I've got the following code from "Android in Practice" book. It's implementation of custom adapter which downloads images from the internet. It uses private class RetrieveImageTask to retrieve pictures.
Can someone explain me why the first thing the adapter class does is to get image from the cache instead of downloading it ? I understand it in a way that, the first time it displays
default image which was set in the beginning of getView(), then sets downloaded image, but does it mean that view is being refreshed constantly by calling getView() ?
And why author sets tag of image to item ID in getView() and then sets it to null in onPostExecute() ?
DealsAdapter
private class DealsAdapter extends ArrayAdapter<Item> {
public DealsAdapter(List<Item> items) {
super(DealList.this, R.layout.list_item, items);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
convertView = inflater.inflate(R.layout.list_item, parent, false);
}
// use ViewHolder here to prevent multiple calls to findViewById (if you have a large collection)
TextView text = (TextView) convertView.findViewById(R.id.deal_title);
ImageView image = (ImageView) convertView.findViewById(R.id.deal_img);
image.setImageBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.ddicon));
Item item = getItem(position);
if (item != null) {
text.setText(item.getTitle());
Bitmap bitmap = app.getImageCache().get(item.getItemId()); //<------HERE
if (bitmap != null) {
image.setImageBitmap(bitmap);
} else {
// put item ID on image as TAG for use in task
image.setTag(item.getItemId());
// separate thread/via task, for retrieving each image
// (note that this is brittle as is, should stop all threads in onPause)
new RetrieveImageTask(image).execute(item.getSmallPicUrl());
}
}
return convertView;
}
}
RetriveImageTask
private class RetrieveImageTask extends AsyncTask<String, Void, Bitmap> {
private ImageView imageView;
public RetrieveImageTask(ImageView imageView) {
this.imageView = imageView;
}
@Override
protected Bitmap doInBackground(String... args) {
Bitmap bitmap = app.retrieveBitmap(args[0]);
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
app.getImageCache().put((Long) imageView.getTag(), bitmap);
imageView.setTag(null);
}
}
}
