Kotlin无法将Gradle的Action类转换为lambda [英] Kotlin not able to convert gradle's Action class to a lambda

问题描述

因此，尽管这对于gradle特定问题来说是相当多的kotlin-dsl，但我认为它总体上适用于kotlin语言本身，因此我将不使用该标记.

So, while this is quite a kotlin-dsl for gradle specific issue, I think it overall applies to the kotlin language itself, so I am not going to use that tag.

在gradle API中，类Action<T>定义为:

In the gradle API, the class Action<T> is defined as:

@HasImplicitReceiver
public interface Action<T> {
    /**
     * Performs this action against the given object.
     *
     * @param t The object to perform the action on.
     */
    void execute(T t);
 }

因此理想情况下，这应该在kotlin中有效(因为它是带有SAM的类):

So ideally, this should work in kotlin (because it is a class with a SAM):

val x : Action<String> = {
    println(">> ${it.trim(0)}")
    Unit
}

但是我收到以下两个错误:

But I get the following two errors:

Unresolved reference it
Expected Action<String> but found () -> Unit

最后，甚至Action<String> = { input: String -> ... }都不起作用.

Fwiw, even Action<String> = { input: String -> ... } doesn't work.

现在这是真正有趣的部分.如果我在IntelliJ中执行以下操作(顺便说一句，可行):

Now here's the really intriguing part. If I do the following in IntelliJ (which btw, works):

object : Action<String> {
    override fun execute(t: String?) {
        ...
    }
}

IntelliJ弹出建议Convert to lambda，当我这样做时，我得到:

IntelliJ pops the suggestion Convert to lambda, which when I do, I get:

val x = Action<String> {
}

更好，但是it仍未解决.现在指定它:

which is better, but it is still unresolved. Specifying it now:

val x = Action<String> { input -> ... }

出现以下错误Could not infer type for input和Expected no parameters.有人可以帮我解决所发生的事情吗?

gives the following errors Could not infer type for input and Expected no parameters. Can someone help me with what is going on?

推荐答案

这是因为gradle中的Action类使用

This is because the Action class in gradle is annotated with HasImplicitReceiver. From the documentation:

将SAM接口标记为lambda表达式/闭包的目标，其中单个参数作为调用的隐式接收者传递(在Kotlin中为 this ，delegate(在Groovy中为c9))，就好像lambda表达式是参数类型的扩展方法.

Marks a SAM interface as a target for lambda expressions / closures where the single parameter is passed as the implicit receiver of the invocation (this in Kotlin, delegate in Groovy) as if the lambda expression was an extension method of the parameter type.

^{(重点是我的)}

因此，下面的编译就可以了:

So, the following compiles just fine:

val x = Action<String> {
    println(">> ${this.trim()}")
}

您甚至可以只写${trim()}并在其前面省略this.

You could even just write ${trim()} and omit the this in front of it.

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