在执行IF块之前测试对象成员是否为null [英] Test object member for null before executing IF block

问题描述

我有以下代码:

class Countries {
    var list: MutableList<String>? = null
}

val countries = Countries()

if (countries.list!!.isNotEmpty()) {

}

在运行时，这将引发异常，因为list为null.我可以改为:

At runtime this will raise an exception because list is null. I can do this instead:

if ((countries.list != null) && countries.list!!.isNotEmpty()) {

}

如果我有一个名为areInitialized的布尔成员，该布尔成员可以为空，那么我可以创建一个像这样的infix函数:

If I had a boolean member called areInitialized that was nullable, I could create a infix function like this:

infix fun Any?.ifTrue(block: () -> Unit) {
    if ((this != null) && this == true) block()
}

然后像这样使用它:

countries.areInitialized ifTrue {

}

但是我似乎无法为可变列表创建类似的内容.

But I can't seem to create something similar for a mutable list.

但是我讨厌不得不在代码的其他部分的成员字段上重复此测试以使null为空. Kotlin中有更简单的方法可以做到这一点吗?

But I hate having to repeat this test for null on an member field in other parts of code. Is there a simpler way in Kotlin to do this?

推荐答案

我会尽可能地坚持标准.因此，在您的示例中，我不会介绍该ifTrue函数，而是使用 takeIf 或 takeUnless 与安全运算符?.结合使用代替，例如:

I would try to stick to the standard as often as you can. So in your example I wouldn't have introduced that ifTrue-function, but rather used takeIf or takeUnless in combination with the safe operator ?. instead, e.g.:

countries?.takeIf { it.areInitialized == true }
         ?.also { 
             /* do something with countries */ 
         }

或者，如果必须返回一个值，则将also与let交换(或查看其他

Or if you must return a value, exchange also with let (or see the other scope functions).

然后，同样适用于countries中的列表:

The same then also works for the list within countries:

countries?.takeUnless { it.list.isNullOrEmpty() }
         ?.also { 
             /* do something with countries */ 
             it.list!!.forEach(::println)
         }

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