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RxJava 3 adapter for Retrofit 2
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/*
 * Copyright (C) 2015 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package hu.akarnokd.rxjava3.retrofit;

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.*;

import io.reactivex.rxjava3.annotations.Nullable;
import io.reactivex.rxjava3.core.*;
import retrofit2.*;
import retrofit2.HttpException;

/**
 * A {@linkplain retrofit2.CallAdapter.Factory call adapter} which uses RxJava 3 for creating observables.
 * 
 * Adding this class to {@link Retrofit} allows you to return an {@link Observable},
 * {@link Flowable}, {@link Single}, {@link Completable} or {@link Maybe} from service methods.
 * 

 * interface MyService {
 *   @GET("user/me")
 *   Observable<User> getUser()
 * }
 * 
 * There are three configurations supported for the {@code Observable}, {@code Flowable},
 * {@code Single}, {@link Completable} and {@code Maybe} type parameter:
 * 
 * Direct body (e.g., {@code Observable}) calls {@code onNext} with the deserialized body
 * for 2XX responses and calls {@code onError} with {@link HttpException} for non-2XX responses and
 * {@link IOException} for network errors.
 * Response wrapped body (e.g., {@code Observable>}) calls {@code onNext}
 * with a {@link Response} object for all HTTP responses and calls {@code onError} with
 * {@link IOException} for network errors
 * Result wrapped body (e.g., {@code Observable>}) calls {@code onNext} with a
 * {@link Result} object for all HTTP responses and errors.
 * 
 */
public final class RxJava3CallAdapterFactory extends CallAdapter.Factory {
  /**
   * Returns an instance which creates synchronous observables that do not operate on any scheduler
   * by default.
   * @return the new adapter instance
   */
  public static RxJava3CallAdapterFactory create() {
    return new RxJava3CallAdapterFactory(null, false);
  }

  /**
   * Returns an instance which creates asynchronous observables. Applying
   * {@link Observable#subscribeOn} has no effect on stream types created by this factory.
   * @return the new adapter instance
   */
  public static RxJava3CallAdapterFactory createAsync() {
    return new RxJava3CallAdapterFactory(null, true);
  }

  /**
   * Returns an instance which creates synchronous observables that
   * {@linkplain Observable#subscribeOn(Scheduler) subscribe on} {@code scheduler} by default.
   * @param scheduler the scheduler to run the network operations on
   * @return the new adapter instance
   */
  public static RxJava3CallAdapterFactory createWithScheduler(Scheduler scheduler) {
    if (scheduler == null) throw new NullPointerException("scheduler == null");
    return new RxJava3CallAdapterFactory(scheduler, false);
  }

  private final @Nullable Scheduler scheduler;
  private final boolean isAsync;

  private RxJava3CallAdapterFactory(@Nullable Scheduler scheduler, boolean isAsync) {
    this.scheduler = scheduler;
    this.isAsync = isAsync;
  }

  @Override public @Nullable CallAdapter get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    Class rawType = getRawType(returnType);

    if (rawType == Completable.class) {
      // Completable is not parameterized (which is what the rest of this method deals with) so it
      // can only be created with a single configuration.
      return new RxJava3CallAdapter<>(Void.class, scheduler, isAsync, false, true, false, false,
          false, true);
    }

    boolean isFlowable = rawType == Flowable.class;
    boolean isSingle = rawType == Single.class;
    boolean isMaybe = rawType == Maybe.class;
    if (rawType != Observable.class && !isFlowable && !isSingle && !isMaybe) {
      return null;
    }

    boolean isResult = false;
    boolean isBody = false;
    Type responseType;
    if (!(returnType instanceof ParameterizedType)) {
      String name = isFlowable ? "Flowable"
          : isSingle ? "Single"
          : isMaybe ? "Maybe" : "Observable";
      throw new IllegalStateException(name + " return type must be parameterized"
          + " as " + name + " or " + name + "");
    }

    Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);
    Class rawObservableType = getRawType(observableType);
    if (rawObservableType == Response.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException("Response must be parameterized"
            + " as Response or Response");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
    } else if (rawObservableType == Result.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException("Result must be parameterized"
            + " as Result or Result");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
      isResult = true;
    } else {
      responseType = observableType;
      isBody = true;
    }

    return new RxJava3CallAdapter<>(responseType, scheduler, isAsync, isResult, isBody, isFlowable,
        isSingle, isMaybe, false);
  }
}