I haven't read through the whole thread, but FWIW there's a very good
introduction to list view 'lazy loading' techniques in Beginning
Android: http://www.apress.com/book/view/1430224193.
Look in the sample code under fancylists -- well documented in the
book.
Sam Dutton
On Dec 8 2009, 2:34 pm, Samuh <samuh.va...@gmail.com> wrote:
> I trying a hand at ListViews and my current experiment is aimed at
> displaying some data in a ListView. The data to be displayed in each
> row is simple: an image and some text. The images come from a remote
> server and the textual data is hardcoded.
>
> I have a class that downloads images using AsyncTask and caches the
> list of images fetched as SoftReferences in a LinkedHashMap. I am also
> passing a reference of the view to this class, so when the image
> download/cache read is complete the class will set appropriate Bitmap
> in the view.
>
> Following is my code:
>
> //class BasicAdapter extends ArrayAdapter<RowData>
> ...
> static class ViewHolder{
> TextView text;
> ImageView icon;
> }
>
> @Override
> public View getView(int position, View convertView, ViewGroup parent)
> {
> ViewHolder holder;
> if(convertView == null){
> convertView =
> mInflater.inflate(R.layout.listview_row,null);
>
> holder = new ViewHolder();
> holder.text = (TextView)
> convertView.findViewById(R.id.row_txt);
> holder.icon = (ImageView)
> convertView.findViewById(R.id.row_img);
>
> convertView.setTag(holder);
> }
> else{
> holder = (ViewHolder) convertView.getTag();
> }
>
> holder.text.setText(mDataSet.get(position).getMText());
>
> //set default icon
> holder.icon.setImageResource(R.drawable.icon);
>
> // set the actual image from the cache
> String imageUrl = mDataSet.get(position).getMUrl();
> WriteThroughCache.getImage(holder.icon, imageUrl);
>
> return convertView;
>
> }
>
> // My helper class that manages cache and image Download
>
> public class WriteThroughCache{
> static LinkedHashMap<String,SoftReference<Bitmap>> imageCache =
> new LinkedHashMap<String,SoftReference<Bitmap>>();
>
> public static void getImage(ImageView icon, String imageUrl){
> if(imageCache.containsKey(imageUrl)){
> Bitmap image = imageCache.get(imageUrl).get();
> if(image != null){
> icon.setImageBitmap(image);
> return;
> }
> }
>
> new LoadImageInBackground().execute(new
> Object[]{icon,imageUrl});
>
> }
>
> static class LoadImageInBackground extends AsyncTask<Object, Void,
> Bitmap>{
> ImageView mIcon = null;
> String mUrl = null;
> @Override
> protected Bitmap doInBackground(Object... params) {
> mIcon = (ImageView)params[0];
> mUrl = (String) params[1];
> Bitmap image = BitmapFactory.decodeStream
> (GetMethodExecutor.getResponseStream(mUrl));
> return image;
> }
> @Override
> protected void onPostExecute(Bitmap result) {
> super.onPostExecute(result);
> mIcon.setImageBitmap(result);
> imageCache.put(mUrl, new
> SoftReference<Bitmap>(result));
> }
>
> }
>
> }
>
> Problem: The applications seems to work just fine. However, when I
> touch scroll the screen sometimes, the rows display the default image
> and "Loading.." text. These are never refreshed.
> But if I fling the listview, everything works normal then.
>
> I am not sure of what I am missing here, please guide.
>
> Thanks.
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