依赖

implementation 'com.squareup.retrofit2:retrofit:2.5.0'
implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.5.0'
implementation 'io.reactivex.rxjava2:rxjava:2.2.8'
implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'

Retrofit版本2.5.0

RxJava版本2.2.8

converter-gson：用于将来自服务端的json数据转换为Java对象

adapter-rxjava：用于实现Retrofit和RxJava的组合

rxandroid：用于使用AndroidSchedulers.mainThread()

示例

EyeKeyService.java

package com.bu_ish.retrofitrxjava;

import io.reactivex.Single;
import retrofit2.http.Field;
import retrofit2.http.FormUrlEncoded;
import retrofit2.http.POST;

public interface EyeKeyService {
    String BASE_URL = "http://api.eyekey.com/face/";
    String APP_ID = "f89ae61fd63d4a63842277e9144a6bd2";
    String APP_KEY = "af1cd33549c54b27ae24aeb041865da2";

    @FormUrlEncoded
    @POST("Check/checking")
    Single<FaceCheckingReply> checkFace(@Field("app_id") String appId, @Field("app_key") String appKey, @Field("url") String url);
}

MainActivity.java

package com.bu_ish.retrofitrxjava;

import android.app.ProgressDialog;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

import io.reactivex.SingleObserver;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.schedulers.Schedulers;
import retrofit2.Retrofit;
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;
import retrofit2.converter.gson.GsonConverterFactory;

public class MainActivity extends AppCompatActivity {
    private ProgressDialog pdHttpRequest;
    private EyeKeyService service;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        pdHttpRequest = new ProgressDialog(this);
        pdHttpRequest.setCancelable(false);
        findViewById(R.id.btnCheckFace).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String picUrl = "https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1554372634204&di=a4804907c80d38d4997291e0cb546f71&imgtype=0&src=http%3A%2F%2Fi1.sinaimg.cn%2Fent%2F2011%2F1012%2FU3593P28DT20111012165707.jpg";
                service.checkFace(EyeKeyService.APP_ID, EyeKeyService.APP_KEY, picUrl)
                        .subscribeOn(Schedulers.io())
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe(new SingleObserver<FaceCheckingReply>() {
                            @Override
                            public void onSubscribe(Disposable d) {
                                pdHttpRequest.show();
                            }

                            @Override
                            public void onSuccess(FaceCheckingReply faceCheckingReply) {
                                pdHttpRequest.dismiss();
                                String code = faceCheckingReply.getResCode();
                                if (code.equals("0000")) {
                                    String faceId = faceCheckingReply.getFaces().get(0).getFaceId();
                                    new AlertDialog.Builder(MainActivity.this).setTitle("Face Id").setMessage(faceId).show();
                                } else {
                                    new AlertDialog.Builder(MainActivity.this).setTitle(code).setMessage(faceCheckingReply.getMessage()).show();
                                }
                            }

                            @Override
                            public void one rror(Throwable e) {
                                pdHttpRequest.dismiss();
                                new AlertDialog.Builder(MainActivity.this).setTitle("Error").setMessage(e + "").show();
                            }
                        });
            }
        });
        service = new Retrofit.Builder()
                .baseUrl(EyeKeyService.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build()
                .create(EyeKeyService.class);
    }
}

其中的addConverterFactory(GsonConverterFactory.create())用于实现json转换为Java对象

若不添，则抛出如下异常

Caused by: java.lang.IllegalArgumentException: Could not locate ResponseBody converter for class com.bu_ish.retrofitrxjava.FaceCheckingReply.

addCallAdapterFactory(RxJava2CallAdapterFactory.create())用于实现将EyeKeyService中的函数类型设定为RxJava中的被观察者类型

若不添，则抛出如下异常

Caused by: java.lang.IllegalArgumentException: Could not locate call adapter for io.reactivex.Single<com.bu_ish.retrofitrxjava.FaceCheckingReply>.

subscribeOn(Schedulers.io())用于把网络请求切换至子线程

若不添，则抛出异常NetworkOnMainThreadException

observeOn(AndroidSchedulers.mainThread())用于指定在主线程中处理服务端的响应数据

参考博文：RxJava线程切换——ObserveOn和SubscribeOn的区别