How to get LiveData from Room?

问题

I'm trying to get LiveData from Room. So my RecycleView can have Live updates if anything in database is changed

I have tried with out LiveData and that works, but when i add LiveData that always shows this error.

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()

I googled and looked on this site for solution, but every on with this issue have used rxjava, rxandroid, rxkotlin or ArrayList. And for them solution is to replace ArrayList with List, and for RX to try coroutine. Well i am using Coroutine and List and still no progress.

This is my ClubDao

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

In Club i have this attributes

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 should be List, but i convert those to and from json with TypeConverters

TypeConverter is here

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)
    }
}

And my DB

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()
    }
}

In my fragment i need to observe all the clubs from database via ViewModel and Repository and send them to RecycleView

Now i am getting error:

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()

Does anyone know solution for this?

EDIT:

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
}

Top Level 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'
}

回答1:

As mentioned in the comments, remove suspend. When a method returns an observable, there is no reason to make it suspend since it just returns and object, does not run any query until it is observed.

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

Room's coroutines integration brings ability to return suspend values but when the value itself is asnyc, there is no reason to use it.

回答2:

I found a good solution to the problem with using rxJava with room and livedata. The approach is similar to using Retrofit with RxJava, using LiveDataReactiveStreams, which by default uses flowable.

In your approach i would recomend is:

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

changed to

DAO:

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

Repository:

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()")
}

ViewModel:

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

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

then you can observe the livedata object as usual in your view.

Hope what i wrote is understandable, if not i will try to make it better :)

来源：https://stackoverflow.com/questions/56833407/how-to-get-livedata-from-room