retrofit2原理学习（未完待续）

最简单的okhttp+retrofit2使用demo

retrofit
= new
Retrofit.Builder()

        .baseUrl("https://api.douban.com/v2/")

        .build();
service=
retrofit.create(RetrofitService.class);
Call<Book> call =  service.getSearchBook("金瓶梅",null,
0, 1);
call.enqueue(new
Callback<Book>(){

    @Override

    public void onResponse(Call<Book>call,
Response<Book> response) {

        text_tv.setText(response.body()+"");

    }

    @Override

    public void onFailure(Call<Book>call,
Throwable t) {

    }

});
这里retrofit使用了build模式，点进去可以看到

public Builder() {
  this(Platform.get());
}

Builder函数

Builder(Platform platform) {
  this.platform = platform;
  // 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());
}

这里使用了一个Platform.get()函数，查阅资料可以得知，这个Platform.get()是获得平台信息的函数（平台可以是android或者是java8）。除此之外还有一个converterFactories.add(new BuiltInConverters());这个函数的主要作用是添加了一个Converter.Factory工厂类。在retrofit的build函数中有解释

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<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
  adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

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

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

没理解错的话，应该是一个把返回结果转换的工厂。另外那个CallAdapter.Factory应该是添加Call或者Rxjava中Observer的类，默认情况的话，应该是Call。

而这个默认添加的BuiltInConverters再请求情况的状态下，主要去除future I/O，其实啥也没做就是吧response又吐出来了。

回到retrofit的build，这里这个baseUrl是必须的，不然会报错，源码里是这样的

public Builder baseUrl(String baseUrl) {
  checkNotNull(baseUrl, "baseUrl == null");
  HttpUrl httpUrl = HttpUrl.parse(baseUrl);
  if (httpUrl == null) {
    throw new IllegalArgumentException("Illegal URL: " + baseUrl);
  }
  return baseUrl(httpUrl);
}

这样的话，一个retrofit对象就新建好了。

然后就到了新建接口的对象service了，源码里是这样的

@SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
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);
          }
          ServiceMethod serviceMethod = loadServiceMethod(method);
          OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
          return serviceMethod.callAdapter.adapt(okHttpCall);
        }
      });
}

首先这个validateEagerly来自于Retrofit对象的自定义。好像预设做了点什么，实际测了一下好像并没有什么区别。

讲真这里这个原理我不是很懂。好像利用了反射和代理模式相关的知识，需要单独学一下，暂时先放一下。

这里的SetviceMethod是retrofit的主要功能实现载体。

ServiceMethod:从接口方法到Http请求(OKHttp的Call)的转换器,也是使用的构造器模式。