使用 Transformations.map 在 ViewModel 中处理来自 Room 数据库的 LiveData

Question

我有以下简单的数据库：

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
    ...
}

目前我只想获取并处理这个非空数据库的最后一个条目（没有 NameListDao 中的 changing/adding 方法）。

在一个片段中，很容易得到，例如使用 observe 方法显示（在 TextView 中通过数据绑定）最后一个数据库条目：

viewModel.allNames.observe(this, Observer<List<NameList?>> { 
    nameList ->  binding.textView.text = nameList[nameList.lastIndex]!!.name 
})

据我所知，在ViewModel中需要使用Transformations.map或Transformations.switchMap来处理LiveData。为了获取数据库的最后一个条目并将其传递给方法 doSomethingWithName()，我尝试了以下两（四）种带有转换的方法（none 有效）。但是，在所有情况下，传递的参数显然都是空的。（我也通过 [0] 尝试了第一个条目，但我得到了相同的结果）

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
    }
}

我在这里错过了什么？

Answer 1

如果要获取列表中的最后一个值，考虑到列表可能也是空的...

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
    }

}

使用 Transformations.map 在 ViewModel 中处理来自 Room 数据库的 LiveData

Processing LiveData from Room database in ViewModel with Transformations.map

android

kotlin

android-room

android-livedata

android-viewmodel