使用 launch() 范围时无法为 LiveData 变量赋值，但可以使用 runBlocking() 为 LiveData 赋值

Question

我有一个LiveData 变量，我想在收到来自HTTP request 的响应后为其赋值。我在launch {} 范围内分配了这个值。但是，当我测试代码以查看该值是否已实际分配时，测试失败并指出该值为null。

我尝试了另一种选择，即使用runBlocking {} 范围而不是launch {} 范围，并且我的测试用例通过了。用于联网的库是Retrofit

以下是使用launch() 并使测试用例失败的代码。

private var _state = MutableLiveData<State>()
val state: LiveData<State>
  get() = _state 

private var job = Job()

private val uiScope = CoroutineScope(job + Dispatchers.Main)

init {
    executeRequest(....)
}

// Test cases fails with this function
private fun executeRequest(params) {
  uiScope.launch {
    val deferred = repository.getApi()
        .requestAsync(params)

    try {
        val response = deferred.await()

        _state.value = StateSuccess(response)
    } catch (e: Exception) {
        _state.value = StateError("Failure: $e.message")
    }
  }
}

以下是使用runBlocking() 并使测试用例通过的代码。

private var _state = MutableLiveData<State>()
val state: LiveData<State>
  get() = _state 

private var job = Job()

private val uiScope = CoroutineScope(job + Dispatchers.Main)

init {
    executeRequest(....)
}

// Test cases passes with this function
private fun executeRequest(params) {
  runBlocking {
    val deferred = repository.getApi()
        .requestAsync(params)

    try {
        val response = deferred.await()

        _state.value = StateSuccess(response)
    } catch (e: Exception) {
        _state.value = StateError("Failure: $e.message")
    }
  }
}

这是测试用例

val viewModel = MyViewModel(fakeClientRepository)

val observer = Observer<State> {}

try {
    viewModel.state.observeForever(observer)

    assert(viewModel.state.value == StateSuccess("success"))
} finally {
    viewModel.state.removeObserver(observer)
}

为什么launch() 没有将值赋给LiveData 变量？

Answer 1

尝试使用Dispatchers.IO 调度程序，然后将值分配给LiveData，使用postValue() 而不是value 即

private fun executeRequest(params) {
  uiScope.launch {
    withContext(Dispatcher.IO){
 val deferred = repository.getApi()
        .requestAsync(params)
    try {
        val response = deferred.await()
        _state.postValue(RESPONSE)
    } catch (e: Exception) {
        _state.postValue(StateError("Failure: $e.message"))
    }

    }
  }
}

Answer 2

您的函数需要成为挂起函数。这是因为您正在调用将在函数体内进行暂停操作，并且为了做到这一点，调用函数也必须是暂停函数。挂起函数是非阻塞的，一旦触发就可以进行管理——例如启动、暂停、恢复和取消。

   private suspend  fun executeRequest(params){
       job = CoroutineScope(Dispatchers.IO).launch {
       val response = repository.getApi()
            .requestAsync(params).await()

        withContext(Dispatchers.Main) {
          try {
             _state.value = StateSuccess(response)
          } catch (e: Exception) {
             _state.value = StateError("Failure: $e.message")
          }
        }
      }
   }

Answer 3

您的测试用例在请求完成之前完成，LiveData 的值发生变化。使用runBlocking、executeRequest（以及您的构造函数）仅在块执行时返回；使用launch，它会在单独的Job 中启动它并立即返回。

如果您确实需要在构造函数中执行此操作，可以将 Job 分配给字段，例如

private var _state = MutableLiveData<State>()
val state: LiveData<State>
  get() = _state 

private var job = Job()

private val uiScope = CoroutineScope(job + Dispatchers.Main)

internal val requestJob = executeRequest(....)

private fun executeRequest(params): Job = uiScope.launch {
    try {
        val deferred = repository.getApi()
            .requestAsync(params)

        val response = deferred.await()

        _state.value = StateSuccess(response)
    } catch (e: Exception) {
        _state.value = StateError("Failure: $e.message")
    }
}

然后在检查值之前在您的测试用例中调用requestJob.await()，例如

val viewModel = MyViewModel(fakeClientRepository)

val observer = Observer<State> {}

runBlocking { 
    viewModel.requestJob.await()
}

try {
    viewModel.state.observeForever(observer)

    assert(viewModel.state.value == StateSuccess("success"))
} finally {
    viewModel.state.removeObserver(observer)
}