如何在Android Activity上使用Kotlin扩展功能正确执行空检查

Question

I'm new to Kotlin and trying to convert one of the many Android Util methods we have in our existing codebase into a Kotlin extension function.

This is the Kotlin code:

fun Activity?.isAlive(): Boolean {
    return !(this?.isFinishing ?: false)
}

Which is meant to be the equivalent of this Java method:

public static boolean isAlive(Activity activity) {
    return activity != null && !activity.isFinishing();
}

However, I'm still getting NPEs in the Kotlin code whenever an Activity is null . Any thoughts on where I'm going wrong?

Answer 1

I suppose you get NPE not in the isAlive() function but somewhere after, when the Activity is referenced. This is likely caused by the fact that .isAlive() returns true on null receiver.

That's because if the receiver is null , this?.isFinishing ?: false chooses the right branch false , thus !(this?.isFinishing ?: false) is true .

Try changing your function in either way so that it returns false on null receiver, for example:

fun Activity?.isAlive(): Boolean = !(this?.isFinishing ?: true)

Answer 2

I would suggest writing

fun Activity?.isAlive(): Boolean = this != null && !this.isFinishing 

I find the separate null check and condition easier to read than the combined versions.

Answer 3

虽然可接受的答案很好用，但我建议改用相等检查，因为这样会导致可读性更高的代码：

fun Activity?.isAlive(): Boolean = this?.isFinishing == false