The Code A is from CameraXBasic
I can't understand completely the code private val volumeDownReceiver = object : BroadcastReceiver().
I think the Code B will work well, but in fact it failed.
What does the keyword object mean in Kotlin ?
Code A
private val volumeDownReceiver = object : BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent) {
...
}
}
Code B
private val volumeDownReceiver = BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent) {
...
}
}
In Code A val volumeDownReceiver = object : BroadcastReceiver() refers to creating an object of an anonymous class that inherits from type BroadcastReceiver.
In Code B val volumeDownReceiver = BroadcastReceiver() tries to instantiate a new instance of an abstract class and that's why it's failing.
Edit: link to docs: https://kotlinlang.org/docs/reference/object-declarations.html#object-expressions
Related
I'm trying to instanciate a Room database in my main activity in Android Studio, following codelabs and tutorials, but my app always crash. Here's a part of the code:
My database (AppDatabase.kt):
#Database(entities = [Device::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
abstract fun deviceDao(): DeviceDao
companion object {
#Volatile
private var INSTANCE: AppDatabase? = null
fun getDatabase(context: Context): AppDatabase {
return INSTANCE ?: synchronized(this) {
val instance = Room.databaseBuilder(
context.applicationContext,
AppDatabase::class.java,
"item_database"
)
.fallbackToDestructiveMigration()
.build() // <---- The crash occurs here
INSTANCE = instance
return instance
}
}
}
}
And here's the activity from which I'm trying to instantiate it:
class NavigationActivity() : AppCompatActivity() {
private lateinit var binding: ActivityNavigationBinding
private val db by lazy { AppDatabase.getDatabase(this) }
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityNavigationBinding.inflate(layoutInflater)
setContentView(binding.root)
Log.d("instantiation", "$db") // <----- Called from here
val navView: BottomNavigationView = binding.navView
val navController = findNavController(R.id.nav_host_fragment_activity_navigation)
val appBarConfiguration = AppBarConfiguration(
setOf(
R.id.navigation_devices, R.id.navigation_logs, R.id.navigation_settings
)
)
setupActionBarWithNavController(navController, appBarConfiguration)
navView.setupWithNavController(navController)
}
}
Finally, here's the error message, which doesn't helps me much:
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Package.getName()' on a null object reference
at androidx.room.Room.getGeneratedImplementation(Room.java:82)
at androidx.room.RoomDatabase$Builder.build(RoomDatabase.java:1486)
at AppDatabase$Companion.getDatabase(AppDatabase.kt:24)
I tried a lot of things, including ViewModel, Repository and more, but got the crash systematically, at the same point.
Here's also the part of my build.gradle file where I import Room, maybe I'm wrong in some version or whatever...
plugins {
id 'com.android.application'
id 'org.jetbrains.kotlin.android'
id 'kotlin-android'
id 'kotlin-kapt'
}
[...]
def roomVersion = "2.4.2"
implementation("androidx.room:room-runtime:$roomVersion")
kapt("androidx.room:room-compiler:$roomVersion")
implementation "androidx.room:room-ktx:$roomVersion"
Make sure package declaration on top of the class is declared, for example:
package com.macrosystems.clean.ui.core.view
import android.content.Context
import android.content.Intent
import android.graphics.Color
etc...
I am struggling to use Live data on an MVVM pattern. The app is supposed to:
Fetch data from an API (which it does correctly)
Store that data in the Live data object from the ViewModel
Then the fragment calls the Observer method to fill the recyclerView.
The problem comes in point 3, it does nothing, and I cannot find the solution.
Here is the relevant code. (If I'm missing something, I will try to answer as quickly as possible)
Main Activity:
class MainActivity : AppCompatActivity() {
private lateinit var binding: ActivityMainBinding
private val viewModel: SharedViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
// Custom button to fetch data from api and log the Live Data value.
binding.refreshFab.setOnClickListener {
viewModel.fetchPlayerData()
Log.d("gabs", "${viewModel.livePlayerlist.value}")
}
}
}
ViewModel:
class SharedViewModel(app: Application): AndroidViewModel(app) {
// val playerDao = LaRojaDB.getDatabase(app).playerDao()
lateinit var playerList: Players
val livePlayerlist: MutableLiveData<MutableList<Players.PlayersItem>> by lazy {
MutableLiveData<MutableList<Players.PlayersItem>>()
}
fun fetchPlayerData() {
CoroutineScope(Dispatchers.IO).launch {
val response = MyService.getLaRojaService().getAllPlayers()
withContext(Dispatchers.Main) {
if (response.isSuccessful) {
val body = response.body()
if(!body.isNullOrEmpty()){
playerList = body
val playerArrayList = mutableListOf<Players.PlayersItem>()
playerList.forEach {
playerArrayList.add(it)
}
livePlayerlist.value = playerList
}
}
}
}
}
}
The fragment that displays the recycler view: (Fragment is already showing, I set up a textView as a title to make sure since I'm new using fragments as well.)
