如何从 Room 获取 LiveData？

Question

我正在尝试从 Room 获取 LiveData。因此，如果数据库中的任何内容发生更改，我的 RecycleView 可以进行实时更新

我试过不使用 LiveData 并且效果很好，但是当我添加 LiveData 时总是显示此错误。

error: Not sure how to convert a Cursor to this method's return type     (androidx.lifecycle.LiveData<java.util.List<com.example.models.Club>>).
public abstract java.lang.Object     getAll(@org.jetbrains.annotations.NotNull()

我用谷歌搜索并在这个网站上寻找解决方案，但遇到这个问题的每个人都使用了 rxjava、rxandroid、rxkotlin 或 ArrayList。对他们来说，解决方案是用 List 替换 ArrayList，并让 RX 尝试协程。好吧，我正在使用 Coroutine 和 List，但仍然没有任何进展。

这是我的 ClubDao

ClubDao
@Query("SELECT * FROM club")
suspend fun getAll(): LiveData<List<Club>>

在俱乐部我有这个属性

Club
@Entity
data class Club(@PrimaryKey var id: Int,
            @ColumnInfo(name = "logo_url") var logoUrl: String,
            @ColumnInfo(name = "name") var name: String,
            @ColumnInfo(name = "town") var town: String,
            @ColumnInfo(name = "address") var address: String,
            @ColumnInfo(name = "contact_name") var contactName: String,
            @ColumnInfo(name = "phone_numbers") var phoneNumbers: String,
            @ColumnInfo(name = "email") var email: String)

phoneNumbers 应该是列表，但我使用 TypeConverters

将它们与 json 相互转换

类型转换器在这里

TypeConverter
class ConvertersDB {
    @TypeConverter
    fun fromString(value: String): ArrayList<String> {
        val listType = object : TypeToken<ArrayList<String>>() {

        }.type
        return Gson().fromJson(value, listType)
    }

    @TypeConverter
    fun fromArrayList(list: ArrayList<String>): String {
        val gson = Gson()
        return gson.toJson(list)
    }
}

还有我的数据库

DataBase
@Database(entities = [Club::class], version = 1, exportSchema = false)
@TypeConverters(ConvertersDB::class)
abstract class AppDatabase : RoomDatabase() {
    abstract fun clubDao(): ClubDao

    companion object {
        @Volatile
        private var instance: AppDatabase? = null
        private val LOCK = Any()

        operator fun invoke(context: Context) = instance ?: synchronized(LOCK) {
            instance ?: buildDatabase(context).also { instance = it }
        }

        private fun buildDatabase(context: Context) = Room.databaseBuilder(context,
                AppDatabase::class.java, "pss.db")
                .allowMainThreadQueries()
                .build()
    }
}

在我的片段中，我需要通过 ViewModel 和 Repository 从数据库中观察所有俱乐部，并将它们发送到 RecycleView

现在出现错误：

error: Not sure how to convert a Cursor to this method's return type     (androidx.lifecycle.LiveData<java.util.List<com.example.models.Club>>).
public abstract java.lang.Object     getAll(@org.jetbrains.annotations.NotNull()

有人知道这个的解决方案吗？

编辑：

Gradle
apply plugin: 'com.android.application'
apply plugin: 'androidx.navigation.safeargs'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlin-kapt'


android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.overswayit.plesnisavezsrbije"
        minSdkVersion 24
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    dataBinding {
        enabled true
    }
    packagingOptions {
        exclude 'META-INF/atomicfu.kotlin_module'
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.3.31"
    implementation 'com.jakewharton:butterknife:10.1.0'
    implementation 'com.google.android.material:material:1.1.0-alpha07'
    implementation 'androidx.cardview:cardview:1.0.0'
    implementation 'androidx.appcompat:appcompat:1.1.0-beta01'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'androidx.legacy:legacy-support-v4:1.0.0'

    // Room components
    implementation "androidx.room:room-runtime:$rootProject.roomVersion"
    implementation "androidx.room:room-ktx:$rootProject.roomVersion"
    kapt "androidx.room:room-compiler:$rootProject.roomVersion"
    androidTestImplementation "androidx.room:room-testing:$rootProject.roomVersion"

