首次运行时未定义迭代器值

Question

在我的onViewCreated() 里面我的MythFragment 我执行这些步骤。

1. 预填充 Room 数据库。
2. 从数据库中获取所有记录到MutableLiveData<List<..>>
3. 初始化这些数据的迭代器
4. 单击“下一步”按钮检查条件it.hasNext() == true

由于某种原因，仅在程序的第一次运行期间，it.hasNext() 给了我false。我希望这是真的，因为步骤 1-3 应该已经确保列表已填充并且迭代器指向第一个元素。

有趣的是，MythView 上的任何后续导航都会正确检索元素，it.hasNext() 给我true。

MythFragment.kt

class MythFragment : Fragment() {

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        viewModel = ViewModelProviders.of(this).get(MythViewModel::class.java)
        viewModel.populateMyths()
        viewModel.fetchFromDatabase()

        buttonNextMyth.setOnClickListener {
            mythEvaluation.visibility = View.GONE
            buttonIsThisMythTruthful.visibility = View.VISIBLE

            viewModel.mythsIterator.let {
                if (it.hasNext()) {
                    val myth = it.next()
                    mythText.text = myth.myth
                    mythEvaluation.text = myth.evaluation
                } else {
                    Toast.makeText(activity, "There is no myth, because it.hasNext() is false!)", Toast.LENGTH_SHORT).show()
                    val action = MythFragmentDirections.actionMenuFragment()
                    Navigation.findNavController(view).navigate(action)
                }
            }
        }
    }
}

MythViewModel.kt

class MythViewModel(application: Application) : BaseViewModel(application) {

    private val myths = MutableLiveData<List<Myth>>()
    lateinit var mythsIterator: Iterator<Myth>

    fun populateMyths() {
        launch {
            val dao = MythDatabase(getApplication()).mythDao()

            if (dao.getRowCount() > 0)
                return@launch

            val mythList = arrayListOf(
                Myth("This is the myth 1", "This is the evaluation of the myth 1"),
                Myth("This is the myth 2", "This is the evaluation of the myth 2"),
                Myth("This is the myth 3", "This is the evaluation of the myth 3"),
                Myth("This is the myth 4", "This is the evaluation of the myth 4"),
                Myth("This is the myth 5", "This is the evaluation of the myth 5")
            )

            dao.insertAll(
                *mythList.toTypedArray()
            )
        }
    }

    fun fetchFromDatabase() {
        launch {
            val mythList = MythDatabase(getApplication()).mythDao().getAllMyths()
            myths.value = mythList
            myths.value?.let {
                mythsIterator = it.iterator()
            }
        }
    }
}

我认为问题可能出在并发（协程）上，但我不明白我做错了什么。

Answer 1

由于populateMyths() 和fetchFromDatabase() 两个函数都在启动新的协程，这两个协程将并行运行。所以第一次调用fetchFromDatabase()，它可能是在dao.insertAll() 发生在populateMyths() 之前检索数据。 也许您应该重新考虑/澄清通过启动这些协程来实现的目标。