简介

Retrofit是Square公司开发的一款针对Android网络请求的框架，Retrofit2底层基于OkHttp实现的。
在gradle中添加

compile 'com.squareup.retrofit2:retrofit:2.3.0'    compile 'com.squareup.retrofit2:converter-gson:2.3.0'

实现简单的POST请求

1. 首先创建业务接口，代码如下

public interface LoginService {    @POST("user/auth")    Call login(@Query("username") String username, @Query("password") String password);}

这里对几种类型的注解做下说明：
@GET 和@POST分别对应get和post请求
@Query代表请求参数，另外还有@Body，@Field，@Part，@Path，@Header等注解

2.完成LoginService的配置

Retrofit retrofit = new Retrofit.Builder()                .baseUrl(baseUrl)                .addConverterFactory(GsonConverterFactory.create())                .build();   LoginService loginService = retrofit.create(LoginService.class);

3.调用接口方法，完成请求

Call<User> call = loginService.login("nanguangtailang","123");        call.enqueue(new Callback<User>() {            @Override            public void onResponse(Call<User> call, Response<User> response) {                Log.d("d",response.body().toString());            }           @Override            public void onFailure(Call<User> call, Throwable t) {                Log.d("f",t.getMessage());            }        });

到这里我们的第一次简单使用Retrofit之旅已经结束了，接下来我们看看Retrofit关键实现的源码(主要关注第二步中出现的类和方法)
在创建Retrofit实例使用了builder模式，Builder是Retrofit中的一个嵌套类，代码如下

public static final class Builder {    private final Platform platform;    private @Nullable okhttp3.Call.Factory callFactory;    private HttpUrl baseUrl;    private final List converterFactories = new ArrayList<>();    private final List adapterFactories = new ArrayList<>();    private @Nullable Executor callbackExecutor;    private boolean validateEagerly;    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());    }    public Builder() {      this(Platform.get());    }

可以看到这个类中的几个重要属性 Platform、okhttp3.Call.Factory、List< Converter.Factory >、List< CallAdapter.Factory > 、Executor
platform 代表我们目前使用的平台，Android、iOS等 ，okhttp3.Call.Factory 是我们传入的 okhttpClient,作用是代理Retrofit发出http请求

接下来我们看看

 LoginService loginService = retrofit.create(LoginService.class);

这行代码执行时发生了什么
这是create()方法的内部实现

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