Android ViewModel协程模式

Question

I would like to hear some critics on the way I have implemeneted viewmodel data fetching using coroutine. My goal is clean way to write ViewModels. But isn't it too big of overhead? I haven't found so far some clean solution. Let me know your tips. I wanted to avoid to write if (!::users.isInitialized) as it is in the official tutorial .

class LiveDataLoader <T>{
    val liveData = MutableLiveData<T>()
    var loaded:Boolean = false
}

abstract class CustomViewModel : ViewModel(){
    private val viewModelJob = Job()
    private val uiScope = CoroutineScope(Dispatchers.Main + viewModelJob)

    fun <T> get(container: LiveDataLoader<T>, loader: ()->T): LiveData<T>{
        if (!container.loaded){
            container.loaded = true
            uiScope.launch{
                container.liveData.postValue(loader.invoke())
            }
        }
        return container.liveData;
    }

    override fun onCleared() {
        super.onCleared()
        viewModelJob.cancel()
    }
}

class ActivityTodoGroupsViewModel() : CustomViewModel(){
    private val groups = LiveDataLoader<MutableList<TaskGroupWithTasks>>()
    private val tasks = LiveDataLoader<MutableList<TodoTask>>()

    private lateinit var tasksx: MutableLiveData<MutableList<TodoTask>>

    fun getTaskGroups() = get(groups){
        AppDatabase.db.toDoTasksDAO.getGroupsWithItems()
    }

    fun getUpcomingTasks() = get(tasks){
        val calendar = Calendar.getInstance()
        calendar.add(Calendar.DAY_OF_YEAR, -7)
        AppDatabase.db.toDoTasksDAO.getRecentTasks(calendar)
    }

}

Answer 1

In general it's always good to not expose the Mutable* types too much. This ensures data consistency. Also it seems like you're trying a achieve something similar to lazy , so why not use it.

So you could consider an extension function to CoroutineScope to convert a suspending function to a simple LiveData :

fun <V> CoroutineScope.liveData(
        provider: suspend () -> V
) = lazy<LiveData<V>> {
    MutableLiveData<V>().apply {
        launch {
            postValue(provider())
        }
    }
}

If you also have your ViewModel implement CoroutineScope , you could use it as:

val taskGroups() by liveData {
    AppDatabase.db.toDoTasksDAO.getGroupsWithItems()
}