仅在变量为null时分配变量 [英] assign variable only if it is null

问题描述

在Ruby上有这样的东西:

on Ruby one have something like this:

@var ||= 'value' 

基本上，这意味着仅当尚未分配@var时(例如，如果@var为nil)，@var将被分配'value'

basically, it means that @var will be assigned 'value' only if @var is not assigned yet (e.g. if @var is nil)

我正在Kotlin上寻找相同的东西，但到目前为止，最接近的东西是Elvis运算符.有那样的东西，我错过了文档吗?

I'm looking for the same on Kotlin, but so far, the closest thing would be the elvis operator. Is there something like that and I missed the documentation?

推荐答案

我能想到的最短的方法确实是使用elvis运算符:

The shortest way I can think of is indeed using the elvis operator:

value = value ?: newValue

如果您经常这样做，则可以选择使用委派的属性 ，仅在null:

If you do this often, an alternative is to use a delegated property, which only stores the value if its null:

class Once<T> {

    private var value: T? = null

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T? {
        return value
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
        this.value = this.value ?: value
    }
}

您现在可以创建一个使用该属性的属性，如下所示:

You can now create a property that uses this like so:

var value by Once<String>()

fun main(args: Array<String>) {
    println(value) // 'null'
    value = "1"
    println(value) // '1'
    value = "2"
    println(value) // '1'
}

请注意，这不是线程安全的，不允许将其设置回null.另外，这确实会评估new表达式，而简单的elvis运算符版本则不会.

Note that this is not thread-safe and does not allow setting back to null. Also, this does evaluate the new expression while the simple elvis operator version might not.

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