    // Room and RxJava
    implementation "androidx.room:room-rxjava2:$rootProject.roomVersion"

    // Lifecycle components
    implementation "androidx.lifecycle:lifecycle-extensions:$rootProject.archLifecycleVersion"
    kapt "androidx.lifecycle:lifecycle-compiler:$rootProject.archLifecycleVersion"
    androidTestImplementation "androidx.arch.core:core-testing:$rootProject.androidxArchVersion"

    // ViewModel Kotlin support
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$rootProject.archLifecycleVersion"

    //LiveData Kotlin
    implementation "androidx.lifecycle:lifecycle-livedata:$rootProject.archLifecycleVersion"
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$rootProject.archLifecycleVersion"

    implementation "androidx.lifecycle:lifecycle-runtime-ktx:$rootProject.archLifecycleVersion"

    // Coroutines
    api "org.jetbrains.kotlinx:kotlinx-coroutines-core:$rootProject.coroutines"
    api "org.jetbrains.kotlinx:kotlinx-coroutines-android:$rootProject.coroutines"

    //RxJava
    implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
    implementation "io.reactivex.rxjava2:rxjava:2.2.6"
    implementation 'com.jakewharton.rxbinding3:rxbinding:3.0.0-alpha2'

    implementation "org.jetbrains.kotlin:kotlin-stdlib:1.3.31"
    annotationProcessor 'com.jakewharton:butterknife-compiler:10.1.0'
    implementation 'com.squareup.picasso:picasso:2.71828'
    implementation 'com.kaopiz:kprogresshud:1.0.5'
    implementation 'com.squareup:otto:1.3.8'
    implementation 'agency.tango.android:avatar-view:0.0.2'
    implementation 'agency.tango.android:avatar-view-picasso:0.0.2'
    implementation 'com.mikhaellopez:circularimageview:3.2.0'
    implementation 'com.googlecode.libphonenumber:libphonenumber:8.1.0'

    // Data Binding
    kapt "com.android.databinding:compiler:3.1.4"

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation project(path: ':tuple')

    // Navigation Component
    implementation 'androidx.navigation:navigation-fragment-ktx:2.0.0'
    implementation 'androidx.navigation:navigation-ui-ktx:2.0.0'

    //Gson
    implementation 'com.google.code.gson:gson:2.8.5'
}

kapt {
    generateStubs = true
}

顶级Gradle

buildscript {

    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.31"
        classpath "android.arch.navigation:navigation-safe-args-gradle-plugin:1.0.0"


        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

ext {
    roomVersion = '2.1.0'
    archLifecycleVersion = '2.2.0-alpha01'
    androidxArchVersion = '2.0.0'
    coroutines = '1.2.0'
}

Answer 1

如评论中所述，删除suspend。当一个方法 returns 是一个可观察对象时，没有理由让它暂停，因为它只是 returns 和对象，在被观察之前不会运行任何查询。

@Query("SELECT * FROM club")
fun getAll(): LiveData<List<Club>>

Room 的协程集成带来了 return 暂停值的能力，但是当值本身是 asnyc 时，没有理由使用它。

Answer 2

我找到了将 rxJava 与 room 和 livedata 一起使用的很好的解决方案。该方法类似于使用 Retrofit 和 RxJava，使用 LiveDataReactiveStreams，它默认使用可流动的。

在你的方法中我推荐的是：

@Query("SELECT * FROM club")
suspend fun getAll(): LiveData<List<Club>>

改为

DAO:

@Query("SELECT * FROM club")
fun getAll(): (Any rxJava object) Flowable<List<Club>>

存储库：

fun getAllClubs() : LiveData<List<Club>>{
   return LiveDataReactiveStreams.fromPublisher(dao.getAll()"do any transformation or 
   other functionality you want and then if using Maybe for example transform to 
   flowable by .toFlowable()")
}

视图模型：

val clubList : MediatorLiveData<List<Club>>()

fun setClubData(){
   clubList.addSource(repository.getAllClubs(), {
   clubList.value = it
   })
}

然后就可以在视图中像往常一样观察livedata对象了

希望我写的是可以理解的，如果不是我会努力让它变得更好:)

如何从 Room 获取 LiveData？

How to get LiveData from Room?

android

kotlin

android-room

android-livedata

android-viewmodel