如果 Coroutine 的 Deferred 类型为 null 会怎样？

Question

我在我的代码中使用嵌套的协程块。当我尝试将延迟类型的结果传递给变量时，我得到了一个空值。因此，它会导致转换问题，即 kotlin.TypeCastException: null cannot be cast to non-null type kotlin.collections.ArrayList in getNearbyHealthInstitutions() 方法的返回行。我相信，我在某个时候做了正确的实现，但是我缺少什么来从 Deferred 的结果中获取空值？有趣的是，当我调试它时，它确实返回了预期值。我认为这应该是并发问题，或者我不知道为什么它首先在调试模式下工作。有什么想法吗？

// Invocation point where resides in a callback
GlobalScope.launch(Dispatchers.Main) {
    nearbyHealthInstitutionSites.value = getNearbyHealthInstitutions()
}

private suspend fun getNearbyHealthInstitutions(radius: Meter = DEFAULT_KM_RADIUS) : ArrayList<Hospital> {
    return CoroutineScope(Dispatchers.IO).async {
        val list = getHealthInstitutions()
        val filteredList = list?.filter { it.city == state?.toUpperCase() } as MutableList<Hospital>
        Log.i(MTAG, "nearby list is $filteredList")
        Log.i(MTAG, "nearby list's size is ${filteredList.size}")

        var deferred: Deferred<MutableList<Hospital>>? = null

        addAllNearbyLocations(onEnd = { nearbyHealthInstitutions ->
            deferred = async {
                findNearbyOfficialHealthInstitutions(
                    officialHealthInstitutionList = filteredList as ArrayList<Hospital>,
                    nearbyHealthInstitutions = nearbyHealthInstitutions
                )
            }
        })

        val result = deferred?.await()

        return@async result as ArrayList<Hospital>
    }.await()
}

private suspend fun findNearbyOfficialHealthInstitutions(officialHealthInstitutionList: ArrayList<Hospital>, nearbyHealthInstitutions: MutableList<Hospital>): MutableList<Hospital> {
        return GlobalScope.async(Dispatchers.Default) {
            val result = mutableListOf<Hospital>()

            officialHealthInstitutionList.forEach {
                nearbyHealthInstitutions.forEach { hospital ->
                    StringSimilarity.printSimilarity(it.name, hospital.name)

                    val similarity = StringSimilarity.similarity(it.name, hospital.name.toUpperCase())

                    if (similarity > SIMILARITY_THRESHOLD) {
                        Log.i(MTAG, "findNearbyOfficialHealthInstitutions() - ${it.name} and ${hospital.name.toUpperCase()} have %$similarity")
                        result.add(hospital)
                    }

                }
            }

            Log.i(TAG, "------------------------------------------")
            result.forEach {
                Log.i(MTAG, "findNearbyOfficialHealthInstitutions() - hospital.name is ${it.name}")
            }

            return@async result
        }.await()
    }

Answer 1

由于addAllNearbyLocations() 是异步的，您的协程需要等待回调被调用才能继续执行。您可以为此使用suspendCoroutine API。

val result = suspendCoroutine { continuation ->
    addAllNearbyLocations(onEnd = { nearbyHealthInstitutions ->
          findNearbyOfficialHealthInstitutions(
                officialHealthInstitutionList = filteredList as ArrayList<Hospital>,
                nearbyHealthInstitutions = nearbyHealthInstitutions
          ).let { found -> continuation.resume(found) }
    })
}

另外，您应该使用List 而不是ArrayList 或MutableList，您应该始终使用通用接口而不是该接口的特定实现。这也消除了一些强制转换（理想情况下，这段代码中应该没有强制转换）。