foo(ints: List<Int>) {
    ints.forEach {
        if (it == 0) return
        print(it)
    }
}

^{Here, return will finish the execution of foo, not just the lambda.}

But if you want to return from any other function (a lambda or an outer fun) you have to specify it as a label at return statement:

fun foo(ints: List<Int>) {
    ints.forEach {
        if (it == 0) return@forEach // implicit label for lambda passed to forEach
        print(it)
    }
}

fun foo(ints: List<Int>): List<String> {
    val result = ints.map f@{
        if (it == 0) return@f "zero" // return at named label
        if (it == -1) return emptyList() // return at foo
        "number $it" // expression returned from lambda
    }
    return result
}

foo(listOf(1, -1, 1)) // []
foo(listOf(1, 0, 1)) // ["number 1", "zero", "number 1"]

只有local和inline函数支持从lambda返回Non-local return(即从外部函数返回)，因为如果lambda没有内联(或将函数放在对象内部)，则不能保证只在封闭函数内调用它(例如，它可以存储在变量中，稍后调用)，在这种情况下，非本地返回没有意义.

There is also a similar syntax for qualified this, which is used to reference receivers of outer scopes: this@outer.