retrofit的实现原理(一)

retrofit框架实现的这么巧妙,虽然我们不需要再造一个轮子,但研究下轮子的实现还是很有帮助的.

retrofit有几个关键的地方.

1.用户自定义的接口和接口方法.(由动态代理创建对象.)

2.converter转换器.(把response转换为一个具体的对象)

3.注解的使用.

 

让我们跟随Api来看吧.

RestAdapter restAdapter = new RestAdapter.Builder().setEndpoint(API_URL).build(); 

　　build()其内部实现是这样的:

 public RestAdapter build() {
      if (endpoint == null) {
        throw new IllegalArgumentException("Endpoint may not be null.");
      }
      ensureSaneDefaults();
      return new RestAdapter(endpoint, clientProvider, httpExecutor, callbackExecutor,
          requestInterceptor, converter, profiler, errorHandler, log, logLevel);
    }

　　当用户没有设置自定义的converter,client, httpExecutor(http访问执行的线程-->只对异步的retrofit有效.), callBackExecutor(异步的callBack执行的线程), errorHandler, log, RequestInterceptor的时候,就会使用retrofit默认的配置.

   private void ensureSaneDefaults() {
      if (converter == null) {
        converter = Platform.get().defaultConverter();
      }
      if (clientProvider == null) {
        clientProvider = Platform.get().defaultClient();
      }
      if (httpExecutor == null) {
        httpExecutor = Platform.get().defaultHttpExecutor();
      }
      if (callbackExecutor == null) {
        callbackExecutor = Platform.get().defaultCallbackExecutor();
      }
      if (errorHandler == null) {
        errorHandler = ErrorHandler.DEFAULT;
      }
      if (log == null) {
        log = Platform.get().defaultLog();
      }
      if (requestInterceptor == null) {
        requestInterceptor = RequestInterceptor.NONE;
      }
    }
  }

　　Platform.java

  private static final Platform PLATFORM = findPlatform();

  static Platform get() {
    return PLATFORM;
  }

  private static Platform findPlatform() {
    try {
      Class.forName("android.os.Build");//只要android.os.Build的class可以正常找到,证明是在android平台
      if (Build.VERSION.SDK_INT != 0) {
        return new Android();
      }
    } catch (ClassNotFoundException ignored) {
    }

    if (System.getProperty("com.google.appengine.runtime.version") != null) {
      return new AppEngine();   //google的app Engine平台
    }

    return new Base();
  }　　

retrofit的Android类继承了Platform, 根据android的特性对配置项做了处理.

private static class Android extends Platform {
    @Override Converter defaultConverter() {
      return new GsonConverter(new Gson());  //默认的转换器是Gson
    }

    @Override Client.Provider defaultClient() {
      final Client client;
      if (hasOkHttpOnClasspath()) {  //有okhttp的路径就使用 Okhttp
        client = OkClientInstantiator.instantiate();
      } else if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {
        client = new AndroidApacheClient(); //没有okhttp,且版本小于2.3 使用HttpClient
      } else {
        client = new UrlConnectionClient();  //没有okhttp,且版本大于等于2.3 使用urlConnection.
      }
      return new Client.Provider() {
        @Override public Client get() {
          return client;
        }
      };
    }

    @Override Executor defaultHttpExecutor() {   //网络访问执行的线程.
      return Executors.newCachedThreadPool(new ThreadFactory() {  //一个cached的线程池.可以复用老线程且线程长时间不用会自动回收. 线程池中线程不够会生成新线程.
        @Override public Thread newThread(final Runnable r) {
          return new Thread(new Runnable() {
            @Override public void run() {
              Process.setThreadPriority(THREAD_PRIORITY_BACKGROUND);    //设置线程的优先级 为最低
              r.run();
            }
          }, RestAdapter.IDLE_THREAD_NAME);
        }
      });
    }

    @Override Executor defaultCallbackExecutor() { //异步执行的线程.
      return new MainThreadExecutor();
    }

    @Override RestAdapter.Log defaultLog() {  //通过Log.d("Retrofit",String)打印log
      return new AndroidLog("Retrofit");
    }
  }

private static boolean hasOkHttpOnClasspath() {
  try {
    Class.forName("com.squareup.okhttp.OkHttpClient"); //是否可以找到OkHttpClient类.
    return true;
  } catch (ClassNotFoundException ignored) {
  }
  return false;
}

可以发现上面默认的Http的Executor是一个线程池.

而CallBack的Executor是在主线程执行的. 由绑定MainLooper的Handler提交到主线程执行.

public final class MainThreadExecutor implements Executor {
  private final Handler handler = new Handler(Looper.getMainLooper()); //关联主线程的Handler

  @Override public void execute(Runnable r) {
    handler.post(r);                                 //提交到主线程执行
  }
}

public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

  private static Message getPostMessage(Runnable r) {   //把runnable封装到Message中.
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }