Retrofit 1.9源码学习

Retrofit的建立是通过建造者模式，可以自己设置组件，比如Log，Json转换器等，如果不设置，就用默认的。

@Override Client.Provider defaultClient() {
  final Client client;
  if (hasOkHttpOnClasspath()) {
    client = OkClientInstantiator.instantiate ();
  } else if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD ) {
    client = new AndroidApacheClient();
  } else {
    client = new UrlConnectionClient();
  }
  return new Client.Provider() {
    @Override public Client get () {
      return client;
    }
  };
}

@Override Executor defaultHttpExecutor() {
  return Executors.newCachedThreadPool(new ThreadFactory() {
    @Override public Thread newThread (final Runnable r) {
      return new Thread(new Runnable() {
        @Override public void run() {
          Process.setThreadPriority( THREAD_PRIORITY_BACKGROUND );
          r .run();
        }
      }, RestAdapter.IDLE_THREAD_NAME);
    }
  });
}

@Override Executor defaultCallbackExecutor() {
  return new MainThreadExecutor();
}

@Override RestAdapter.Log defaultLog() {
  return new AndroidLog("Retrofit") ;

}

在这里重点介绍下

final Executor httpExecutor ;

final Executor callbackExecutor;

httpExecuter在这里是用于网络请求的，callbackExecutor是用于向Android主线程传递消息，执行返回语句的。比如

/** Executor that runs tasks on Android's main thread. */
public final class MainThreadExecutor implements Executor {
  private final Handler handler = new Handler(Looper.getMainLooper()) ;

  @Override public void execute (Runnable r) {
    handler .post(r) ;
  }

}

到底如何执行，是在这里

Callback<?> callback = (Callback<?>) args[args. length - 1 ] ;
httpExecutor .execute( new CallbackRunnable(callback , callbackExecutor , errorHandler ) {
  @Override public ResponseWrapper obtainResponse () {
    return (ResponseWrapper) invokeRequest( interceptorTape , methodInfo , args ) ;
  }

});

abstract class CallbackRunnable< T > implements Runnable {
  private final Callback< T > callback ;
  private final Executor callbackExecutor ;
  private final ErrorHandler errorHandler ;

  CallbackRunnable(Callback< T > callback , Executor callbackExecutor , ErrorHandler errorHandler) {
    this . callback = callback ;
    this . callbackExecutor = callbackExecutor ;
    this . errorHandler = errorHandler ;
  }

  @SuppressWarnings ( "unchecked" )
  @Override public final void run () {
    try {
      final ResponseWrapper wrapper = obtainResponse() ;
      callbackExecutor .execute( new Runnable() {
        @Override public void run () {
          callback .success(( T ) wrapper . responseBody , wrapper . response ) ;
        }
      }) ;
    } catch (RetrofitError e) {
      Throwable cause = errorHandler .handleError(e) ;
      final RetrofitError handled = cause == e ? e : unexpectedError(e.getUrl() , cause) ;
      callbackExecutor .execute( new Runnable() {
        @Override public void run () {
          callback .failure( handled ) ;
        }
      }) ;
    }
  }

  public abstract ResponseWrapper obtainResponse() ;

httpExecutor先执行obtainResponse，再调用callbackExecutor，这样就能够异步请求网络数据，并在Android主线程返回，从而可以更新界面等操作了。