我認為問題是我不知道如何很好地使用協程。在 Maps Activity 中，您將看到我訪問 PointsDao 掛起函式，該函式回傳我想用來在我的 Google Maps Activity 中創建標記的物件串列。
@AndroidEntryPoint
類 MapsActivity : AppCompatActivity(), OnMapReadyCallback {
private lateinit var mMap: GoogleMap
private lateinit var binding: ActivityMapsBinding
private lateinit var requestPermissionLauncher: ActivityResultLauncher<Array<String>>
private val permissions = arrayOf(Manifest.permission.ACCESS_COARSE_LOCATION, Manifest.permission.ACCESS_FINE_LOCATION)
private lateinit var fusedLocationClient: FusedLocationProviderClient
private val mapsViewModel: MapsViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMapsBinding.inflate(layoutInflater)
setContentView(binding.root)
requestPermissionLauncher = registerForActivityResult(
ActivityResultContracts.RequestMultiplePermissions()
) {
permissions ->
if (permissions.getOrDefault(Manifest.permission.ACCESS_FINE_LOCATION, false)) {
Log.d("fine_location", "Permission granted")
} else {
Log.d("fine_location", "Permission not granted")
getBackToMainActivity()
Toast.makeText(this, "Necessites acceptar els permisos de geolocalització per a realitzar la ruta", Toast.LENGTH_LONG).show()
}
if (permissions.getOrDefault(Manifest.permission.ACCESS_COARSE_LOCATION, false)) {
Log.d("coarse_location", "Permission granted")
} else {
Log.d("coarse_location", "Permission not granted")
getBackToMainActivity()
Toast.makeText(this, "Necessites acceptar els permisos de geolocalització per a realitzar la ruta", Toast.LENGTH_LONG).show()
}
}
// Obtain the SupportMapFragment and get notified when the map is ready to be used.
val mapFragment = supportFragmentManager
.findFragmentById(R.id.map) as SupportMapFragment
mapFragment.getMapAsync(this)
fusedLocationClient = LocationServices.getFusedLocationProviderClient(this)
requestLocationPermissions()
}
/**
* Manipulates the map once available.
* This callback is triggered when the map is ready to be used.
* This is where we can add markers or lines, add listeners or move the camera.
*/
override fun onMapReady(googleMap: GoogleMap) {
mMap = googleMap
CoroutineScope(Dispatchers.Main).launch {
val listOfPoints = getRoutePoints()
for (point in listOfPoints) {
mMap.addMarker(MarkerOptions().position(LatLng( point.latitude, point.longitude)))
if (point == listOfPoints[0]) {
mMap.animateCamera(CameraUpdateFactory.newLatLngZoom(LatLng(point.latitude, point.longitude), 18f))
}
}
}
}
private fun requestLocationPermissions() {
when (PackageManager.PERMISSION_GRANTED) {
ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) -> {
Log.d("fine_location", "Permission already granted")
}
ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) -> {
Log.d("coarse_location", "Permission already granted")
}
else -> {
requestPermissionLauncher.launch(permissions)
}
}
}
private fun getBackToMainActivity() {
val intent = Intent(this, MainActivity::class.java)
startActivity(intent)
}
private fun getRouteId(): Int {
return intent.getIntExtra("routeId", 0)
}
// Gets the points from room repository through ViewModel
private fun getRoutePoints(): List<PointOfInterest> {
val route = getRouteId()
var points = emptyList<PointOfInterest>()
CoroutineScope(Dispatchers.IO).launch {
points = mapsViewModel.getRoutePoints(route)
}
return points
}
這是我用于此活動的 ViewModel：
@HiltViewModel
