Android 网络框架之Retrofit2使用详解及从源码中解析原理

http://blog.youkuaiyun.com/guiman/article/details/51480497
http://www.jianshu.com/p/097947afddaf

主要的类：
Retrofit:总类 负责各模块的组装
ServiceMethod:将注解转为http请求参数
CallAdapter:适配器类 转换为具体的Call的子类 执行网络请求操作
Converter:对返回的结果进行处理

二、调用步骤

        //创建Retrofit对象
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(API_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        //动态生成一个代理对象
        GitHub github = retrofit.create(GitHub.class);
        //生成一个OKHttpCall的代理对象
        final retrofit2.Call<ResponseBody> call = github.contributors("square", "retrofit");

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                try {
                    System.out.println("----->"+response.body().string());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
            }
        });

2.1 Retrofit.build
会创建：
callFactory = new OkHttpClient();
//用于回调到主线程 在android中为Handler
callbackExecutor = platform.defaultCallbackExecutor();
//创建ExecutorCallAdapterFactory对象
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

2.2 retrofit.create

public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override public Object invoke(Object proxy, Method method, Object... args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            //获取ServiceMethod 会对注解进行解析
            ServiceMethod serviceMethod = loadServiceMethod(method);
            //创建OkHttpCall 用于执行实际的网络请求
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            //将okHttpCall传入ExecutorCallAdapterFactory.ExecutorCallbackCall 并返回
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });
  }

loadServiceMethod会调用ServiceMethod.build
在其中会调用createCallAdapter获取ExecutorCallAdapterFactory的实例
调用createResponseConverter获取BuiltInConverters的实例

重点是创建OkHttpCall，并传入ExecutorCallAdapterFactory.ExecutorCallbackCall。 会在里面执行实际的网络请求

2.3 call.enqueue(new Callback<ResponseBody>(){})
call是通过Retrofit.create获得的ExecutorCallAdapterFactory.ExecutorCallbackCall的实例，
执行call.enqueue将执行

static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override public void enqueue(final Callback<T> callback) {
      if (callback == null) throw new NullPointerException("callback == null");

      delegate.enqueue(new Callback<T>() {
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
              } else {
                callback.onResponse(ExecutorCallbackCall.this, response);
              }
            }
          });
        }

delegate为create方法中传入的OkHttpCall实例，callbackExecutor为Retrofit.build中创建的Handler对象

2.4 返回结果
最终的执行在OKHttpCall.java的enqueue方法中

call.enqueue(new okhttp3.Callback() {
      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
          throws IOException {
        Response<T> response;
        try {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          callFailure(e);
          return;
        }
        callSuccess(response);
      }

调用parseResponse方法，实际执行Converter的responseBodyConverter方法解析返回结果，并最终使用Handler返回