I have the following simple database:
NameList.kt
@Entity(tableName = "name_list")
data class NameList(
@PrimaryKey(autoGenerate = true)
var id: Long = 0L,
@ColumnInfo(name = "name")
var name: String = "")
NameListDao.kt
@Dao
interface NameListDao{
@Insert
fun insert(nameList: NameList)
@Update
fun update(nameList: NameList)
@Query("SELECT * FROM name_list ORDER BY id DESC")
fun getAll(): LiveData<List<NameList>>
}
NameListRepository.kt
class NameListRepository(private val nameListDao: NameListDao){
val allNames: LiveData<List<NameList>> = nameListDao.getAll()
}
MyViewModel.kt
class MyViewModel(app: Application): AndroidViewModel(app){
private val nameListDao = NameListDatabase.getInstance(app).nameListDao
private val repository: nameListRepository = nameListRepository(nameListDao)
private val allNames: LiveData <List<NameList>> = repository.allNames
...
}
For the time being I want to get and process only the last entry of this non-empty database (without changing/adding methods in NameListDao
).
Within a fragment, it is easy to get and eg display (in a TextView
via data binding) the last database entry using the observe
method:
viewModel.allNames.observe(this, Observer<List<NameList?>> {
nameList -> binding.textView.text = nameList[nameList.lastIndex]!!.name
})
As far as I found out, in ViewModel
one needs to use Transformations.map
or Transformations.switchMap
to process LiveData
. I tried the following two (four) approaches with Transformations (none of which worked) in order to get the last entry of the database and pass it on to the method doSomethingWithName()
. However, in all cases the passed parameter was apparently empty. (I also tried the first entry via [0]
, but I got the same results though)
class MyViewModel(app: Application): AndroidViewModel(app){
private val nameListDao = NameListDatabase.getInstance(app).nameListDao
private val repository: nameListRepository = nameListRepository(nameListDao)
private val allNames: LiveData <List<NameList>> = repository.allNames
private var _lastName = ""
init{
Transformations.map(allNames, ::getLastName1) //first try
//Transformations.switchMap(allNames, ::getLastName2) //second try
doSomethingWithName(_lastName) //commented when executed in getLastName()
}
private fun doSomethingWithName(name: String){ ... }
private fun getLastName1(nameList: List<NameList>){
val names: MutableList<String> = ArrayList()
nameList.forEach { names.add(it.name) }
val lastName = names[names.lastIndex]
//doSomethingWithName(lastName) //third try
_lastName = lastName
}
private fun getLastName2(nameList: List<NameList>): LiveData<NameList>{
val liveData: MutableLiveData<NameList> = MutableLiveData(nameList[nameList.lastIndex])
val lastName = liveData.value!!.name
//doSomethingWithName(lastName) //fourth try
_lastName = lastName
return liveData
}
}
What am I missing here?
If you want to get the last value in the list, considering the list might also be empty...
class MyViewModel(app: Application): AndroidViewModel(app){
private val nameListDao = NameListDatabase.getInstance(app).nameListDao
private val repository: nameListRepository = nameListRepository(nameListDao)
private val allNames: LiveData <List<NameList>> = repository.allNames
private var _lastName = ""
private val theLatestName = MutableLiveData<NameList?>()
init {
allNames.observe(getAReferenceToaLifeCycleOwner, Observer{
theLastestName.postValue(it.lastOrNull())
})
//make sure to get rid of the observer once this instance
//of the viewmodel is destroyed
}
}
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