Usando Paging Library 3 com LiveData de Java

Quero usar o Paging Library 3 em conjunto com o LiveData do Java. A documentação explica como usar, Guava Futures, RxJava Singles and Kotlin Coroutinesmas não como usar com o LiveDataJava. Provavelmente posso As várias PagingSourceclasses fornecem load, loadSinglee loadFuture.

O loadexemplo em Kotlin carrega dados usando retrofit com uma co-rotina e pode, portanto, retornar um LoadResultobjeto. Mas com LiveData, eu preciso fazer uma chamada assíncrona de retrofit e definir o valor no LiveDataobjeto. Não existe um loadmétodo de utilitário separado para o LiveDataque existe para RxJavae Guava. Então, como eu posso conseguir isso usando LiveDataa partir de Java?

package org.metabrainz.mobile.data.repository;

import androidx.annotation.NonNull;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.paging.PagingSource;

import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import org.metabrainz.mobile.data.sources.Constants;
import org.metabrainz.mobile.data.sources.api.MusicBrainzServiceGenerator;
import org.metabrainz.mobile.data.sources.api.SearchService;
import org.metabrainz.mobile.data.sources.api.entities.mbentity.MBEntity;
import org.metabrainz.mobile.data.sources.api.entities.mbentity.MBEntityType;
import org.metabrainz.mobile.presentation.features.adapters.ResultItem;
import org.metabrainz.mobile.presentation.features.adapters.ResultItemUtils;

import java.util.ArrayList;
import java.util.List;

import kotlin.coroutines.Continuation;
import okhttp3.ResponseBody;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class SearchPagingSource extends PagingSource<Integer, ResultItem> {

    @NonNull
    private final static SearchService service = MusicBrainzServiceGenerator
            .createService(SearchService.class, true);
    @NonNull
    private MBEntityType entity;
    @NonNull
    private String searchTerm;

    public SearchPagingSource(@NonNull MBEntityType entity, @NonNull String searchTerm) {
        this.entity = entity;
        this.searchTerm = searchTerm;
    }

    @NotNull
    @Override
    public LiveData<LoadResult<Integer, ResultItem>> load(@NotNull LoadParams<Integer> loadParams,
                         @NotNull Continuation<? super LoadResult<Integer, ResultItem>> continuation) {

        Integer pageSize = loadParams.getLoadSize();
        Integer offset = loadParams.getKey() == null ? 0 : loadParams.getKey();

        MutableLiveData<LoadResult<Integer, ResultItem>> resultsLiveData = new MutableLiveData<>();
        service.searchEntity(entity.name, searchTerm, pageSize.toString(),
                String.valueOf(offset * pageSize))
                .enqueue(new Callback<ResponseBody>() {
                    @Override
                    public void onResponse(@NonNull Call<ResponseBody> call,
                                           @NonNull Response<ResponseBody> response) {
                        try {
                            List<ResultItem> data = ResultItemUtils
                                    .getJSONResponseAsResultItemList(response.body().string(), entity);

                            LoadResult.Page<Integer, ResultItem> loadResult
                                    = new LoadResult.Page<>(data, Math.max(0, offset - pageSize),
                                    offset + pageSize, LoadResult.Page.COUNT_UNDEFINED,
                                    LoadResult.Page.COUNT_UNDEFINED);
                            resultsLiveData.setValue(loadResult);
                        } catch (Exception e) {
                            e.printStackTrace();
                            LoadResult.Error<Integer, ResultItem> error = new LoadResult.Error<>(e);
                            resultsLiveData.setValue(error);
                        }
                    }

                    @Override
                    public void onFailure(@NonNull Call<ResponseBody> call, @NonNull Throwable t) {

                    }
                });
        return resultsLiveData;
    }


}

No entanto, isso trava no tempo de execução

org.metabrainz.android E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.metabrainz.android, PID: 2222
    java.lang.ClassCastException: androidx.lifecycle.MutableLiveData cannot be cast to androidx.paging.PagingSource$LoadResult at androidx.paging.PageFetcherSnapshot.doInitialLoad(PageFetcherSnapshot.kt:302) at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invokeSuspend(PageFetcherSnapshot.kt:149) at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invoke(Unknown Source:10) at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invokeSuspend(CancelableChannelFlow.kt:35) at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invoke(Unknown Source:10) at kotlinx.coroutines.flow.ChannelFlowBuilder.collectTo$suspendImpl(Builders.kt:327)
        at kotlinx.coroutines.flow.ChannelFlowBuilder.collectTo(Unknown Source:0)
        at kotlinx.coroutines.flow.internal.ChannelFlow$collectToFun$1.invokeSuspend(ChannelFlow.kt:33)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
        at kotlinx.coroutines.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:321)
        at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:26)
        at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109)
        at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:158)
        at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch(Builders.common.kt:54)
        at kotlinx.coroutines.BuildersKt.launch(Unknown Source:1)
        at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch$default(Builders.common.kt:47) at kotlinx.coroutines.BuildersKt.launch$default(Unknown Source:1)
        at androidx.lifecycle.BlockRunner.maybeRun(CoroutineLiveData.kt:174)
        at androidx.lifecycle.CoroutineLiveData.onActive(CoroutineLiveData.kt:240)
        at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:437) at androidx.lifecycle.LiveData$LifecycleBoundObserver.onStateChanged(LiveData.java:395)
        at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361) at androidx.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:300) at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:339) at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:145) at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:131) at androidx.lifecycle.ReportFragment.dispatch(ReportFragment.java:68) at androidx.lifecycle.ReportFragment.dispatch(ReportFragment.java:144) at androidx.lifecycle.ReportFragment.onStart(ReportFragment.java:109) at android.app.Fragment.performStart(Fragment.java:2637) at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1312) at android.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1549) at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1611) at android.app.FragmentManagerImpl.dispatchMoveToState(FragmentManager.java:3039) at android.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:2996) at android.app.FragmentController.dispatchStart(FragmentController.java:189) at android.app.Activity.performStart(Activity.java:7007) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2867) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2986) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1671)
        at android.os.Handler.dispatchMessage(Handler.java:108)
        at android.os.Looper.loop(Looper.java:206)
        at android.app.ActivityThread.main(ActivityThread.java:6784)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:852)

Eu abri uma solicitação de recurso para enviar uma fonte de paginação LiveData com a biblioteca de paginação. Recebi a seguinte resposta dos desenvolvedores do Google.

As amostras de Java Guava em d.android.com são para Guava + LiveData, no equivalente da corrotina usamos: ListenableFuture da Guava como uma primitiva assíncrona que retorna um único resultado (equivalente a uma Coroutine ou RxJava Single) e LiveData para vários resultados / fluxo de resultados (equivalente a Fluxo de Kotlin ou RxJava observável).

Fui recomendado para escrever algo nas linhas do seguinte trecho de código I se eu quisesse usar o LiveData.

abstract class SearchPagingSource extends RxPagingSource<Integer, ResultItem>() {

    @NotNull
    public abstract LiveData<LoadResult<Integer, ResultItem>> loadLiveData(params: LoadParams<Key>);

    @NotNull
    @Override
    public Single<LoadResult<Integer, ResultItem>> loadSingle(@NotNull LoadParams<Integer> loadParams) {
        return loadLiveData(params).toRxJavaSingle(); // You must implement this bit!
    }
}

O LiveDatausado no snippet acima deve ser umSingleLiveEvent

PS: Os desenvolvedores do Google estão abertos para reconsiderar sua posição sobre o envio disso na própria biblioteca se mais desenvolvedores solicitarem. O tíquete do rastreador de problemas do Google relevante é este .