Dao class 的实例未正确初始化

Question

我想使用 Kotlin Coroutine 将数据库中的数据加载到 RecyclerView 中，但它不会在第一次启动时加载数据，但当应用程序重新启动时它会正确加载数据。存储库 class:

中似乎有问题

NoteDao.kt 是：

@Dao
interface NoteDAO {

    @Query("SELECT * FROM NoteTable ORDER BY noteDate DESC")
    fun findAllNotes(): List<Note>

}

NoteRepository.kt

class NoteRepository(application: Application) {

    private var allNotes = MutableLiveData<List<Note>>()

    private val noteDAO = NoteDatabase.getDatabase(application).getNoteDao() // Issue is caused by this line because data is not assigned to noteDAO 

    fun getAllNotes(): MutableLiveData<List<Note>> {
        CoroutineScope(Dispatchers.IO).launch {
            val noteData = noteDAO.findAllNotes()
            allNotes.postValue(NoteData)
        }
        return allNotes   // it also returns empty List with size = 0
    }

}

适配器 class 是

class NoteAdapter(val context: Context): RecyclerView.Adapter<NoteAdapter.NoteHolder>() {

    private var allNotes: List<Note> = ArrayList()

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): NoteHolder {
        val itemView = LayoutInflater.from(parent.context)
            .inflate(R.layout.Note_item, parent, false)
        return NoteHolder(itemView)
    }

    override fun onBindViewHolder(holder: NoteHolder, position: Int) {
        val currentNote = allNotes[position]
        holder.title.text = currentNote.title
        holder.date.text = DateUtils.getDate(Date(currentNote.date))

    }

    override fun getItemCount(): Int {
        return allNotes.size
    }

    fun addNotes(allNotes: List<Note>) {
        this.allNotes = allNotes
        notifyDataSetChanged()
    }

    inner class NoteHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        var title: LabelTextView = itemView.findViewById(R.id.item_title)
        var date: LabelTextView = itemView.findViewById(R.id.item_date)
    }

}

并且 ViewModel 是

class NoteViewModel(
    application: Application
): AndroidViewModel(application) {

    private var repository = NoteRepository(application)

    private var _allNotes = repository.getAllNotes()

    val allNotes: LiveData<List<Note>>
        get() = _allNotes
}

最后附加到 MainActivity

class MainActivity : AppCompatActivity(){

    private lateinit var mNoteViewModel: NoteViewModel
    private lateinit var mNoteAdapter: NoteAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)    

        mNoteAdapter = NoteAdapter(this)

        mNoteViewModel = ViewModelProviders.of(this).get(NoteViewModel::class.java)
        mNoteViewModel.allNotes.observe(
            this, Observer {
                Log.e("MAIN_ACTIVITY", "Called...") // it is printed in Log but data is not recieved from repository
                mNoteAdapter.addNotes(it)
            }
        )

        rv_log.layoutManager = GridLayoutManager(this, 2, GridLayoutManager.VERTICAL, false)

        mNoteAdapter.notifyDataSetChanged()
        rv_log.adapter = mNoteAdapter
    }

}

RecyclerView 中的所有数据都没有加载和显示。

Answer 1

似乎 getAllNotes() 的问题意味着 CoroutineScope 无法正常工作。您应该使用 Flow API 并将 Dao 方法更改为

@Query("SELECT * FROM NoteTable ORDER BY noteDate DESC")
fun findAllNotes(): Flow<List<Note>>

在此之后你必须更改 NoteRepository class 并将 init 块放入：

init {
    CoroutineScope(Dispatchers.IO).launch {
         val noteData = noteDAO.findAllNotes()
         noteData.collect{ // it is related to Flow API
             allNotes.postValue(it)
         }
    }
}

return allNotes getAllNotes()

fun getAllNotes(): MutableLiveData<List<Note>> {            
    return allNotes 
}

现在你必须检查它是否有效？

Answer 2

我不同意已接受的答案。 CoroutineScope.

不是问题

原始代码返回空 List 的原因是因为您没有将 DAO 函数声明为 suspend 函数。由于 getAllNotes() 不是 suspend 函数，它不会暂停调用协程，直到 Room returns 得到结果，因此 returns 是一个空列表。

如果您要将 Flow 转换为 LiveData，我也建议使用 asLiveData()。

Dao class 的实例未正确初始化

instance of Dao class is not being initialized correctly

android

kotlin

android-mvvm

android-viewmodel

kotlin-coroutines