以下代码中正确使用了asynctask吗？

Question

i am trying to use asynctask to download images from internet.i have this in my custom pager adapter which sets imageview with the image downloaded.

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 public class pageradapter extends PagerAdapter {
    Button load_img;
    ImageView imgview;
    Bitmap bitmap;



    Context mContext;
    LayoutInflater mLayoutInflater;
    List<String> l = MainActivity.list;
    ImageLoader mImageLoader;

    public pageradapter(Context context) {
        mContext = context;


    }

    @Override
    public int getCount() {
        return 4;
    }


    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        ImageLoader mImageLoader = ImageLoader.getInstance();
        mLayoutInflater = ((LayoutInflater) container.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE));
        View view = mLayoutInflater.inflate(R.layout.img, container, false);
          imgview = (ImageView) view.findViewById(R.id.imageView3);

   new LoadImage().execute(l.get(position));


        container.addView(view);

        return view;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((LinearLayout) object);
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == object;
    }

    public class LoadImage extends AsyncTask<String, String, Bitmap> {

        ProgressDialog pDialog = new ProgressDialog(mContext);
        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            pDialog.setMessage("Loading Image ....");
            pDialog.show();

        }
        protected Bitmap doInBackground(String... args) {

            try {
                bitmap = BitmapFactory.decodeStream((InputStream) new URL(args[0]).getContent());

            } catch (Exception e) {
                e.printStackTrace();
            }
            return bitmap;
        }

        protected void onPostExecute(Bitmap image) {

            if(image != null){
                imgview.setImageBitmap(image);
                pDialog.dismiss();

            }else{

                pDialog.dismiss();
                Toast.makeText(mContext, "Image Does Not exist or Network Error", Toast.LENGTH_SHORT).show();

            }
        }
    }

}

images do not appear in the view pager.

Answer 1

You should change your LoadImage class.

 public class LoadImage extends AsyncTask<String, Void, Bitmap> {  
    ImageView view;

    public LoadImage(ImageView view){
        this.view = view;
    }

    ProgressDialog pDialog = new ProgressDialog(mContext);
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        pDialog.setMessage("Loading Image ....");
        pDialog.setCancelable(false);
        pDialog.show();

    }
    protected Bitmap doInBackground(String... args) {

        try {
            URL URL = new URL(args[0]);
            URLConnection connection = URL.openConnection();
            connection.connect();
            InputStream input = new BufferedInputStream(URL.openStream());
            return BitmapFactory.decodeStream(input);
        } catch (Exception e) {}
        return null;
    }

    protected void onPostExecute(Bitmap image) {
        if (pDialog.isShowing())
            pDialog.dismiss();

        if(image != null){
            view.setImageBitmap(image);

        }else{
            Toast.makeText(mContext, "Image Does Not exist or Network Error", Toast.LENGTH_SHORT).show();
        }
    }
}

Use it like this:

new LoadImage(imgview).execute(l.get(position));

Answer 2

I would argue that this particular case is not a best practice for use of an AsyncTask. Instead, try using a library called Glide that is excellent and downloading images and placing them in an ImageView. You'll write a lot less code and have fewer problems with a better user experience.

Answer 3

To set image from url to ImageView use library like http://square.github.io/picasso/

Its so simple to use just add dependency in project

Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView); 

Answer 4

Here is some libs for loading image from URL and it will store image in to memory as cache.

UIL : flexible and highly customizable instrument for image loading, caching and displaying. It provides a lot of configuration options and good control over the image loading and caching process.

PICASSO : Handling ImageView recycling and download cancelation in an adapter. Complex image transformations with minimal memory use. Automatic memory and disk caching.

i personally prefer picasso.

eg picasso

Picasso.with(context)
  .load(url)
  .resize(50, 50)
  .centerCrop()
  .into(imageView)