Kotlin如何使setOnClickListener接受函数作为参数 [英] How kotlin makes setOnClickListener accept functions as parameter

问题描述

在kotlin中，我们可以这样使用setOnClickListener():

In kotlin, we can use setOnClickListener() like this:

view.setOnClickListener { println("Hello") }

但是，如果我定义自己的接口，则只能传递这样的匿名对象:

But if I define my own interface, I can only pass anonymous object like this:

obj.setMyListener(object: MyListener() {
    ...
})

我只是想知道它们如何使setOnClickListener()接受函数而不是匿名对象.

I just wondering how they make setOnClickListener() accept a function rather than an anonymous object.

推荐答案

根据有关 Java互操作，对于用Java定义的功能接口，您可以使用SAM转换.

According to Kotlin documentations about Java interop, for a functional interface defined in Java, you can use a SAM conversion.

就像Java 8一样，Kotlin支持SAM转换.这意味着 Kotlin函数文字可以自动转换为 Java接口的单个​​非默认方法的实现， 只要接口方法的参数类型与 Kotlin函数的参数类型.

Just like Java 8, Kotlin supports SAM conversions. This means that Kotlin function literals can be automatically converted into implementations of Java interfaces with a single non-default method, as long as the parameter types of the interface method match the parameter types of the Kotlin function.

val runnable = Runnable { println("This runs in a runnable") }

val executor = ThreadPoolExecutor()
// Java signature: void execute(Runnable command)
executor.execute { println("This runs in a thread pool") }

但是，Kotlin具有功能类型，因此SAM转换不适用于Kotlin中定义的接口:

However, Kotlin has functional types, therefore SAM conversion doesn't work for interfaces defined in Kotlin:

还请注意，此功能仅适用于Java互操作.自科特林以来 具有适当的功能类型，可以将功能自动转换为 Kotlin接口的实现是不必要的，因此 不支持.

Also note that this feature works only for Java interop; since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported.

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可能的解决方案:

1. 在Java中定义接口.如果它是可以从Java代码中使用的库/代码，则很有用.

2. 使方法接受函数作为参数:

1. Define the interface in Java. Useful if it is a library/code that may be used from a Java code.

2. Make the method receive a function as argument:

// Example listener receives a bool and return unit.  
fun setMyListener(listener: (isChecked: Bool) -> Unit) { ... }  
// Usage:  
obj.setMyListener { isChecked -> }

使用类型别名(仅Kotlin 1.1+支持):

Use type alias (only supported in Kotlin 1.1+):

typealias MyListener = (Bool) -> Unit
fun setMyListener(listener: MyListener) { ... }

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