前言

Retrofit是一个RESTful的Http网络请求框架的封装。注意这里并没有说它是网络请求框架，主要原因在于网络请求的工作并不是Retrofit来完成的。

Retrofit 2.0开始内置OkHttp，前者专注于接口的封装，后者专注于网络请求的高效，二者分工协作。

使用步骤

1、 添加Retrofit库的依赖

implementation 'com.squareup.retrofit2:retrofit:2.3.0'
implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'
implementation 'com.squareup.okhttp3:logging-interceptor:3.4.1'

2、创建 用于描述网络请求 的接口

public interface ApiService {

    String HOST = "http://www.wanandroid.com";

    @GET("banner/json")
    Observable<BannerEntity> getBannerList();

    @GET("article/list/{page}/json")
    Observable<HomeEntity> getHomePageList(@Path("page") int page);
}

3、创建Retrofit实例

public class RetrofitHelper {

    private ApiService apiService;
    private static final int DEFAULT_TIMEOUT = 20;
    private volatile static RetrofitHelper instance = null;

    private Retrofit.Builder builder;

    public static RetrofitHelper getInstance() {
        if (instance == null) {
            synchronized (RetrofitHelper.class) {
                if (instance == null)
                    instance = new RetrofitHelper();
            }
        }
        return instance;
    }

    private RetrofitHelper() {
        builder = new Retrofit.Builder();
    }


    private OkHttpClient getClient() {
        String PATH_DATA = BaseApplication.getInstance().getCacheDir().getAbsolutePath() + File.separator + "data";
        String PATH_CACHE = PATH_DATA + "/NetCache";
        File cacheFile = new File(PATH_CACHE);
        Cache cache = new Cache(cacheFile, 1024 * 1024 * 50);

        OkHttpClient.Builder builder = new OkHttpClient.Builder();
        builder.connectTimeout(DEFAULT_TIMEOUT, TimeUnit.SECONDS);
        builder.addNetworkInterceptor(getInterceptor());
        builder.addInterceptor(getInterceptor());
        loggingInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
        builder.addInterceptor(loggingInterceptor);
        builder.cache(cache);
        //设置超时
        builder.connectTimeout(DEFAULT_TIMEOUT, TimeUnit.SECONDS);
        builder.readTimeout(20, TimeUnit.SECONDS);
        builder.writeTimeout(20, TimeUnit.SECONDS);
        //错误重连
        builder.retryOnConnectionFailure(true);
        //cookie认证
        builder.cookieJar(new CookiesManager());

        return builder.build();
    }

    private Interceptor getInterceptor() {
        Interceptor cacheInterceptor = new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                Request request = chain.request();
                if (!CommonUtils.isNetworkConnected()) {
                    request = request.newBuilder()
                            .cacheControl(CacheControl.FORCE_CACHE)
                            .build();
                }
                Response response = chain.proceed(request);
                if (CommonUtils.isNetworkConnected()) {
                    int maxAge = 0;
                    // 有网络时, 不缓存, 最大保存时长为0
                    response.newBuilder()
                            .header("Cache-Control", "public, max-age=" + maxAge)
                            .removeHeader("Pragma")
                            .build();
                } else {
                    // 无网络时，设置超时为4周
                    int maxStale = 60 * 60 * 24 * 28;
                    response.newBuilder()
                            .header("Cache-Control", "public, only-if-cached, max-stale=" + maxStale)
                            .removeHeader("Pragma")
                            .build();
                }
                return response;
            }
        };
        return cacheInterceptor;
    }

    public ApiService getApiService(String url) {
        builder.baseUrl(url)
                .client(getClient())
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();
        apiService = builder.build().create(ApiService.class);
        return apiService;
    }

    private HttpLoggingInterceptor loggingInterceptor = new HttpLoggingInterceptor(new HttpLoggingInterceptor.Logger() {
        @Override
        public void log(String message) {
            Logger.d("net log ==", message);
        }
    });


}

4、发送请求

 RetrofitHelper.getInstance().getApiService(Constants.ANDROID_URL).bannerList
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(object : BaseObserver<BannerEntity>() {
                    override fun onSuccess(bean: BannerEntity) {
                        if (bean.errorCode == 0) {
                            mView?.setBanner(bean)
                        }
                    }
                    override fun onFail(e: Throwable) {
                        Log.e(tag, e.message)
                    }
                })

public abstract class BaseObserver<T> implements Observer<T> {

    public abstract void onSuccess(T t);

    public abstract void onFail(Throwable e);

    @Override
    public void onSubscribe(@NonNull Disposable d) {}

    @Override
    public void onNext(@NonNull T t) {
        onSuccess(t);
    }

    @Override
    public void onError(@NonNull Throwable e) {
        onFail(e);
    }

    @Override
    public void onComplete() {

    }
}