RTFSC系列之Retrofit

摘要：从开始，的底层实现已替换成了，也因此成为了平台上主流的网络框架之一。，负责把接口方法注解转换为表示包括等，负责把转换为，并且包装了的接口调用。负责把转为，使其具备线程调度能力。其内部接口负责把请求响应结果转换可识别的数据格式。

Retrofit是Square开源的一个适用于Android和Java的类型安全的HTTP网络框架。其本质是对OKHttp的进一步封装，通过接口的方式进行网络请求，结合了注解、动态代理等技术实现模块解耦，简化了网络请求的上层逻辑。从Android4.4开始，http的底层实现已替换成了OKHttp，Retrofit也因此成为了Android平台上主流的网络框架之一。

http://square.github.io/retro...

Step1：声明Retrofit API请求接口

    public interface GitHub {
        @GET("/repos/{owner}/{repo}/contributors")
        Call> contributors(
            @Path("owner") String owner,
            @Path("repo") String repo);
    }

请求类型：以@GET/POST/PUT/DELETE注解声明

请求相对URL：以请求方法注解内的Value值表示

请求参数：以接口方法@Path参数表示

Step2：构建Retrofit对象并创建API接口实例

    // Create a very simple REST adapter which points the GitHub API.
    Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("https://api.github.com")
        .addConverterFactory(GsonConverterFactory.create())
        .build();

    // Create an instance of our GitHub API interface.
    GitHub github = retrofit.create(GitHub.class);

通过baseUrl接口声明接口请求域名；

配置对象的序列化和反序列化转换工厂；

Step3：通过API接口方法创建Call实例并发起请求

    // Create a call instance for looking up Retrofit contributors.
    Call> call = github.contributors("square", "retrofit");

    // Fetch and print a list of the contributors to the library.
    List contributors = call.execute().body();

Retrofit的实现就是这么简单优雅，业务层只需要像调用普通接口一样即可完成网络请求。

首先要构建一个Retrofit实例，其作用是把接口方法注解转换为OKHttp请求。

Retrofit.Builder.build

public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }

      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List converterFactories =
          new ArrayList<>(1 + this.converterFactories.size());

      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
      converterFactories.addAll(this.converterFactories);

      return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
    }
  }

callFactory，生产并处理请求的工厂（默认创建OKHttpClient）

callbackExecutor，用以处理callback线程调度（Android默认回调到主线程）

callAdapterFactories，用以生产CallAdapter以处理Response类型转换（默认使用ExecutorCallAdapterFactory）

converterFactories，用以生产Converter处理http请求响应的数据转换（默认使用BuiltInConverters）

Retrofit实例化完成，创建API接口动态代理实现。

Retrofit.create

public  T create(final Class 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, @Nullable 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);
            }
            ServiceMethod serviceMethod =
                (ServiceMethod) loadServiceMethod(method);
            OkHttpCall