没有 livedata 的 Android Studio Room 数据

问题描述

我得到了以下 Room 数据库，想在 textview 中输出随机用户的名称。不幸的是，运行代码会产生输出： kotlin.unit 在 textview 中。我的文件看起来像这样：

MainActivity.kt

class MainActivity : AppCompatActivity() {

    private lateinit var mNameViewModel: NameViewModel

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

        mNameViewModel = ViewModelProvider(this).get(NameViewModel::class.java)

        val btn = findViewById<Button>(R.id.btn_addName)
        val tv = findViewById<TextView>(R.id.tv_showName)

        btn.setOnClickListener {
            val text = findViewById<EditText>(R.id.et_enterName)?.text.toString()
            val name = Name(0,text)

            // Add Data to Database
            mNameViewModel.addName(name)
            Toast.makeText(applicationContext,"Successfully added $text.",Toast.LENGTH_LONG).show()

            val randomName = mNameViewModel.getRandomName()

            // Without .toString() I get an error,with it it displays kotlin.unit
            tv.text = randomName.toString() 
        }

    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_showName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.133" />

    <EditText
        android:id="@+id/et_enterName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        android:hint="Name"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.244" />

    <Button
        android:id="@+id/btn_addName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.379" />

</androidx.constraintlayout.widget.ConstraintLayout>

名称.kt

@Entity(tableName = "name_data")
data class Name (
    @PrimaryKey(autoGenerate = true) val id: Int,@ColumnInfo(name = "name") val name: String
)

NameDao.kt

@Dao
interface NameDao {

    @Insert
    fun addName(name: Name)

    @Query("SELECT name FROM name_data ORDER BY RANDOM() LIMIT 1")
    fun getRandomName(): String
}

NameDatabase.kt

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

    abstract fun nameDao(): NameDao

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

        fun getDatabase(context: Context): NameDatabase{
            val tempInstance = INSTANCE
            if(tempInstance != null){
                return tempInstance
            }
            synchronized(this){
                val instance = databaseBuilder(
                    context.applicationContext,NameDatabase::class.java,"name_data"
                ).build()
                INSTANCE = instance
                return instance
            }
        }
    }
}

NameRepository.kt

class NameRepository(private val nameDao: NameDao) {

     fun getRandomName() { nameDao.getRandomName() }

     fun addName(name: Name) { nameDao.addName(name) }
}

NameViewModel.kt

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

    private val repository: NameRepository

    init {
        val nameDao = NameDatabase.getDatabase(application).nameDao()
        repository = NameRepository(nameDao)
    }

    fun addName(name: Name) {
        viewModelScope.launch(Dispatchers.IO){
            repository.addName(name)
        }
    }

    fun getRandomName() {
        viewModelScope.launch(Dispatchers.IO){
            repository.getRandomName()
        }
    }
}

这是按下按钮时textview的输出方式。

尽管数据库已填充。

感谢任何帮助显示数据。谢谢！

解决方法

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