Retrofit API to retrieve a png image

Question

Hi I am new to Retrofit framework for Android. I could get JSON responses from REST services using it but I don't know how to download a png using retrofit. I am trying to download the png from this url: http://wwwns.akamai.com/media_resources/globe_emea.png. What should be response Object to be specified in the Callback<> to achieve this.

Answer 1

Of course we usually use Picasso to load image, but sometimes we really need use Retrofit to load a special image (like fetch a captcha image), you need add some header for request, get some value from header of response (of course you can also use Picasso + OkHttp, but in a project you have already use Retrofit to handle most of net requests), so here introduce how to implement by Retrofit 2.0.0 (I have already implemented in my project).

The key point is that you need use okhttp3.ResponseBody to receive response, else Retrofit will parse the response data as JSON, not binary data.

codes:

public interface Api {
    // don't need add 'Content-Type' header, it's useless
    // @Headers({"Content-Type: image/png"})
    @GET
    Call<ResponseBody> fetchCaptcha(@Url String url);
}

Call<ResponseBody> call = api.fetchCaptcha(url);
call.enqueue(new Callback<ResponseBody>() {
        @Override
        public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
            if (response.isSuccessful()) {
                if (response.body() != null) {
                    // display the image data in a ImageView or save it
                    Bitmap bmp = BitmapFactory.decodeStream(response.body().byteStream());
                    imageView.setImageBitmap(bmp);
                } else {
                    // TODO
                }
            } else {
                // TODO
            }
        }

        @Override
        public void onFailure(Call<ResponseBody> call, Throwable t) {
            // TODO
        }
    });

Answer 2

As mentioned you shouldn't use Retrofit to actually download the image itself. If your goal is to simply download the content without displaying it then you could simply use an Http client like OkHttp which is another one of Square's libraries.

Here's a few lines of code which would have you download this image. You could then read the data from the InputStream.

    OkHttpClient client = new OkHttpClient();

    Request request = new Request.Builder()
            .url("http://wwwns.akamai.com/media_resources/globe_emea.png")
            .build();

    client.newCall(request).enqueue(new Callback() {
        @Override
        public void onFailure(Request request, IOException e) {
            System.out.println("request failed: " + e.getMessage());
        }

        @Override
        public void onResponse(Response response) throws IOException {
            response.body().byteStream(); // Read the data from the stream
        }
    });

Even though Retrofit isn't the man for the job to answer your question, the signature of your Interface definition would like this. But again don't do this.

public interface Api {
    @GET("/media_resources/{imageName}")
    void getImage(@Path("imageName") String imageName, Callback<Response> callback);
}

Answer 3

Declare it returning Call for instance:

@GET("/api/{api}/bla/image.png")
Call<ResponseBody> retrieveImageData();

then convert it to Bitmap yourself:

ResponseBody body = retrofitService.retrieveImageData().execute().body();
        byte[] bytes = body.bytes();
Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);

Answer 4

Retrofit is a REST library, you can use Retrofit only to get image URL but for displaying Image you should use Picasso: http://square.github.io/picasso/