写之前不得提下接口，接口的设计是这样的

@ActivityScope
public class RequestParam<T> {
    public HeaderEntity header;
    public T            body;

    @Inject
    public RequestParam(HeaderEntity header, T body) {
        this.header = header;
        this.params = body;
    }
}

简单说下，接口请求是application/json;，每个请求参数里面包含一个通用的 header里面有一些api、设备和token信息，通常这部分是定义到 Http请求头header里面的，可能我们的接口比较任性...

在header里面我们写一个OkHttp拦截器 然后加header 就可以了，请求体里面拦截器就无能为力了。所以我们用到了请求转换器。

我们在Retrofit的ApiService把 参数类req 定义为 body,通过自定义转换器给body加上header组装成 包含header和body 的完整对象param。
以一个登录接口为例，说明下。
定义在ApiService的方法

    @POST(LOGIN_URL)
    Observable<ResultWrapper<UserLoginResultDto>> login(@Body UserLoginSearchDto dto);

@ActivityScope
public class UserLoginSearchDto {
    public String  account;
    @NeedMD5
    public String  password;//md5 加密
    @Inject
    public UserLoginSearchDto() {}
}

    public void login(BaseActivity activity, UserLoginSearchDto dto, Subscriber<UserLoginResultDto> subscriber) {
        service.login(dto)
               .map(new GetResponseData<>())
               //持久化
               .doOnNext(user -> App.getApp().getAppComponent().userManager().persist(user))
               .compose(activity.bindUntilEvent(ActivityEvent.DESTROY))
               .compose(applySchedulers())
               .subscribe(subscriber);
    }

调用，可以看到，我们的参数UserLoginSearchDto并没有定义header信息。这时自定义转换器就上场了。Retrofit 的代理实现类会通过请求转化器会把我们在ApiService定义的@Body UserLoginSearchDto dto序列化为json。

public class RequestConvertFactory extends Converter.Factory {


    public static RequestConvertFactory create() {
        return create(new Gson());
    }

    public static RequestConvertFactory create(Gson gson) {
        return new RequestConvertFactory(gson);
    }

    private final Gson gson;

    private RequestConvertFactory(Gson gson) {
        if (gson == null) throw new NullPointerException("gson == null");
        this.gson = gson;
    }

    @Override
    public Converter<?, RequestBody> requestBodyConverter(
            Type type,
            Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
//RequestParam 就是包含 header 和 body 的完整参数类
        TypeToken<RequestParam<Type>> typeToken = new TypeToken<RequestParam<Type>>() {};
        TypeAdapter<RequestParam<Type>> adapter = gson.getAdapter(typeToken);
        return new MyGsonRequestBodyConverter<>(gson, adapter);
    }
}

public class MyGsonRequestBodyConverter<T> implements Converter<T, RequestBody> {
    private static final MediaType MEDIA_TYPE = MediaType.parse("application/json; charset=UTF-8");
    private static final Charset   UTF_8      = Charset.forName("UTF-8");

    private final Gson                         gson;
    private final TypeAdapter<RequestParam<T>> adapter;

    MyGsonRequestBodyConverter(Gson gson, TypeAdapter<RequestParam<T>> adapter) {
        this.gson = gson;
        this.adapter = adapter;
    }

    @Override
    public RequestBody convert(T value) throws IOException {
        Buffer buffer = new Buffer();
        Writer writer = new OutputStreamWriter(buffer.outputStream(), UTF_8);
        JsonWriter jsonWriter = gson.newJsonWriter(writer);
        //head头参数
        HeaderEntity header = App.getApp().getAppComponent().header();
        //实际的参数实体
        RequestParam<T> param = new RequestParam<>(header, value);
        //时间戳 md5
        header.refreshVkey();

        UserManager userManager = App.getApp().getAppComponent().userManager();

        boolean login = value instanceof UserLoginSearchDto;
        header.token = login ? null : userManager.getToken();
        header.userId = login ? 0 : userManager.getUserId();

        //处理 base64
        Class<?> paramClass = value.getClass();
        Field[] fields = paramClass.getFields();
        Field needMD5 = null;
        for (int i = 0; i < fields.length; i++) {
            Field field = fields[i];
            NeedMD5 annotation = field.getAnnotation(NeedMD5.class);
            if (annotation != null) {
                needMD5 = field;
                break;
            }
        }
        if (needMD5 != null) {
            try {
                needMD5.set(value, EncryptUtil.encryptMD5(String.valueOf(needMD5.get(value))));
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
        adapter.write(jsonWriter, param);
        jsonWriter.close();
        //把组装好的完整 param序列化
        return RequestBody.create(MEDIA_TYPE, buffer.readByteString());
    }
}

别忘了注册自定义转换器RequestConvertFactory.create

 Retrofit retrofit = new Retrofit
                .Builder()
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .addConverterFactory(RequestConvertFactory.create(gson))
                .addConverterFactory(GsonConverterFactory.create(gson))
        ...