如何在Android的ContentProvider中处理Room功能？

Question

我正在尝试通过

SQLite

中的

Room

库提供来自

ContentProvider

的数据，并且我的应用程序使用

Hilt

进行依赖项注入。

@Entity(tableName = "files")
data class FileEntity(
    @PrimaryKey(autoGenerate = false)
    val url: String,
    val fileName: String
)

@Dao
interface FilesDao {
    @Query(value = "SELECT * FROM files ORDER BY fileName")
    suspend fun getFiles(): List<FileEntity>
}

@Database(entities = [FileEntity::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
    abstract val filesDao: FilesDao
}

@Module
@InstallIn(SingletonComponent::class)
object RoomModule {
    @Provides
    @Singleton
    fun provideAppDatabase(
        @ApplicationContext context: Context
    ): AppDatabase = Room.databaseBuilder(
        context, AppDatabase::class.java, "app_database"
    ).build()

    @Provides
    @Singleton
    fun provideFilesDao(
        appDatabase: AppDatabase
    ): FilesDao = appDatabase.filesDao
}

通过上面的代码，我可以在

FilesDao

中接收

DataSource class

的实例，并通过

suspended functions

正常拨打电话。例如：

data class FileModel(
    val url: String,
    val fileName: String
)

interface FilesDataSource {
    suspend fun getFiles(): List<FileModel>
}

class FilesDataSourceImpl @Inject constructor(
    private val filesDao: FilesDao
) : FilesDataSource {
    override suspend fun getFiles(): List<FileModel>{
        val filesEntity = filesDao.getFiles()
        return filesEntity.toFilesModel()
    }

    private fun List<FileEntity>.toFilesModel() = map { fileEntity ->
        FileModel(
            url = fileEntity.url,
            fileName = fileEntity.fileName
        )
    }
}

要使用

Hilt

通过依赖注入将实例传递给

ContentProvider

我知道有必要创建一个自定义

EntryPoint

。例如：

@EntryPoint
@InstallIn(SingletonComponent::class)
interface ContentProviderEntryPoint {
    // var appDatabase: AppDatabase
    var filesDao: FilesDao
}

在

ContentProvider

中，到目前为止我有以下代码：

class MyProvider : ContentProvider() {
    private val appContext: Context by lazy {
        context?.applicationContext ?: throw IllegalStateException()
    }

    private val files: List<FileModel> by lazy {
        val hiltEntryPoint = EntryPointAccessors.fromApplication(
            context = appContext,
            entryPoint = ContentProviderEntryPoint::class.java
        )

        hiltEntryPoint.filesDao.getFiles() // this is a suspend function and I have no idea how to deal...
    }

    override fun onCreate(): Boolean {
        return true
    }

    override fun query(
        uri: Uri,
        projection: Array<out String>?,
        selection: String?,
        selectionArgs: Array<out String>?,
        sortOrder: String?
    ): Cursor {
        TODO()
    }

    override fun getType(uri: Uri): String {
        TODO()
    }

    override fun openAssetFile(
        uri: Uri,
        mode: String
    ): AssetFileDescriptor? {
        TODO()
    }

    override fun insert(
        uri: Uri,
        values: ContentValues?
    ): Uri = throw UnsupportedOperationException()

    override fun delete(
        uri: Uri,
        selection: String?,
        selectionArgs: Array<out String>?
    ): Int = throw UnsupportedOperationException()

    override fun update(
        uri: Uri,
        values: ContentValues?,
        selection: String?,
        selectionArgs: Array<out String>?
    ): Int = throw UnsupportedOperationException()
}

现在我的疑问，如何处理

suspended function

里面的

ContentProvider

？

补充：
我见过一些在

DAO class

中创建新函数（未暂停）以返回

Cursor

的示例。例如：

@Dao
interface FilesDao {
    @Query(value = "SELECT * FROM files")
    fun getFilesCursor(): Cursor

    // other suspend functions for datasource layer
}

这样我就可以在

ContentProvider

中调用新函数，但是 Room 会抛出异常：

class MyProvider : ContentProvider() {
    private val appContext: Context by lazy {
        context?.applicationContext ?: throw IllegalStateException()
    }

    private val filesCursor: Cursor by lazy {
        val hiltEntryPoint = EntryPointAccessors.fromApplication(
            context = appContext,
            entryPoint = ContentProviderEntryPoint::class.java
        )

        hiltEntryPoint.filesDao.getFilesCursor()
    }

    override fun onCreate(): Boolean {
        println(filesCursor) // just to see the output, but this throw an IllegalStateException
        return true
    }

    // ...
}

java.lang.IllegalStateException：无法在主线程上访问数据库，因为它可能会长时间锁定 UI。

我的

ContentProvider

目标是为WhatsApp提供贴纸（文件）。我已经有一个应用程序的工作版本，可以直接从 JSON 文件 读取内容，遵循与

Hilt

相同的逻辑，问题是现在我正在迁移到使用

Room

I'我无法处理这种情况。

Answer 1

suspended functions

：

runBlocking

但是您应该从 

override fun query(
    uri: Uri,
    projection: Array<out String>?,
    selection: String?,
    selectionArgs: Array<out String>?,
    sortOrder: String?
): Cursor {
    return runBlocking {
        // your code here
    }
}

调用

query

，因为

CoroutineScope(Dispatchers.IO)

会中断地阻塞当前线程，直到其完成