我什么时候应该使用 let {} 什么时候使用 != null

Question

你会如何以更 kotlinic 的方式编写以下代码？

var returnValue = ...
val s3data = presignedUrl.body()
if (s3data != null) {
    val uploadImage = api.uploadImage(s3data.bucketUrl, s3data.awsAccessKeyId, s3data.policy, s3data.key, s3data.signature, body).execute()
    if (!uploadImage.isSuccessful) {
        crashReporterService.sendIssue("Failed uploading file", "Failed uploading file ${uploadImage.raw()}")
        returnValue = Result.FAILURE
    } else {
        returnValue = Result.SUCCESS
    }
} else {
    crashReporterService.sendIssue("Failed uploading image", "Error - ${presignedUrl.raw()}")
    returnValue = Result.FAILURE
}

return returnValue

我可以使用 let，但我觉得它让代码更难理解

Answer 1

if/else is an expression in Kotlin，所以下面肯定更像 Kotlinesque：

val s3data = presignedUrl.body()
return if (s3data != null) {
    val uploadImage = api.uploadImage(s3data.bucketUrl, s3data.awsAccessKeyId, s3data.policy, s3data.key, s3data.signature, body).execute()
    if (!uploadImage.isSuccessful) {
        crashReporterService.sendIssue("Failed uploading file", "Failed uploading file ${uploadImage.raw()}")
        Result.FAILURE
    } else {
        Result.SUCCESS
    }
} else {
    crashReporterService.sendIssue("Failed uploading image", "Error - ${presignedUrl.raw()}")
    Result.FAILURE
}

Answer 2

公共共享代码 - 在这种情况下，错误报告和 return 失败结果 - 可以合并到 local function.
可空性导致 return（在这种情况下，s3data 可以为空）通常可以用 returning ?: elvis operator.
当一遍又一遍地键入同一个变量时（在本例中，访问 s3data），run block is appropriate. If confused, see
正如另一个答案中所述，if/else 块是 Kotlin 中的一个表达式。

因此，我会发现以下实现最理想化，需要为本地函数正确命名参数：

fun foo() {
    fun failure(p0: String, p1: String) = crashReporterService.sendIssue(p0, p1).let { Result.FAILURE }
    val s3data = presignedUrl.body() ?: return failure("Failed uploading image", "Error - ${presignedUrl.raw()}")
    val uploadImage = s3data.run { api.uploadImage(bucketUrl, awsAccessKeyId, policy, key, signature, body).execute() }

    return if (uploadImage.isSuccessful) {
        Result.SUCCESS
    } else {
        failure("Failed uploading file", "Failed uploading file ${uploadImage.raw()}")
    }
}

你的问题接近于代码审查，所以你可能也会很高兴知道之前有一个 dedicated Stack Exchange network just for that. Do read A guide to Code Review for Stack Overflow users。

我什么时候应该使用 let {} 什么时候使用 != null

When should I use let {} and when just plain != null

kotlin

kotlin-null-safety