class PlayerListFragment : Fragment() {
private var _binding: FragmentPlayerListBinding? = null
private val binding get() = _binding!!
private val model: SharedViewModel by viewModels()
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
_binding = FragmentPlayerListBinding.inflate(inflater, container, false)
binding.rvPlayerList.layoutManager = LinearLayoutManager(activity)
----> // This is the observer that does not update the UI** <----
model.livePlayerlist.observe( viewLifecycleOwner, {
binding.rvPlayerList.adapter = PlayerAdapter(it)
})
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_player_list, container, false)
}
}
Thank you all in advance, hope I can finally learn what is causing the issue!
I think you don't need to switch Coroutine contexts. A few changes I'd expect if I were reviewing this code:
This should all be in the same IO context. You then postValue to your liveData.
fun fetchPlayerData() {
viewModelScope.launch(Dispatchers.IO) {
val xx = api.fetch()
...
_playerState.postValue(xx) //see below
}
}
Additionally, it's preferred not to expose mutable state, so your ViewModel should not expose the MutableLiveData (which shouldn't really be lazy). But it's also better to encapsulate the state in a sealed class:
//delete this
val livePlayerlist: MutableLiveData<MutableList<Players.PlayersItem>> by lazy {
MutableLiveData<MutableList<Players.PlayersItem>>()
}
Should be: (names are just pseudo code, I have no idea what this code is about)
sealed class PlayerDataState {
data class ListAvailable(data: List<Players.PlayersItem>>): PlayerDataState
object Loading(): PlayerDataState
}
And your new LiveData:
private val _playerState = MutableLiveData<PlayerDataState>()
val playerState: LiveData<PlayerDataState>() get() = _playerState
Finally when observing from the UI, you just...
model.playerState.observe(viewLifecycleOwner, {
when (it) {
is Loading -> ...
is ListAvailable -> binding.rvPlayerList.adapter = PlayerAdapter(it.data)
}
}
I am working on an android app which request Mars photos ans use it to display it on screen.
To make an request.And trying to use A public Api object that exposes the lazy-initialized Retrofit service.
below is source code with error
import retrofit2.Retrofit
import retrofit2.converter.scalars.ScalarsConverterFactory
import retrofit2.http.GET
class MarsApiService {
public val retrofit = Retrofit.Builder()
.addConverterFactory(ScalarsConverterFactory.create())
.baseUrl(Companion.BASE_URL)
.build()
interface MarsApiService{
#GET("photos")
fun getPhotos(): String
}
object MarsApi {
val retrofitService: MarsApiService by lazy { retrofit.create(MarsApiService::class.java) }
}
companion object {
private const val BASE_URL = "https://android-kotlin-fun-mars-server.appspot.com"
}
}
17th line the code inside object MarsApi pops up errors Unresolved reference : retrofit. The call to create() function on a Retrofit object is expensive and the app needs only one instance of Retrofit API service. So, i exposed the service to the rest of the app using object declaration.
What I have tried:
The code is working if i bring code inside object MarsApi out but doing so may result in multiple instance of retrofit.
code1
interface MarsApiService {
#GET("photos")
suspend fun getPhotos(): String
companion object {
private const val BASE_URL = "https://android-kotlin-fun-mars-server.appspot.com"
val marsApiService: MarsApiService by lazy {
Retrofit.Builder()
.addConverterFactory(ScalarsConverterFactory.create())
.baseUrl(BASE_URL)
.build().create(MarsApiService::class.java)
}
}
}
code2
private const val BASE_URL = "https://android-kotlin-fun-mars-server.appspot.com/"
private val retrofit = Retrofit.Builder()
.addConverterFactory(ScalarsConverterFactory.create())
.baseUrl(BASE_URL)
.build()
interface MarsApiService {
#GET("photos")
suspend fun getPhotos(): String
}
object MarsApi {
val marsApiService: MarsApiService by lazy { retrofit.create(MarsApiService::class.java) }
}
run
fun main() = runBlocking {
val rs = marsApiService.getPhotos()
println(rs)
}
In your code, retrofit by lazy reference not resolved cause of retrofit is property of class, move the retrofit globally like using (Companion) object or like code2 (highest hierarchy?)
Hope this link
make you clear the difference between object and companion object and when to use them. Happy learning
It was the same problem that I had.
