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A Retrofit CallAdapter for Guava's ListenableFuture.
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/*
 * Copyright (C) 2016 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 retrofit2.adapter.guava;

import com.google.common.util.concurrent.AbstractFuture;
import com.google.common.util.concurrent.ListenableFuture;
import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import javax.annotation.Nullable;
import retrofit2.Call;
import retrofit2.CallAdapter;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;

/**
 * A {@linkplain CallAdapter.Factory call adapter} which creates Guava futures.
 *
 * Adding this class to {@link Retrofit} allows you to return {@link ListenableFuture} from
 * service methods.
 *
 * 

 * interface MyService {
 *   @GET("user/me")
 *   ListenableFuture<User> getUser()
 * }
 * 
 *
 * There are two configurations supported for the {@code ListenableFuture} type parameter:
 *
 * 
 *   Direct body (e.g., {@code ListenableFuture}) returns the deserialized body for 2XX
 *       responses, sets {@link retrofit2.HttpException HttpException} errors for non-2XX responses,
 *       and sets {@link IOException} for network errors.
 *   
Response wrapped body (e.g., {@code ListenableFuture>}) returns a {@link
 *       Response} object for all HTTP responses and sets {@link IOException} for network errors
 * 
 */
public final class GuavaCallAdapterFactory extends CallAdapter.Factory {
  public static GuavaCallAdapterFactory create() {
    return new GuavaCallAdapterFactory();
  }

  private GuavaCallAdapterFactory() {}

  @Override
  public @Nullable CallAdapter get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != ListenableFuture.class) {
      return null;
    }
    if (!(returnType instanceof ParameterizedType)) {
      throw new IllegalStateException(
          "ListenableFuture return type must be parameterized"
              + " as ListenableFuture or ListenableFuture");
    }
    Type innerType = getParameterUpperBound(0, (ParameterizedType) returnType);

    if (getRawType(innerType) != Response.class) {
      // Generic type is not Response. Use it for body-only adapter.
      return new BodyCallAdapter<>(innerType);
    }

    // Generic type is Response. Extract T and create the Response version of the adapter.
    if (!(innerType instanceof ParameterizedType)) {
      throw new IllegalStateException(
          "Response must be parameterized" + " as Response or Response");
    }
    Type responseType = getParameterUpperBound(0, (ParameterizedType) innerType);
    return new ResponseCallAdapter<>(responseType);
  }

  private static final class BodyCallAdapter implements CallAdapter> {
    private final Type responseType;

    BodyCallAdapter(Type responseType) {
      this.responseType = responseType;
    }

    @Override
    public Type responseType() {
      return responseType;
    }

    @Override
    public ListenableFuture adapt(final Call call) {
      CallCancelListenableFuture future = new CallCancelListenableFuture<>(call);

      call.enqueue(
          new Callback() {
            @Override
            public void onResponse(Call call, Response response) {
              if (response.isSuccessful()) {
                future.set(response.body());
              } else {
                future.setException(new HttpException(response));
              }
            }

            @Override
            public void onFailure(Call call, Throwable t) {
              future.setException(t);
            }
          });

      return future;
    }
  }

  private static final class ResponseCallAdapter
      implements CallAdapter>> {
    private final Type responseType;

    ResponseCallAdapter(Type responseType) {
      this.responseType = responseType;
    }

    @Override
    public Type responseType() {
      return responseType;
    }

    @Override
    public ListenableFuture> adapt(final Call call) {
      CallCancelListenableFuture> future = new CallCancelListenableFuture<>(call);

      call.enqueue(
          new Callback() {
            @Override
            public void onResponse(Call call, Response response) {
              future.set(response);
            }

            @Override
            public void onFailure(Call call, Throwable t) {
              future.setException(t);
            }
          });

      return future;
    }
  }

  private static final class CallCancelListenableFuture extends AbstractFuture {
    private final Call call;

    CallCancelListenableFuture(Call call) {
      this.call = call;
    }

    @Override
    public boolean set(@org.checkerframework.checker.nullness.qual.Nullable T value) {
      return super.set(value);
    }

    @Override
    public boolean setException(Throwable throwable) {
      return super.setException(throwable);
    }

    @Override
    protected void interruptTask() {
      call.cancel();
    }
  }
}