RefreshShows() in Repository get called every time onCreate() callback of Activity get called

问题

I have following project in Github : https://github.com/Ali-Rezaei/TVMaze

I have started to using Koin as dependency injection framework in a sample app :

class TVMazeApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        startKoin {
            androidContext(this@TVMazeApplication)
            modules(networkModule)
            modules(persistenceModule)
            modules(repositoryModule)
            modules(viewModelModule)
        }
    }
}

This is my repository class :

class ShowRepository(
    private val dao: ShowDao,
    private val api: TVMazeService,
    private val context: Context
) {

    /**
     * A list of shows that can be shown on the screen.
     */
    val shows = resultLiveData(
        databaseQuery = {
            Transformations.map(dao.getShows()) {
                it.asDomainModel()
            }
        },
        networkCall = { refreshShows() })

    /**
     * Refresh the shows stored in the offline cache.
     */
    private suspend fun refreshShows(): Result<List<Show>> =
        try {
            if (isNetworkAvailable(context)) {
                val shows = api.fetchShowList().await()
                dao.insertAll(*shows.asDatabaseModel())
                Result.success(shows)
            } else {
                Result.error(context.getString(R.string.failed_internet_msg))
            }
        } catch (err: HttpException) {
            Result.error(context.getString(R.string.failed_loading_msg))
        }
}

And my ViewModel :

class MainViewModel(
    repository: ShowRepository
) : ViewModel() {

    private val _shows = repository.shows
    val shows: LiveData<Result<List<Show>>>
        get() = _shows
}

And I observe LiveData in my Activity :

viewModel.shows.observe(this, Observer { result ->
            when (result.status) {
                Result.Status.SUCCESS -> {
                    binding.loadingSpinner.hide() 
                    viewModelAdapter.submitList(result.data)
                }
                Result.Status.LOADING -> binding.loadingSpinner.show()
                Result.Status.ERROR -> {
                    binding.loadingSpinner.hide()
                    Snackbar.make(binding.root, result.message!!, Snackbar.LENGTH_LONG).show()
                }
            }
        })

When I click on Back button, Activity get destroyed ( but instance of app still exist as I can access it from recent apps). What I expect is a call to refreshShows() method when I start the app again, but it never get called.

But when I destroy instance of app by clearing from recent app and start the app, refreshShows() get called.

What should I do to have a call on refreshShows() every time onCreate() callback of Activity get called?

fun <T, A> resultLiveData(databaseQuery: () -> LiveData<T>,
                          networkCall: suspend () -> Result<A>): LiveData<Result<T>> =
    liveData(Dispatchers.IO) {
        emit(Result.loading<T>())
        val source = databaseQuery.invoke().map { Result.success(it) }
        emitSource(source)

        val result = networkCall.invoke()
        if (result.status == Result.Status.ERROR) {
            emit(Result.error<T>(result.message!!))
            emitSource(source)
        }
    }

回答1:

Your refreshShows() in your repository is only get called when a new network request is done. The idea of your livedata is to provide the latest result when its fragment/activity is recreated, so when your screen rotates or you resume an activity it doesnt triggers another request as the livedata already have the latest result and you dont have a stateful connection with your network database/server (if you were observing data from Room it would receive the latest change if any).

The simpliest way I find to "fix" this, is to actually have your viewmodel val shows to be a fun, like this:

class MainViewModel(
    repository: ShowRepository
) : ViewModel() {

    private val _shows = repository.shows()
    val shows: LiveData<Result<List<Show>>>
        get() = _shows
}

However using like this, everytime the screen rotates a new network call will be made thus calling your refreshShows()

来源：https://stackoverflow.com/questions/62092123/refreshshows-in-repository-get-called-every-time-oncreate-callback-of-activi