自定义适配器重复

我有这个CustomAdapter,用我的数据填充列表。问题是Imageview被下载和绘制了很多次。例子:

我在服务器上搜索视频列表:

(Video1) Title 1 Description 1

(Video2) Title 2 Description 2

(Video3) Title 3 Description 3

..

当加载时,将加载Video1中的图像,然后在同一Imageview中加载Video2Image,然后再次为列表中的每个视频加载,与列表中的视频相同。当我滚动适配器时,再次下载所有图像。有一些方法可以解决这个问题,我不理解这个问题。

自定义适配器.java

public class CustomAdapter extends ArrayAdapter<Video> {
    // declaring our ArrayList of items
    private ArrayList<Video> objects;

    public CustomAdapter(Context context, int textViewResourceId, ArrayList<Video> objects) {
        super(context, textViewResourceId, objects);
        this.objects = objects;
    }

    public View getView(int position, View convertView, ViewGroup parent){

        View v = convertView;

        if (v == null) {
            LayoutInflater inflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = inflater.inflate(R.layout.list_row, null);
        }

        Video i = objects.get(position);

        if (i != null) {

            TextView title = (TextView) v.findViewById(R.id.title);
            TextView description = (TextView) v.findViewById(R.id.description);
            ImageView imagen = (ImageView) v.findViewById(R.id.list_image);

            title.setText(i.getTitulo());
            description.setText(i.getDescripcion());

            //Creamos imagen descargada y la seteamos
            new DownloadImageTask(imagen).execute(i.getUrlimagen());
            BitmapDrawable drawable = (BitmapDrawable) imagen.getDrawable();
            Bitmap bitmap = drawable.getBitmap();
            imagen.setImageBitmap(bitmap);

            Log.i("Debug", "Creando una imagen para: " + i.getTitulo());

            v.setTag(R.id.id_url, i.getUrl().trim());//1.Url
            v.setTag(R.id.id_titulo,i.getTitulo().trim());//2.Título
            v.setTag(R.id.id_video,i.getId().trim());//3.ID

        }

        return v;

    }

    private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
        ImageView bmImage;

        public DownloadImageTask(ImageView bmImage) {
            this.bmImage = bmImage;
        }

        protected Bitmap doInBackground(String... urls) {
            String urldisplay = urls[0];
            Bitmap mIcon11 = null;
            try {
                InputStream in = new java.net.URL(urldisplay).openStream();
                BitmapFactory BitmapFactory = null;
                mIcon11 = BitmapFactory.decodeStream(in);
            } catch (Exception e) {
                e.printStackTrace();
            }
            return mIcon11;
        }

        protected void onPostExecute(Bitmap result) {
            if(result!=null)
                bmImage.setImageBitmap(result);
        }
    }

    public ArrayList getValues(){
        return objects;
    }

}

对不起,我的英语。