任务是选择一个日记时，打开一个带有此日记注释的活动。（一对多）这就是数据库中实体的样子：
@Entity(tableName = "word_table")
data class Word(@ColumnInfo(name = "word") val word: String,
@ColumnInfo(name = "description") val description : String
)
{
@ColumnInfo(name = "id")
@PrimaryKey(autoGenerate = true)
var id : Long = 0
}
@Entity(tableName = "note_table")
data class Note(@ColumnInfo(name = "note_name") val note : String,
@ColumnInfo(name = "text") val text : String,
@ColumnInfo(name = "diaryId") val diaryId : Long
){
@PrimaryKey(autoGenerate = true)
var idNote : Long = 0
}
在NoteRepository.kt中使用数据类
data class NotesAndWords (@Embedded val word : Word,
@Relation(parentColumn = "id", entityColumn = "diaryId")
val notes : List<Note>)
以及WordDao.kt中的查询
@Transaction
@Query("SELECT * from word_table ")
fun getSomeNotes() : LiveData<List<NotesAndWords>>
我得到数据并将其保存在NoteRepository类中：
class NoteRepository (private val wordDao : WordDao) {
var allNotes : LiveData<List<NotesAndWords>> = wordDao.getSomeNotes()
suspend fun insertNote(note : Note)
{
wordDao.insertNote(note)
}
}
然后通过NoteViewModel.kt将数据传递到NoteActivity.kt：
class NoteViewModel(application: Application) : AndroidViewModel(application) {
private val repository: NoteRepository
val allNotes: LiveData<List<NotesAndWords>>
init {
val wordsDao = WordRoomDatabase.getDatabase(application, viewModelScope).wordDao()
repository = NoteRepository(wordsDao)
allNotes = repository.allNotes
}
fun insertNote(note: Note) = viewModelScope.launch {
repository.insertNote(note)
}
}
（（NoteActivity.kt）]
class NoteActivity : AppCompatActivity() {
private val newWordActivityRequestCode = 1
private lateinit var noteViewModel: NoteViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_note)
val adapter = NoteListAdapter(this, intent.getLongExtra("tag", -1) ){
val intent = Intent(this, ClickedActivity::class.java)
intent.putExtra("tag", it.note)
startActivity(intent)
}
recyclerview1.adapter = adapter
recyclerview1.layoutManager = LinearLayoutManager(this)
noteViewModel = ViewModelProvider(this).get(NoteViewModel::class.java)
noteViewModel.allNotes.observe(this, Observer {
adapter.setNotes(it)
})
val fab = findViewById<FloatingActionButton>(R.id.fab)
fab.setOnClickListener {
val intent = Intent(this, NewWordActivity::class.java)
startActivityForResult(intent, newWordActivityRequestCode)
}
}
override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
super.onActivityResult(requestCode, resultCode, data)
if (requestCode == newWordActivityRequestCode && resultCode == Activity.RESULT_OK)
{
data?.getStringArrayListExtra(NewWordActivity.EXTRA_REPLY)?.let {
val note = Note(it[0], it[1], intent.getLongExtra("tag", -1))
noteViewModel.insertNote(note)
}
}
else
{
Toast.makeText(applicationContext, R.string.empty_not_saved,
Toast.LENGTH_LONG).show()
}
}
然后，在适配器中，我使用MutableMap来转换列表，以便键是名称ID，值是根据请求选择的注释（附加到特定的日记）
NoteListAdapter.kt：
class NoteListAdapter internal constructor(
context: Context,
val wordId: Long,
private val listener : (Note) -> Unit
) : RecyclerView.Adapter<NoteListAdapter.NoteViewHolder>() {
private val inflater: LayoutInflater = LayoutInflater.from(context)
//private val mContext = context
private var notes = emptyList<NotesAndWords>() // Cached copy of words
private var notesMapped = mutableMapOf<Long, List<Note>>()
inner class NoteViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
private val noteItemView: TextView = itemView.findViewById(R.id.textView1)
private val noteDescriptionView: TextView = itemView.findViewById(R.id.textView)
fun bindView(note: Note, listener : (Note) -> Unit) {
noteItemView.text = note.diaryId.toString()
noteDescriptionView.text = note.text
itemView.setOnClickListener {
listener(note)
}
}
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): NoteViewHolder {
val itemView = inflater.inflate(R.layout.recyclerview_layout, parent,
false)
return NoteViewHolder(itemView)
}
override fun onBindViewHolder(holder: NoteViewHolder, position: Int) {
holder.bindView(notesMapped[wordId]!![position], listener)
}
internal fun setNotes(notes: List<NotesAndWords>) {
this.notes = notes
for (i in this.notes) {
notesMapped[i.word.id] = i.notes
}
notifyDataSetChanged()
}
override fun getItemCount() = notesMapped[wordId]!!.size
}
数据库：
@Database(entities = [Word::class, Note::class], version = 2, exportSchema = false)
abstract class WordRoomDatabase : RoomDatabase() {
abstract fun wordDao(): WordDao
private class WordDatabaseCallback(private val scope: CoroutineScope) : RoomDatabase.Callback()
{
override fun onOpen(db: SupportSQLiteDatabase) {
super.onOpen(db)
INSTANCE?.let { database ->
scope.launch {
populateDatabase(database.wordDao())
}
}
}
suspend fun populateDatabase(wordDao: WordDao) {
//wordDao.deleteAll()
//wordDao.deleteAllNotes()
}
}
companion object {
@Volatile
private var INSTANCE: WordRoomDatabase? = null
fun getDatabase(context: Context, scope:CoroutineScope): WordRoomDatabase {
val tempInstance = INSTANCE
if (tempInstance != null) {
return tempInstance
}
val instance = Room.databaseBuilder(context.applicationContext,
WordRoomDatabase::class.java, "word_database")
.addCallback(WordDatabaseCallback(scope))
//.fallbackToDestructiveMigration()
.build()
INSTANCE = instance
return instance
}
}
}
我已经创建了几本日记和一个便笺，使用按钮来创建新的日记和便笺。现在，如果您依次选择多个日记，则在尝试选择日记时，适配器将在此行中发出NullPointerException：
override fun getItemCount() = notesMapped[wordId]!!.size如果notesMapped始终具有wordId键，为什么会引发此异常？
NoteActivity是从另一个活动调用的，并且日记ID传递给了它
此库在GitHub上：https://github.com/Lomank123/RoomDatabase
任务是选择一个日记时，打开一个带有此日记注释的活动。 （一对多）数据库中的实体如下所示：@Entity（tableName =“ word_table”）数据...
您好，地图是空的，地图返回的是空值，您正在检查空对象的大小。同样好的做法是，不要使用地图，而只使用注释列表，然后直接传递该列表。