I solved it just excluding the class MarsApiService
I didn´t use companion object too.
The right code is below
import retrofit2.Retrofit
import retrofit2.converter.scalars.ScalarsConverterFactory
import retrofit2.http.GET
private const val BASE_URL = "https://android-kotlin-fun-mars-server.appspot.com"
public val retrofit = Retrofit.Builder()
.addConverterFactory(ScalarsConverterFactory.create())
.baseUrl(Companion.BASE_URL)
.build()
interface MarsApiService{
#GET("photos")
fun getPhotos(): String
}
object MarsApi {
val retrofitService: MarsApiService by lazy {
retrofit.create(MarsApiService::class.java)
}
}
I am trying to figure out if I can work with Kotlin and Spark,
and use the former's data classes instead of Scala's case classes.
I have the following data class:
data class Transaction(var context: String = "", var epoch: Long = -1L, var items: HashSet<String> = HashSet()) :
Serializable {
companion object {
#JvmStatic
private val serialVersionUID = 1L
}
}
And the relevant part of the main routine looks like this:
val transactionEncoder = Encoders.bean(Transaction::class.java)
val transactions = inputDataset
.groupByKey(KeyExtractor(), KeyExtractor.getKeyEncoder())
.mapGroups(TransactionCreator(), transactionEncoder)
.collectAsList()
transactions.forEach { println("collected Transaction=$it") }
With TransactionCreator defined as:
class TransactionCreator : MapGroupsFunction<Tuple2<String, Timestamp>, Row, Transaction> {
companion object {
#JvmStatic
private val serialVersionUID = 1L
}
override fun call(key: Tuple2<String, Timestamp>, values: MutableIterator<Row>): Transaction {
val seq = generateSequence { if (values.hasNext()) values.next().getString(2) else null }
val items = seq.toCollection(HashSet())
return Transaction(key._1, key._2.time, items).also { println("inside call Transaction=$it") }
}
}
However, I think I'm running into some sort of serialization problem,
because the set ends up empty after collection.
I see the following output:
inside call Transaction=Transaction(context=context1, epoch=1000, items=[c])
inside call Transaction=Transaction(context=context1, epoch=0, items=[a, b])
collected Transaction=Transaction(context=context1, epoch=0, items=[])
collected Transaction=Transaction(context=context1, epoch=1000, items=[])
I've tried a custom KryoRegistrator to see if it was a problem with Kotlin's HashSet:
class MyRegistrator : KryoRegistrator {
override fun registerClasses(kryo: Kryo) {
kryo.register(HashSet::class.java, JavaSerializer()) // kotlin's HashSet
}
}
But it doesn't seem to help.
Any other ideas?
Full code here.
It does seem to be a serialization issue.
The documentation of Encoders.bean states (Spark v2.4.0):
collection types: only array and java.util.List currently, map support is in progress
Porting the Transaction data class to Java and changing items to a java.util.List seems to help.
I have de folowing classes :
[DataContract]
public class MyProject
{
[DataMember(Name = "Branches")]
private SortedSet<ModuleFilter> branches = new SortedSet<ModuleFilter>(new ModuleFilterComparer());
[DataMember(Name="VbuildFilePath")]
private string buildprogram = null;
}
I can serialize it to a file with :
DataContractSerializer x = new DataContractSerializer(p.GetType());
using (System.Xml.XmlWriter writer = System.Xml.XmlWriter.Create(p.GetFilePath()))
{
x.WriteObject(writer, p);
}
But when I try to read it back with the folowing piece of code, it fails unless I add a dummy implementation of IComparable to the ModuleFilter object
DataContractSerializer x = new DataContractSerializer(typeof(MyProject));
using (System.Xml.XmlReader reader = System.Xml.XmlReader.Create(filePath))
{
p = (MyProject)x.ReadObject(reader);
}
Why does not the deserializer use the provided IComparer of the SortedSet member ?
Thank you
It is because DataContractSerializer uses default constructor of SortedSet to initialize field.
Solution 1: recreate field after deserialization with needed comparer
[DataContract]
public class MyProject : IDeserializationCallback
{
//...
void IDeserializationCallback.OnDeserialization(Object sender)
{
branches = new SortedSet<ModuleFilter>(branches, new ModuleFilterComparer());
}
}
Solution 2: use your own sorted set implementation instead of SortedSet<ModuleFilter>
public class ModuleFilterSortedSet : SortedSet<ModuleFilter>
{
public ModuleFilterSortedSet()
: base(new ModuleFilterComparer())
{
}
public ModuleFilterSortedSet(IComparer<ModuleFilter> comparer)
: base(comparer)
{
}
}