RxAndroid and Retrofit: Unable to create call adapter for io.reactivex.Observable<retrofit2.Response<okhttp3.ResponseBody>>

Question

I am trying use rxJava, rxAndroid, Retrofit2, and OkHTTP3 to download a file from a URL endpoint. My code is unable to create the call adapter for an "Observable< retrofit2.Response< okhttp3.ResponseBody>>". These methods are new to me so I believe I'm missing an important concept here. Any direction or points is greatly appreciated.

FATAL EXCEPTION: main Process: com.example.khe11e.rxdownloadfile, PID: 14130 java.lang.IllegalArgumentException: Unable to create call adapter for io.reactivex.Observable> for method RetrofitInterface.downloadFileByUrlRx at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:720) at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:234) at retrofit2.ServiceMethod$Builder.build(ServiceMethod.java:160) at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:166) at retrofit2.Retrofit$1.invoke(Retrofit.java:145) at java.lang.reflect.Proxy.invoke(Proxy.java:393) at $Proxy0.downloadFileByUrlRx(Unknown Source) at com.example.khe11e.rxdownloadfile.MainActivity.downloadImage(MainActivity.java:46) at com.example.khe11e.rxdownloadfile.MainActivity$1.onClick(MainActivity.java:39) at android.view.View.performClick(View.java:5207) at android.view.View$PerformClick.run(View.java:21168) at android.os.Handler.handleCallback(Handler.java:746) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5491) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) Caused by: java.lang.IllegalArgumentException: Could not locate call adapter for io.reactivex.Observable>. Tried: * retrofit2.adapter.rxjava.RxJavaCallAdapterFactory * retrofit2.ExecutorCallAdapterFactory at retrofit2.Retrofit.nextCallAdapter(Retrofit.java:237) at retrofit2.Retrofit.callAdapter(Retrofit.java:201) at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:232) ... 16 more

build.gradle:

compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
compile 'io.reactivex.rxjava2:rxjava:2.0.4'
compile 'com.squareup.retrofit2:retrofit:2.1.0'
compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'

RetrofitInterface.java:

package com.example.khe11e.rxdownloadfile;
import io.reactivex.Observable;
import okhttp3.ResponseBody;
import retrofit2.Call;
import retrofit2.Response;
import retrofit2.http.GET;
import retrofit2.http.Streaming;
import retrofit2.http.Url;

public interface RetrofitInterface {
    // Retrofit 2 GET request for rxjava
    @Streaming
    @GET
    Observable<Response<ResponseBody>> downloadFileByUrlRx(@Url String fileUrl);
}

MainActivity.java:

package com.example.khe11e.rxdownloadfile;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import java.io.File;
import io.reactivex.Observable;
import io.reactivex.ObservableEmitter;
import io.reactivex.ObservableOnSubscribe;
import io.reactivex.Observer;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Function;
import io.reactivex.schedulers.Schedulers;
import okhttp3.OkHttpClient;
import okhttp3.ResponseBody;
import okio.BufferedSink;
import okio.Okio;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.adapter.rxjava.RxJavaCallAdapterFactory;

public class MainActivity extends AppCompatActivity {

Button downloadImgBtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    downloadImgBtn = (Button) findViewById(R.id.downloadImgBtn);
    downloadImgBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            downloadImage();
        }
    });
}

public void downloadImage(){
    RetrofitInterface downloadService = createService(RetrofitInterface.class, "https://www.nasa.gov/");
    downloadService.downloadFileByUrlRx("sites/default/files/iss_1.jpg")
            .flatMap(processResponse())
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(handleResult());
}

public <T> T createService(Class<T> serviceClass, String baseUrl){
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(baseUrl)
            .client(new OkHttpClient.Builder().build())
            .addCallAdapterFactory(RxJavaCallAdapterFactory.create()).build();
    return retrofit.create(serviceClass);
}

public Function<Response<ResponseBody>, Observable<File>> processResponse(){
    return new Function<Response<ResponseBody>, Observable<File>>() {
        @Override
        public Observable<File> apply(Response<ResponseBody> responseBodyResponse) throws Exception {
            return saveToDiskRx(responseBodyResponse);
        }
    };
}

private Observable<File> saveToDiskRx(final Response<ResponseBody> response){
    return Observable.create(new ObservableOnSubscribe<File>() {
        @Override
        public void subscribe(ObservableEmitter<File> subscriber) throws Exception {
            String header = response.headers().get("Content-Disposition");
            String filename = header.replace("attachment; filename=", "");
            new File("/data/data/" + getPackageName() + "/images").mkdir();
            File destinationFile = new File("/data/data/" + getPackageName() + "/images/" + filename);

            BufferedSink bufferedSink = Okio.buffer(Okio.sink(destinationFile));
            bufferedSink.writeAll(response.body().source());
            bufferedSink.close();

            subscriber.onNext(destinationFile);
            subscriber.onComplete();
        }
    });
}

private Observer<File> handleResult(){
    return new Observer<File>() {
        @Override
        public void onSubscribe(Disposable d) {
            Log.d("OnSubscribe", "OnSubscribe");
        }

        @Override
        public void onNext(File file) {
            Log.d("OnNext", "File downloaded to " + file.getAbsolutePath());
        }

        @Override
        public void onError(Throwable e) {
            e.printStackTrace();
            Log.d("Error", "Error " + e.getMessage());
        }

        @Override
        public void onComplete() {
            Log.d("OnComplete", "onCompleted");
        }
    };
}
}

I've tried adding Call as mentioned here so it looks like:

Call<Observable<Response<ResponseBody>>> downloadFileByUrlRx(@Url String fileUrl);

however this causes issues with the flatMap function as it cannot find symbol method flatMap(Function< Response< ResponseBody>,Observable< File>>).

Answer 1

You are using RxJava1 adapter for Retrofit, replace it with RxJava2 variant:

//compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'
compile 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'

UPDATE

Starting with Retrofit version 2.2.0 there is a first-party call adapter for RxJava2:

compile 'com.squareup.retrofit2:retrofit:2.2.0'
compile 'com.squareup.retrofit2:adapter-rxjava2:2.2.0'