MVVM：尝试使用 LiveData 加载数据时，ViewModel 为空

Question

So I'm trying to build a simple MVVM interface to load trips from the database retrieved by the ViewModel into my TripFragment. However, I keep getting this error saying that my TripViewModel is null:

Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observe(androidx.lifecycle.LifecycleOwner, androidx.lifecycle.Observer)' on a null object reference

I can't seem to figure out why it thinks that it's null. I believe the issue is in TripViewModel and has something to do with how the fact that it inherits from AndroidViewModel and that I'm passing the application's context in the constructor.

class TripFragment : Fragment()

private var tripViewModel: TripViewModel? = null
private var textViewTripName: TextView? = null

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    val view =
        inflater.inflate(R.layout.fragment_trip, container, false)

    val recyclerView: RecyclerView = view.recycler_view
    recyclerView.layoutManager = LinearLayoutManager(context)
    recyclerView.setHasFixedSize(true)

    val adapter = TripAdapter()
    recyclerView.adapter = adapter

    tripViewModel = ViewModelProvider(this).get(TripViewModel(activity!!.application)::class.java)


    // This is the line where it crashes, it never executes past this
    tripViewModel!!.getAll().observe(viewLifecycleOwner, Observer<List<Trip>> {
        fun onChanged(trips: List<Trip>) {
            adapter.setTrips(trips)
            Log.d("TripFragment", "Went through observer")
        }
    })

    return view
}

class TripViewModel(application: Application): AndroidViewModel(application)

private var tripRepository: TripRepository = TripRepository(application)
private var allTrips: LiveData<List<Trip>> = getAll()

fun insert(trip: Trip) {
    tripRepository.insert(trip)
}

fun update(trip: Trip) {
    tripRepository.update(trip)
}

fun delete(trip: Trip) {
    tripRepository.delete(trip)
}

fun clear() {
    tripRepository.clear()
}

fun getAll(): LiveData<List<Trip>> {
    return allTrips
}

class TripRepository(application: Application)

private lateinit var tripDao: TripDao
private lateinit var allTrips: LiveData<List<Trip>>

init {
    CoroutineScope(IO).launch {
        tripDao = AppDatabase.getDatabase(application.applicationContext).tripDao()
        allTrips = tripDao.getAll()
    }
}

fun insert(trip: Trip) {
    CoroutineScope(IO).launch {
        tripDao.insert(trip)
    }
}

fun update(trip: Trip) {
    CoroutineScope(IO).launch {
        tripDao.update(trip)
    }
}

fun delete(trip: Trip) {
    CoroutineScope(IO).launch {
        tripDao.delete(trip)
    }
}

fun clear() {
    CoroutineScope(IO).launch {
        tripDao.clear()
    }
}

fun getAll(): LiveData<List<Trip>> {
    return allTrips
}

Trip entity

@Entity
data class Trip(var title: String, var startDate: String, var endDate: String?) {

    @PrimaryKey(autoGenerate = true)
    var tid: Long = 0
}

EDIT: I've printed a bunch of debug logs and pinpointed the error at this line in TripRepository .

init {
    CoroutineScope(IO).launch {
        // tripDao is never assigned properly, 
        tripDao = AppDatabase.getDatabase(application.applicationContext).tripDao()
        allTrips = tripDao.getAll()
    }
}

The line tripDao = AppDatabase.getDatabase(application.applicationContext).tripDao() causes an error which turns tripDao into a null variable. The problem has something to do with how I fetch the database, so I've attached my AppDatabase class below.

@Database(entities = [Trip::class], version = 2, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {

    abstract fun tripDao(): TripDao

    companion object {
        // Singleton prevents multiple instances of database opening at the
        // same time.
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            val tempInstance = INSTANCE
            if (tempInstance != null) {
                Log.d("AppDatabase", "Returning existing database")
                return tempInstance
            }
            synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "tripweaver_database"
                )
                    .fallbackToDestructiveMigration()
                    .build()
                INSTANCE = instance
                Log.d("AppDatabase", "Returning new database")
                return instance
            }
        }
    }
}

Answer 1

I couldn't find the solution to fixing my database access code so I restructured my whole codebase according to the architecture components tutorial by Google on Codelabs. Surely enough this helped me fix the issues I was having. Link for whoever needs this tutorial in the future: https://codelabs.developers.google.com/codelabs/android-room-with-a-view-kotlin/#13

Answer 2

In your class TripViewModel , you have:

private var allTrips: LiveData<List<Trip>> = getAll()

fun getAll(): LiveData<List<Trip>> {
    return allTrips
}

So you have a circular arrangement here. allTrips is assigned an initial value of whatever getAll() returns, but it's calling getAll() before allTrips is assigned. So you found a way to assign a null to a non-nullable!

Seems like maybe you meant to put allTrips = tripRepository.getAll() .