public interface ServiceApi { 
   
  //下载文件 
  @GET 
  Call<ResponseBody> downloadPicFromNet(@Url String fileUrl); 
 
} 

@GET 
Observable<ResponseBody> downloadPicFromNet(@Url String fileUrl); 

Retrofit retrofit = new Retrofit.Builder() 
.baseUrl(BASE_URL) 
.addCallAdapterFactory(RxJavaCallAdapterFactory.create()) //添加Rxjava 
.addConverterFactory(GsonConverterFactory.create()) // <span style="font-family: Arial, Helvetica, sans-serif;">定义转化器 可以将结果返回一个json格式</span> 
.build(); 

ServiceApi serviceApi = retrofit.create(ServiceApi.class); 

serviceApi.downloadPicFromNet("http://pic41.nipic.com/20140509/4746986_145156378323_2.jpg") 
.subscribeOn(Schedulers.newThread())//在新线程中实现该方法 
.map(new Func1<ResponseBody, Bitmap>() { 
 
  @Override 
  public Bitmap call(ResponseBody arg0) { 
    if(saveFileToDisc(arg0)) {//保存图片成功 
      Bitmap bitmap = BitmapFactory.decodeFile(getExternalFilesDir(null) + File.separator + "baidu.png"); 
      return bitmap;//返回一个bitmap对象 
    } 
    return null; 
  } 
}) 
.observeOn(AndroidSchedulers.mainThread())//在Android主线程中展示 
.subscribe(new Subscriber<Bitmap>() { 
   
  ProgressDialog dialog = new ProgressDialog(MainActivity.this); 
   
 
  @Override 
  public void onStart() { 
    dialog.show(); 
    super.onStart(); 
  } 
 
  @Override 
  public void onCompleted() { 
    dialog.dismiss(); 
  } 
 
  @Override 
  public void onError(Throwable arg0) { 
    Log.d(TAG, "onError ===== " + arg0.toString()); 
  } 
 
  @Override 
  public void onNext(Bitmap arg0) { 
    imageIv.setImageBitmap(arg0); 
     
  } 
});