class MapsViewModel @Inject constructor(private val repository: RoomRepository): ViewModel() {
suspend fun getRoutePoints(routeId: Int): List<PointOfInterest> {
return repository.getPointsByRouteId(routeId)
}
}
還有道：
@Dao
interface PointsDao
{
@Query("SELECT * FROM points_tbl WHERE route_id = :routeId")
suspend fun getRoutePoints(routeId: Int): List<PointOfInterest>
}
我的跟蹤錯誤：
Process: com.buigues.ortola.touristics, PID: 27515
java.lang.IllegalStateException: Method addObserver must be called on the main thread
at androidx.lifecycle.LifecycleRegistry.enforceMainThreadIfNeeded(LifecycleRegistry.java:317)
at androidx.lifecycle.LifecycleRegistry.addObserver(LifecycleRegistry.java:172)
at androidx.lifecycle.SavedStateHandleController.attachToLifecycle(SavedStateHandleController.java:49)
at androidx.lifecycle.SavedStateHandleController.create(SavedStateHandleController.java:70)
at androidx.lifecycle.AbstractSavedStateViewModelFactory.create(AbstractSavedStateViewModelFactory.java:67)
at androidx.lifecycle.AbstractSavedStateViewModelFactory.create(AbstractSavedStateViewModelFactory.java:84)
at dagger.hilt.android.internal.lifecycle.HiltViewModelFactory.create(HiltViewModelFactory.java:109)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:171)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.kt:139)
at androidx.lifecycle.ViewModelLazy.getValue(ViewModelLazy.kt:44)
at androidx.lifecycle.ViewModelLazy.getValue(ViewModelLazy.kt:31)
at com.buigues.ortola.touristics.ui.MapsActivity.getMapsViewModel(MapsActivity.kt:39)
at com.buigues.ortola.touristics.ui.MapsActivity.getRoutePoints(MapsActivity.kt:123)
at com.buigues.ortola.touristics.ui.MapsActivity.access$getRoutePoints(MapsActivity.kt:31)
at com.buigues.ortola.touristics.ui.MapsActivity$onMapReady$1.invokeSuspend(MapsActivity.kt:85)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)
uj5u.com熱心網友回復：
問題出在getRoutePoints().
CoroutineScope(Dispatchers.IO).launch {
points = mapsViewModel.getRoutePoints(route)
}
在by viewModels()您的視圖模型屬性確實視圖模型的延遲加載。因此，如果您在不在主執行緒上時第一次訪問您的 ViewModel 屬性，它將嘗試在錯誤的執行緒上創建它，從而觸發此崩潰。ViewModels 必須在主執行緒上構建。
CoroutineScope(Dispatchers.IO) 意味著您正在創建一個默認情況下使用后臺 IO 執行緒的協程范圍，因此此代碼在后臺執行緒上運行。
無論如何，您不應該為此創建 CoroutineScope，因為您的 Activity 已經有一個由 Activity 生命周期正確管理的（因此如果 Activity 關閉，它將取消任何正在進行的作業，以避免浪費資源）。
此外，getRoutePoints()是一個掛起功能。你沒有理由Dispatchers.IO在這里使用。按照約定，掛起函式可以安全地從任何調度程式呼叫。（然而，可以撰寫一個打破慣例的，但 Room 設計得當，不會破壞慣例。）
要修復崩潰并正確運行協程，您應該使用lifecycleScope.launch { //.... 但是，您設計的這個功能不會像您期望的那樣作業。它啟動一個協程來檢索一個值，但在該協程完成運行之前它立即回傳，因此在這種情況下將只回傳初始emptyList(). 當您啟動協程時，您正在排隊后臺作業，但當前呼叫 launch 的函式會同步繼續，而無需等待協程結果。如果是這樣，它將是一個阻塞函式。在我的回答中提供了更多相關資訊。
因此，您應該將其設為掛起函式：
// Gets the points from room repository through ViewModel
private suspend fun getRoutePoints(): List<PointOfInterest> {
val route = getRouteId()
return mapsViewModel.getRoutePoints(route)
}
并且您的onMapReady函式也應該固定以使用適當的范圍：
override fun onMapReady(googleMap: GoogleMap) {
mMap = googleMap
lifecycleScope.launch {
val listOfPoints = getRoutePoints()
for (point in listOfPoints) {
mMap.addMarker(MarkerOptions().position(LatLng( point.latitude, point.longitude)))
if (point == listOfPoints[0]) {
mMap.animateCamera(CameraUpdateFactory.newLatLngZoom(LatLng(point.latitude, point.longitude), 18f))
}
}
}
}
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