Room：不确定如何将 Cursor 转换为此方法的返回类型

Posted 2023-04-18

【中文标题】Room：不确定如何将 Cursor 转换为此方法的返回类型

【英文标题】：Room: Not sure how to convert a Cursor to this method's return type

【发布时间】：2019-12-12 21:09:20

【问题描述】：

我无法在我的 Dao 中使用 LiveData。删除 LiveData 解决了编译错误。代码如下：

我查看了我的 Gradle 构建文件。我没有使用任何旧的支持库。我的代码也没有其他一般错误。

基类：

@Entity
@JsonClass(generateAdapter = false)
abstract class BaseEntity(
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "_id")
    var id: Long = 0L,

    @ColumnInfo(name = "_created_at")
    @Json(name = "created_at")
    var createdAt: Date = Date(),

    @ColumnInfo(name = "_updated_at")
    @Json(name = "updated_at")
    var updatedAt: Date = Date(),

    @ColumnInfo(name = "_created_by")
    @Json(name = "created_by")
    var createdBy: String = ""
) 
    init 
        val applicationContext = BaseApp.applicationContext() as BaseApp
        val username = applicationContext.sharedPreference?.getString(
            applicationContext.getString(R.string.setting_login_username),
            ""
        )
        if (username != null) 
            createdBy = username
        
    



@Entity
@JsonClass(generateAdapter = true)
open class Queue(
    @NonNull
    @ColumnInfo(name = "name")
    @Json(name = "name")
    var name: String = "",

    @NonNull
    @ColumnInfo(name = "queue_type")
    @Json(name = "queue_type")
    var queueType: QueueType? = null,

    @NonNull
    @ColumnInfo(name = "queue_status")
    @Json(name = "queue_status")
    var queueStatus: QueueStatus? = null,

    @ColumnInfo(name = "server_response")
    @Json(name = "server_response")
    var serverResponse: String = ""
) : BaseEntity()


@Dao
interface QueueDao 
    @Query("SELECT * FROM Queue")
    suspend fun getQueue(): LiveData<List<Queue>>



Complete gradle build file:
===========================
apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

apply plugin: 'kotlin-kapt'

android 
    compileSdkVersion 28
    defaultConfig 
        applicationId "____"
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        javaCompileOptions 
            annotationProcessorOptions 
                arguments = ["room.schemaLocation": "$projectDir/schemas".toString()]
            
        
    
    buildTypes 
        release 
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        
    
    compileOptions 
        sourceCompatibility = '1.8'
        targetCompatibility = '1.8'
    


dependencies 
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.core:core-ktx:1.0.2'
    implementation 'org.jetbrains.kotlin:kotlin-reflect:1.3.41'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'
    implementation 'androidx.lifecycle:lifecycle-runtime-ktx:2.2.0-alpha02'
    implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0-alpha02'

    // Room for database
    def room_version = '2.1.0'
    implementation "androidx.room:room-runtime:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    implementation "androidx.room:room-ktx:2.1.0"

    // moshi json parser
    implementation("com.squareup.moshi:moshi-kotlin:1.8.0")
    implementation("com.squareup.moshi:moshi-adapters:1.8.0")
    kapt("com.squareup.moshi:moshi-kotlin-codegen:1.8.0")

    // zxing-android-embedded
    implementation('com.journeyapps:zxing-android-embedded:3.6.0')transitive = false
    implementation 'com.google.zxing:core:3.3.3'

    // kotlin coroutines and retrofit
    implementation 'com.squareup.retrofit2:retrofit:2.5.0'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.2.1'
    implementation 'com.squareup.retrofit2:converter-moshi:2.5.0'
    implementation 'com.jakewharton.retrofit:retrofit2-kotlin-coroutines-adapter:0.9.2'

    // glide for image viewing/fetching
    implementation 'com.github.bumptech.glide:glide:4.9.0'
    annotationProcessor 'com.github.bumptech.glide:compiler:4.9.0'

    // dexter for permissions
    implementation 'com.karumi:dexter:5.0.0'

    // materialDrawer
    implementation "com.mikepenz:materialdrawer:6.1.1"

    // Google flexbox
    implementation 'com.google.android:flexbox:1.1.0'

    // UI Elements
    implementation "androidx.constraintlayout:constraintlayout:1.1.3"
    implementation "androidx.recyclerview:recyclerview:1.0.0"
    implementation "com.google.android.material:material:1.0.0"
    implementation "androidx.annotation:annotation:1.1.0"

    // tests
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

error: Not sure how to convert a Cursor to this method's return type (androidx.lifecycle.LiveData<java.util.List<data.basequeue.Queue>>).
public abstract java.lang.Object getQueue(@org.jetbrains.annotations.NotNull()

只需从 Dao 中移除 LiveData 即可成功编译代码。

【参考方案1】：

Room 不支持具有暂停功能的 LiveData。此外，LiveData 已经在后台线程上工作，应该直接使用而不使用协程。

从我的代码中删除 suspend 解决了错误。