Kotlin:什么是“return@"?意思是? [英] Kotlin: Whats does "return@" mean?

问题描述

我在我的一个项目中使用 RxJava，我使用 Android Studio 插件和 map flatMap lambda(java 中的 Func1)之一将我的一个类转换为 Kotlin，中间体返回外观像下面的@Func1.

I'm using RxJava in one of my projects, I converted one of my classes to Kotlin using the Android Studio plugin and in one of map flatMap lambda (Func1 in java), intermediates returns looks like the following @Func1.

我不知道这是什么意思.

I have no idea what this means.

something.flatMap(Func1<ArticleCriteria, Observable<Pair<String, String>>> {
    val isTemporaryClone = it.isATemporaryClone
    val isTheOriginalToken = it.tokenIsOriginalHere

    if (isTemporaryClone) {
        if (!isTheOriginalToken) {
            return@Func1 paramsError("Token is always original for temp articles")
        }

        return@Func1 mJobRunner.doNotRun(DeleteArticleJob.TAG)
                            .doOnNext(deletePersonalActionById(articleId))
    }

    runArticleJobAsync(DeleteArticleJob.TAG, it)
})

推荐答案

在 Kotlin 中，return@label 语法用于指定该语句返回的嵌套函数中的哪个函数.

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

它适用于函数字面量 (lambdas) 和局部函数.未标记的 return 语句从最近的(即最里面的)封闭的 fun (忽略 lambdas)返回.考虑这个函数:

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)
    }
}

^{这里，return 将完成 foo 的执行，而不仅仅是 lambda.}

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

但是如果您想从任何其他函数(一个 lambda 或外部 fun)返回，您必须在 return 语句中将其指定为标签:

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"]

非本地返回(即从来自 lambda 的外部函数)仅支持 local 和 inline 函数，因为如果 lambda 没有内联(或者函数被放置在对象内), 并不能保证只在封闭函数内部调用(例如它可以存储在变量中并稍后调用)，并且在这种情况下非局部返回没有意义.

Non-local return (i.e. return from outer functions) from a lambda is only supported for local and inline functions, because if a lambda is not inlined (or a function is placed inside an object), it is not guaranteed to be called only inside the enclosing function (e.g. it can be stored in a variable and called later), and the non-local return would make no sense in this case.

对于限定的this，用于引用外部作用域的接收者:this@outer.

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