綜述

Retrofit到一個(gè)實(shí)際請(qǐng)求/響應(yīng)的過程包括

• 創(chuàng)建ServiceMethod，調(diào)用ServiceMethod.build()
• 調(diào)用OkHttpCall(ServiceMethod,args)創(chuàng)建OkHttpCall對(duì)象，
• 調(diào)用 CallAdapter.adapt(okHttpCall)創(chuàng)建關(guān)聯(lián)執(zhí)行器Executor和Call的應(yīng)用層Call類，Android默認(rèn)為ExecutorCallbackCall

源碼研究

• 創(chuàng)建ServiceMethod

ServiceMethod.build()

  public ServiceMethod build() {
      //通過Retrofit.nextCallAdapter獲取callAdapter，有自定義就用自定義，沒有就用默認(rèn)ExecutorCallAdapterFactory
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      //返回類型驗(yàn)證
      if (responseType == Response.class || responseType == okhttp3.Response.class) {
        throw methodError("'"
            + Utils.getRawType(responseType).getName()
            + "' is not a valid response body type. Did you mean ResponseBody?");
      }
      //通過Retrofit.nextResponseBodyConverter處理轉(zhuǎn)換異常,獲得自定義轉(zhuǎn)換器
      responseConverter = createResponseConverter();
      //解析自定義的注解成URL組件
      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }

      //錯(cuò)誤處理
      ...
      //構(gòu)建參數(shù)替換的輔助數(shù)組parameterHandlers
      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0; p < parameterCount; p++) {
        Type parameterType = parameterTypes[p];
        if (Utils.hasUnresolvableType(parameterType)) {
          throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
              parameterType);
        }

        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
        if (parameterAnnotations == null) {
          throw parameterError(p, "No Retrofit annotation found.");
        }
        
        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }
      //錯(cuò)誤處理
     ...
      return new ServiceMethod<>(this);
    }

• 用戶調(diào)用enqueue方法發(fā)起異步請(qǐng)求

ExecutorCallbackCall.enqueue()

    @Override public void enqueue(final Callback<T> callback) {
      checkNotNull(callback, "callback == null");
      //調(diào)用OkhttpCall的enqueue方法，Executor執(zhí)行響應(yīng)結(jié)果
      delegate.enqueue(new Callback<T>() {
        
        }
      });
    }

OkHttpCall. enqueue

 @Override public void enqueue(final Callback<T> callback) {
    checkNotNull(callback, "callback == null");

    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
          ...
          //這里創(chuàng)建請(qǐng)求報(bào)文獲得realCall
          call = rawCall = createRawCall();
           ...
    }
    //realCall發(fā)起異步請(qǐng)求
    call.enqueue(new okhttp3.Callback() {
           ...
          //轉(zhuǎn)換響應(yīng)報(bào)文
          response = parseResponse(rawResponse);
          ...
   
    });
  }

createRawCall

  private okhttp3.Call createRawCall() throws IOException {
    //創(chuàng)建request報(bào)文
    Request request = serviceMethod.toRequest(args);
    //獲得okhttp.realCall
    okhttp3.Call call = serviceMethod.callFactory.newCall(request);
    if (call == null) {
      throw new NullPointerException("Call.Factory returned null.");
    }
    return call;
  }

serviceMethod.toRequest(args)

  Request toRequest(@Nullable Object... args) throws IOException {
    RequestBuilder requestBuilder = new RequestBuilder(httpMethod, baseUrl, relativeUrl, headers,
        contentType, hasBody, isFormEncoded, isMultipart);
    ...
    @SuppressWarnings("unchecked") // It is an error to invoke a method with the wrong arg types.
    ParameterHandler<Object>[] handlers = (ParameterHandler<Object>[]) parameterHandlers;
    //根據(jù)先前的parameterHandlers進(jìn)行參數(shù)轉(zhuǎn)換
    for (int p = 0; p < argumentCount; p++) {
      handlers[p].apply(requestBuilder, args[p]);
    }
    //創(chuàng)建request報(bào)文
    return requestBuilder.build();
  }

parseResponse

 Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();
    //異常處理，200-299成功狀態(tài)碼，204和205沒有響應(yīng)body
   ...
      //serviceMethod.toResponse用戶轉(zhuǎn)換器轉(zhuǎn)換
      T body = serviceMethod.toResponse(catchingBody);
      return Response.success(body, rawResponse);

  }

總結(jié)

通過梳理大體了解的Retrofit的整體結(jié)構(gòu)和流程，當(dāng)然還有很多的細(xì)節(jié)沒有討論，需要時(shí)可以查看相應(yīng)的源碼。
Retrofit源碼本身并不復(fù)雜，但是Retrofit的架構(gòu)設(shè)計(jì)，解耦所透露的架構(gòu)經(jīng)驗(yàn)，思想，規(guī)范等是非常值得我們學(xué)習(xí)和參照設(shè)計(jì)的。