android - 如何使用 Kotlin、Coroutines、ViewModel、LiveData 处理查询方法的返回类型 room

Question

如何使用 Kotlin、Coroutines、ViewModel、LiveData 处理查询方法的返回类型 room

构建失败，我收到很多指向我的 Dao 类的错误，错误是

错误1:

Not sure how to handle query method's return type (java.lang.Object). DELETE query methods must either return void or int (the number of deleted rows).

错误2:

error: Query method parameters should either be a type that can be converted into a database column or a List / Array that contains such type. You can consider adding a Type Adapter for this. kotlin.coroutines.Continuation<? super kotlin.Unit> continuation);

错误3:

error: Unused parameter: continuation public abstract java.lang.Object clear(@org.jetbrains.annotations.NotNull()

错误4:

error: Type of the parameter must be a class annotated with @Entity or a collection/array of it. kotlin.coroutines.Continuation<? super kotlin.Unit> continuation);

错误5:

error: Not sure how to handle insert method's return type. public abstract java.lang.Object insert(@org.jetbrains.annotations.NotNull()

**这是我的完整代码: https://drive.google.com/drive/folders/1qWoud5XogzkTmpa-GWxLJStfdUPSoV7r?usp=sharing

android kotlin - 协程 Room ViewModel LiveData MainActivity.kt

package com.example.coroutine

import android.os.Bundle
import android.text.method.ScrollingMovementMethod
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import kotlinx.android.synthetic.main.activity_main.*
import java.util.UUID
import kotlin.random.Random


class MainActivity : AppCompatActivity() {

    private lateinit var model: StudentViewModel

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

        // make text view text scrollable
        textView.movementMethod = ScrollingMovementMethod()

        // initialize the student view model
        model = ViewModelProvider(this).get(StudentViewModel::class.java)


        // observe the students live data
        model.students.observe(this, Observer { students->
                textView.text = "Students(${students.size})..."

                students.forEach {
                    textView.append("\n${it.id} | ${it.fullName} : ${it.result}")
                }
            }
        )


        btnInsert.setOnClickListener {
            // generate a new student
            val student = Student(
                null,
                UUID.randomUUID().toString(),
                Random.nextInt(100)
            )

            // insert new student into room database
            model.insert(student)
        }


        btnClear.setOnClickListener {
            // delete all students from room student table
            model.clear()
        }
    }
}

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"
    android:background="#FAE6FA"
    tools:context=".MainActivity">

    <com.google.android.material.button.MaterialButton
        android:id="@+id/btnInsert"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:backgroundTint="#8DB600"
        android:text="Insert"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.button.MaterialButton
        android:id="@+id/btnClear"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:backgroundTint="#E52B50"
        android:text="Clear"
        app:layout_constraintBottom_toBottomOf="@+id/btnInsert"
        app:layout_constraintStart_toEndOf="@+id/btnInsert" />

    <com.google.android.material.textview.MaterialTextView
        android:id="@+id/textView"
        style="@style/TextAppearance.MaterialComponents.Subtitle1"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:textColor="#1B1811"
        android:textStyle="bold"
        android:padding="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnInsert"
        app:layout_constraintVertical_bias="1.0"
        tools:text="TextView" />

</androidx.constraintlayout.widget.ConstraintLayout>

RoomSingleton.kt

package com.example.coroutine

import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase
import android.content.Context

@Database(entities = [Student::class], version = 1)
abstract class RoomSingleton : RoomDatabase() {
    abstract fun studentDao():StudentDao

    companion object {
        private var INSTANCE: RoomSingleton? = null
        fun getInstance(context: Context): RoomSingleton {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(
                    context,
                    RoomSingleton::class.java,
                    "roomdb")
                    .build()
            }
            return INSTANCE as RoomSingleton
        }
    }
}

RoomDao.kt

package com.example.coroutine

import androidx.lifecycle.LiveData
import androidx.room.Dao
import androidx.room.Insert
import androidx.room.OnConflictStrategy
import androidx.room.Query

@Dao
interface StudentDao{
    @Query("SELECT * FROM studentTbl ORDER BY id DESC")
    fun getStudents():LiveData<List<Student>>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(student:Student)

    @Query("DELETE FROM studentTbl")
    suspend fun clear()
}

RoomEntity.kt

package com.example.coroutine

import androidx.room.ColumnInfo
import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity(tableName = "studentTbl")
data class Student(
    @PrimaryKey
    var id:Long?,

    @ColumnInfo(name = "uuid")
    var fullName: String,

    @ColumnInfo(name = "result")
    var result:Int
)

StudentViewModel.kt

package com.example.coroutine

import androidx.lifecycle.AndroidViewModel
import android.app.Application
import androidx.lifecycle.LiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch


class StudentViewModel(application:Application): AndroidViewModel(application){
    private val db:RoomSingleton = RoomSingleton.getInstance(application)

    internal val students : LiveData<List<Student>> = db.studentDao().getStudents()

    fun insert(student: Student){
        viewModelScope.launch(Dispatchers.IO) {
            db.studentDao().insert(student)
        }
    }

    fun clear(){
        viewModelScope.launch(Dispatchers.IO) {
            db.studentDao().clear()
        }
    }
}

Answer 1

您的问题出在插入和删除方法中的关键字 suspend 中。删除后，您的错误就消失了。

@Insert(onConflict = OnConflictStrategy.REPLACE)
   fun insert(student:Student)

    @Query("DELETE FROM studentTbl")
   fun clear()

顺便说一句，您的构建仍然无法成功。要修复其他错误，您应该在应用程序 build.gradle 文件中的 block 插件上添加“kotlin-android-extensions”。

plugins {
    id 'com.android.application'
    id 'kotlin-android-extensions'
    id 'kotlin-android'
    id 'kotlin-kapt'
}

但是这个解决方案已被弃用，您应该使用 viewBinding 代替。看一下这个。 https://developer.android.com/topic/libraries/view-binding/migration