[英]Store lambda in a variable in kotlin
I'm starting developing in Android with kotlin and I have a problem with lambdas.我开始使用 kotlin 在 Android 中进行开发,但我在使用 lambdas 时遇到了问题。 I have a function to set a listener in my view, this looks like this:
我有一个函数可以在我的视图中设置一个监听器,它看起来像这样:
fun setListener(listener: () -> Unit) {
}
The problem is that the code passed as lambda won't be executed in setListener function, it will be executed in another part of my code (specifically when an item of a spinner is selected) so I have to "save" or "store" this lambda into a variable/property so that I'm able to execute it when needed.问题是作为 lambda 传递的代码不会在 setListener 函数中执行,它将在我的代码的另一部分执行(特别是在选择微调器的项目时)所以我必须“保存”或“存储”将此 lambda 转换为变量/属性,以便我能够在需要时执行它。 Any idea about how to do it?
关于如何做到这一点的任何想法?
Edit: I've achieved it by doing:编辑:我已经通过以下方式实现了:
private var listener: (() -> Unit)? = null
fun setListener(listener: () -> Unit) {
this.listener = listener
}
Is there a better way to do it?有没有更好的方法来做到这一点? Thanks
谢谢
Here's how you can do it:您可以这样做:
class Foo {
private var listener: () -> Unit = {}
fun setListener(listener: () -> Unit) {
this.listener = listener
}
}
However, manually writing setters is discouraged in Kotlin.但是,在 Kotlin 中不鼓励手动编写 setter。 Instead, you can just make your property public:
相反,您可以公开您的财产:
class Foo {
var listener: () -> Unit = {}
}
For reference, here are the docs about properties with lots of examples.作为参考,这里是关于带有大量示例的属性的文档。
You can store a function in a property easily.您可以轻松地将函数存储在属性中。 The simplest way:
最简单的方法:
var listener: (() -> Unit)? = null
Usage:用法:
foo.listener = { println("called") }
If you want your property to be set-only, you can create one public property with unusable getter and one private property for internal use.如果您希望您的属性仅设置,您可以创建一个具有不可用 getter 的公共属性和一个供内部使用的私有属性。 Full example:
完整示例:
class Example {
// for internal use
private var _listener: (() -> Unit)? = null
// public set-only
var listener: (() -> Unit)?
@Deprecated(message = "set-only", level = DeprecationLevel.ERROR)
get() = throw AssertionError() // unusable getter
set(value) { _listener = value } // write-through setter
fun somethingHappend() {
_listener?.invoke()
}
}
Here is a simple example.这是一个简单的例子。
fun main(){
val result=::sum //function assign to variable (first method)
val result1: (first: Int, second: Int) -> Int=::sum //function assign to variable (second method)
print(result(5,6))
}
fun sum(first:Int,second:Int):Int{
return first+second;
}
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