使用 Paging 3 从 Room 获取最新数据 Android

Question

我想显示分页为 3 的项目列表，数据来自我的本地数据库和 Room Library。我使用 JSON 文件的房间提供的 RoomDatabase.Callback 将我的数据预填充到房间数据库中。但是我第一次打开应用程序时，列表没有显示任何内容。我必须重新打开我的应用程序，表格中的项目将会显示。

以下是一些代码片段：

quiz.json

[
    {
        "question": "Question 1",
        "type": "en",
        "answer": 0
    },
    {
        "question": "Question 2",
        "type": "en",
        "answer": 1
    }
]

AppDatabase.kt

@Database(
    entities = [QuizEntity::class],
    version = 1,
    exportSchema = false
)
abstract class AppDatabase : RoomDatabase() {

    abstract fun quizDao(): QuizDao

    private class AppDatabaseCallback(
        private val context: Context,
        private val scope: CoroutineScope
    ) : RoomDatabase.Callback() {

        override fun onCreate(db: SupportSQLiteDatabase) {
            super.onCreate(db)
            INSTANCE?.let { database ->
                scope.launch {
                    val quizDao = database.quizDao()
                    quizDao.deleteAll()

                    fillQuizData(context, quizDao)
                }
            }
        }

      suspend fun fillQuizData(context: Context, quizDao: QuizDao) {
            val jsonArray = FileUtil.loadJsonArray(context, R.raw.quiz)
            try {
                jsonArray?.let {
                    for (i in 0 until it.length()) {
                        val item = it.getJSONObject(i)
                        val isAnswer = item.getInt("answer")
                        quizDao.insert(
                            QuizEntity(
                                0,
                                item.getString("question"),
                                item.getString("type"),
                                isAnswer == 1,
                            )
                        )
                    }
                }
            } catch (exception: JSONException) {
                exception.printStackTrace()
            }
        }
    }

    companion object {
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(
            context: Context,
            scope: CoroutineScope
        ): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "MyApplication.db"
                )
                    .addCallback(AppDatabaseCallback(context, scope))
                    .fallbackToDestructiveMigration()
                    .build()
                INSTANCE = instance
                instance
            }
        }
    }
}

QuizDao.kt

@Dao
interface QuizDao {

    @Query("SELECT * FROM quiz WHERE type=:type AND question LIKE '%' || :question || '%' LIMIT :size")
    suspend fun searchQuiz(type: String, size: Int, question: String = ""): List<QuizEntity>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(quizEntity: QuizEntity)

    @Query("DELETE FROM quiz")
    suspend fun deleteAll()
}

QuizRepository.kt

class QuizRepository @Inject constructor(private val quizDao: QuizDao) {

  fun getEnglishQuiz(query: String = ""): Flow < PagingData < QuizEntity >> {
    return Pager(PagingConfig(PAGE_SIZE)) {
      QuizPagingSource(quizDao, "en", query)
    }.flow
  }

  companion object {
    private
    const val PAGE_SIZE = 25
  }
}

QuizViewModel.kt

@HiltViewModel
class QuizViewModel @Inject constructor(private val quizRepository: QuizRepository) : ViewModel() {

    fun getEnglishQuizData(query: String = ""): Flow<PagingData<Quiz>> {
        return quizRepository.getEnglishQuiz(query).map {
            it.map { quiz ->
                Quiz(
                    quiz.id,
                    quiz.question,
                    quiz.type,
                    quiz.isAnswer
                )
            }
        }.flowOn(Dispatchers.IO).cachedIn(viewModelScope)
    }
}

在我的 Activity:

lifecycleScope.launch {
  binding.etSearch.getQueryTextChangeStateFlow()
    .debounce(500)
    .distinctUntilChanged()
    .flatMapLatest {
      query - >
        quizViewModel.getEnglishQuizData(query)
    }
    .collect {
      result - >
        quizAdapter.submitData(result)
    }
}

SearchView 扩展

fun SearchView.getQueryTextChangeStateFlow(): StateFlow < String > {

  val query = MutableStateFlow("")

  setOnQueryTextListener(object: SearchView.OnQueryTextListener {
    override fun onQueryTextSubmit(query: String ? ): Boolean {
      return true
    }

    override fun onQueryTextChange(newText: String): Boolean {
      query.value = newText
      return true
    }
  })

  return query

}

我考虑过为我的 QuizDao 添加一个新函数，它是我 QuizEntity 的 return LiveData 并且我在我的 activity 中观察到它并检查大小是否不一样我将调用函数由 paging 3 库提供，如果我没记错的话，它是 adapter.refreshed() 或 adapter.invalidated()。我没有尝试过，所以我不知道这是否有效。有什么解决办法吗？？抱歉我的英语不好..谢谢.

Answer 1

分页通过失效通知 DB 更新，这会在 Flow 中生成一个新的 PagingData。在你的 activity 中，确保使用 .collectLatest 而不是 .collect，这样你就可以取消上一个（空的）生成并在下游接收到新的后使用它。

例如，

lifecycleScope.launch {
  binding.etSearch.getQueryTextChangeStateFlow()
    ...
    .collectLatest {
      result - >
        quizAdapter.submitData(result)
    }
}

.submitData 暂停直到该代完成。由于 Paging 由收集器的范围提供支持，因此您不应依赖它来急切取消。

使用 Paging 3 从 Room 获取最新数据 Android

Get Latest Data From Room with Paging 3 Android

android

kotlin

android-room

android-paging