在 iOS 上的 KMM 项目中使用后台线程时出现无效的可变性异常

Question

我正在从事 KMM 项目，目前正在开发模型层。为了处理数据，我计划像这样创建一个单例：

@ThreadLocal object Repository {

    private var dao: DataAccessObject? = null

    private val scope = CoroutineScope(Dispatchers.Main)

    fun injectDao(dao: DataAccessObject) {
        scope.async {
            Repository.dao = dao
        }
    }

    suspend fun create(dataObjectType: TypeOfDataObject): DataObject? {
        var dataObject: DataObject? = null

        val job = scope.async {
            dataObject = dao?.create(dataObjectType = dataObjectType)
        }

        job.await()

        return dataObject
    }
}

在您看到的这种实现中，对数据库的请求是在 Main 线程中处理的，这很不好。但它有效并且数据从函数正确返回。下一个明显的步骤是尝试在后台范围内运行它。要做到这一点，我们应该重新声明 scope:

private val scope = CoroutineScope(Dispatchers.Default)

当我们运行代码并从它属于的某个地方调用 create 函数时

Uncaught Kotlin exception: kotlin.native.concurrent.InvalidMutabilityException: mutation attempt of frozen kotlin.native.internal.Ref@5761ad88
2021-02-02 23:54:50.408645+0300 Plendy[28960:2893398] [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed
    at 0   Shared                          0x000000010d51640f kfun:kotlin.Throwable#<init>(kotlin.String?){} + 95 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/Throwable.kt:23:37)
    at 1   Shared                          0x000000010d50f0bd kfun:kotlin.Exception#<init>(kotlin.String?){} + 93 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/Exceptions.kt:23:44)
    at 2   Shared                          0x000000010d50f32d kfun:kotlin.RuntimeException#<init>(kotlin.String?){} + 93 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/Exceptions.kt:34:44)
    at 3   Shared                          0x000000010d5448cd kfun:kotlin.native.concurrent.InvalidMutabilityException#<init>(kotlin.String){} + 93 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/native/concurrent/Freezing.kt:22:60)
    at 4   Shared                          0x000000010d5460af ThrowInvalidMutabilityException + 431 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/native/concurrent/Internal.kt:92:11)
    at 5   Shared                          0x000000010d6470b0 MutationCheck + 128
    at 6   Shared                          0x000000010d5640f8 kfun:kotlin.native.internal.Ref#<set-element>(1:0){} + 104 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/native/internal/Ref.kt:12:5)
    at 7   Shared                          0x000000010d4a1d5b kfun:com.plendy.PlendyCore.Model.KNPlendyData.$create$lambda-1COROUTINE.invokeSuspend#internal + 779 (/Users/petr/Documents/Projects/Plendy/Android/Plendy/PlendyCore/src/commonMain/kotlin/com/plendy/PlendyCore/Model/KNPlendyData.kt:23:13)
    at 8   Shared                          0x000000010d537958 kfun:kotlin.coroutines.native.internal.BaseContinuationImpl#resumeWith(kotlin.Result<kotlin.Any?>){} + 760 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/coroutines/ContinuationImpl.kt:30:39)
    at 9   Shared                          0x000000010d6d7a78 kfun:kotlinx.coroutines.DispatchedTask#run(){} + 2680 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/common/src/internal/DispatchedTask.kt:106:71)
    at 10  Shared                          0x000000010d687fb8 kfun:kotlinx.coroutines.EventLoopImplBase#processNextEvent(){}kotlin.Long + 840 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/common/src/EventLoop.common.kt:274:18)
    at 11  Shared                          0x000000010d6efbbb kfun:kotlinx.coroutines#runEventLoop(kotlinx.coroutines.EventLoop?;kotlin.Function0<kotlin.Boolean>){} + 843 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/native/src/Builders.kt:80:40)
    at 12  Shared                          0x000000010d6f8d39 kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.start$lambda-0#internal + 409 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/native/src/Workers.kt:49:17)
    at 13  Shared                          0x000000010d6f8f30 kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.$start$lambda-0$FUNCTION_REFERENCE.invoke#internal + 64 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/native/src/Workers.kt:47:24)
    at 14  Shared                          0x000000010d6f8f90 kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.$start$lambda-0$FUNCTION_REFERENCE.$<bridge-UNN>invoke(){}#internal + 64 (/opt/buildAgent/work/44ec6e850d5c63f0/kotlinx-coroutines-core/native/src/Workers.kt:47:24)
    at 15  Shared                          0x000000010d545d59 WorkerLaunchpad + 185 (/Users/teamcity/buildAgent/work/f01984a9f5203417/runtime/src/main/kotlin/kotlin/native/concurrent/Internal.kt:69:54)
    at 16  Shared                          0x000000010d64ba4f _ZN6Worker19processQueueElementEb + 3135
    at 17  Shared                          0x000000010d64adf6 _ZN12_GLOBAL__N_113workerRoutineEPv + 54
    at 18  libsystem_pthread.dylib             0x0000000110a86950 _pthread_start + 224
    at 19  libsystem_pthread.dylib             0x0000000110a8247b thread_start + 15

奇怪的是，数据写入了db，意思是dao调用成功，但是数据没有从函数返回，因为异常发生得更早。在这一点上，我不明白异常与什么冻结对象有关？我接下来尝试的是删除 job.await() 行，它完美地工作，除了函数输出中有 null 的原因之外没有任何例外。

所以我的问题是：如何使后台线程中的代码运行仍然能够等待其输出？

Answer 1

您应该包含更多异常信息以帮助弄清楚发生了什么，并且您可以使用 ensureNeverFrozen 来帮助识别何时无意中冻结了某些内容。不过，这样的话，我想我是可以想通的。

在这种情况下，在后台 lambda 中捕获对 dataObject 的引用将冻结它。尝试重新分配它（可能）会引发异常。

var dataObject: DataObject? = null

        val job = scope.async {
            //Trying to assign the frozen dataObject will fail
            dataObject = dao?.create(dataObjectType = dataObjectType)
        }

既然你已经在暂停功能中，为什么不使用类似 withContext 的东西？

suspend fun create(dataObjectType: TypeOfDataObject): DataObject? {

        val dataObject = withContext(Dispatchers.Default) {
            dao?.create(dataObjectType = dataObjectType)
        }

        return dataObject
    }

如果你要走那么远......

suspend fun create(dataObjectType: TypeOfDataObject): DataObject? = withContext(Dispatchers.Default) {
    dao?.create(dataObjectType = dataObjectType)
}

在 iOS 上的 KMM 项目中使用后台线程时出现无效的可变性异常

Invalid Mutability Exception when using background threads in KMM project on iOS

ios

kotlin

kotlin-native

kotlin-coroutines