Retrofit 源码分析，返回结果的转化

        目前Android最流行的http框架莫过于Retrofit + OkHttp的组合了，通过Retrofit我们可以直接将Http请求返回转化成RxJava的Observable, 简直是开发的一大利器！那么下面我们就分析一下Retrofit是怎么将OkHttp的Response转化成Observable的。

首先我们来看一下Retrofit最基本的构建代码：

Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        .client(okHttpClient)
        .build()

其中GsonConverterFactory是创建GsonResponseBodyConverter，将http请求结果通过Gson解析成实体类；而RxJava2CallAdapterFactory是创建RxJava2CallAdapter， 将返回结果转化成Observable等。那我们就从这两个地方下手。

• RxJava2CallAdapter的创建：

        主要是对returnType的分析，初始化RxJava2CallAdapter需要的构造参数。
        关于Java Type，请参考：https://blog.youkuaiyun.com/sinat_29581293/article/details/52227953

public 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 RxJava2CallAdapter(Void.class, scheduler, isAsync, false, true, false, false,
        false, true);
  }
  ...

  Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);
  Class<?> rawObservableType = getRawType(observableType);
  if (rawObservableType == Response.class) {
    ...
    responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
  } else if (rawObservableType == Result.class) {
    ...
    responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
    isResult = true;
  } else {
    responseType = observableType;
    isBody = true;
  }
  return new RxJava2CallAdapter(responseType, scheduler, isAsync, isResult, isBody, isFlowable,
      isSingle, isMaybe, false);
}

•  RxJava2CallAdapter中的转化： 

@Override public Object adapt(Call<R> call) {
 //通过RxJava的订阅来发起请求的执行，在CallEnqueueObservable和CallExecuteObservable中dispose方法中调用了call.cancel(), 因此只要取消了RxJava的订阅就直接取消了okhttp的请求。
  Observable<Response<R>> responseObservable = isAsync
      ? new CallEnqueueObservable<>(call)
      : new CallExecuteObservable<>(call);

 //对执行结果进行进一步处理，这里的observable就是我们最后返回的。
  Observable<?> observable;
  if (isResult) {
    //Observable<Result<Entity>> 类型的返回, Result持有OkHttp的Response
    observable = new ResultObservable<>(responseObservable);
  } else if (isBody) {
    //Observable<Entity> 类型的返回
    observable = new BodyObservable<>(responseObservable);
  } else {
    observable = responseObservable;
  }
  //订阅线程的管理，一般使用RxJava管理，这里为null
  if (scheduler != null) {
    observable = observable.subscribeOn(scheduler);
  }
  //Observable转化成其他的可订阅对象
  if (isFlowable) {
    return observable.toFlowable(BackpressureStrategy.LATEST);
  }
  if (isSingle) {
    return observable.singleOrError();
  }
  if (isMaybe) {
    return observable.singleElement();
  }
  if (isCompletable) {
    return observable.ignoreElements();
  }
  return observable;
}

CallEnqueueObservable中的执行 

@Override protected void subscribeActual(Observer<? super Response<T>> observer) {
  // Since Call is a one-shot type, clone it for each new observer.
  Call<T> call = originalCall.clone();
  CallCallback<T> callback = new CallCallback<>(call, observer);
  observer.onSubscribe(callback);
  call.enqueue(callback); //发起请求
}

OkHttpCall中的请求 

@Override public void enqueue(final Callback<T> callback) {
  //发起okhttp请求
  ...

  call.enqueue(new okhttp3.Callback() {
    @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
        throws IOException {
      Response<T> response;
      try {
        //Http返回结果解析，快要看到GsonConverterFactory登场了
        response = parseResponse(rawResponse);
      } catch (Throwable e) {
        callFailure(e);
        return;
      }
      callSuccess(response);
    }

    @Override public void onFailure(okhttp3.Call call, IOException e) {
      ...
    }

    private void callFailure(Throwable e) {
      ...
    }

    private void callSuccess(Response<T> response) {
      try {
        //CallEnqueueObservable中的callback
        callback.onResponse(OkHttpCall.this, response);
      } catch (Throwable t) {
        t.printStackTrace();
      }
    }
  });
}

Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
  ResponseBody rawBody = rawResponse.body();

  // Remove the body's source (the only stateful object) so we can pass the response along.
  rawResponse = rawResponse.newBuilder()
      .body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
      .build();
  //错误处理
  ...

  ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
  try {
    //解析http请求返回为具体的实体对象，在这里就会实际调用GsonResponseBodyConverter进行Json解析。
    T body = serviceMethod.toResponse(catchingBody);
    return Response.success(body, rawResponse);
  } catch (RuntimeException e) {
    ...
  }
}

接下来，我们看看GsonResponseBodyConverter是如何初始化到serviceMethod中去的。

private Converter<ResponseBody, T> createResponseConverter() {
  Annotation[] annotations = method.getAnnotations();
  try {
    //还是通过Retrofit类中的responseBodyConverter， 这里会获取method中的注解。
    //responseBodyConverter中会调用nextResponseBodyConverter
    return retrofit.responseBodyConverter(responseType, annotations);
  } catch (RuntimeException e) { // Wide exception range because factories are user code.
    throw methodError(e, "Unable to create converter for %s", responseType);
  }
}

 Retrofit类中的nextResponseBodyConverter，根据返回类型和注解匹配对应的Converter，在这里找到GsonConverterFactory并创建GsonResponseBodyConverter

public <T> Converter<ResponseBody, T> nextResponseBodyConverter(
    @Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
  ...

  int start = converterFactories.indexOf(skipPast) + 1;
  for (int i = start, count = converterFactories.size(); i < count; i++) {
    Converter<ResponseBody, ?> converter =
        converterFactories.get(i).responseBodyConverter(type, annotations, this);
    if (converter != null) {
      //noinspection unchecked
      return (Converter<ResponseBody, T>) converter;
    }
  }
  ...
}

 

 在加入GsonConverterFactory之前，会先加载一个BuiltInConverters工厂类，处理requestBodyConverter，来看看BuiltInConverters中responseBodyConverter的实现。

public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
    Retrofit retrofit) {
  if (type == ResponseBody.class) {
    return Utils.isAnnotationPresent(annotations, Streaming.class)
        ? StreamingResponseBodyConverter.INSTANCE
        : BufferingResponseBodyConverter.INSTANCE;
  }
  if (type == Void.class) {
    return VoidResponseBodyConverter.INSTANCE;
  }
  return null;
}

可以看到，如果想不通过GsonConverter直接返回OkHttp的原始ResponseBody，只需要添加一个@Streaming注解就可以了。
@Steaming
@GET
fun request()：Observable<ResponseBody>

 

到此，整个返回结果的转化流程分析结束，我们来理一下执行的流程：
当你通过发起一个请求时，通过动态代理，找到已经加载过的ServiceMethod， 调用ServiceMethod.callAdapter.adapt(OkHttpCall) ，实际就是调用最开始创建的RxJava2CallAdapter中的adapt，然后创建CallEnqueueObservable，通过RxJava的订阅，发起OkHttp的一个请求，然后通过GsonResponseBodyConverter解析好之后，再创建成返回的Observable对象。