Justin Phillips Justin Phillips - 5 months ago 9
Android Question

Knowing when a View in a ListView has gone off the screen?

I have Googled this but can't find an answer, so here goes...

I have a ListView that displays some text and an image. The underlying adapter recycles the views for performance reasons (as per the recommended approach), and I load the images using an AsynchTask as per the recommendation found on the Android Developers site's bitmap page.

Everything works perfectly and smoothly, but I have one issue. When the View in the adapter is recycled, it still has a reference to the old image (ImageView). If the user scrolls slowly, then the AsynchTask has enough time to load the new image and display it, so there is no visible reloading of the new image to the user.

However, if the user scrolls very quickly, the delay in loading the image means they see the old image (that was loaded when the View was being used by another item) before its replaced by the new image.

So, my question is, how can I detect when a View is no longer visible on the screen, so I can then remove the image? This would mean the user sees an empty list view item that will eventually be loaded with the appropriate image, which would look better.

Many thanks in advance. Here is the list view adapter code.

@Override
public View getView(int position, View convertView, ViewGroup parent)
{
View view = convertView;
ListViewHolder viewHolder;

// if this is not a recycled view then create a new view for the data...
if (view == null)
{
view = this.inflater.inflate(R.layout.target_list_view_layout, null, true);

viewHolder = new ListViewHolder();

viewHolder.manufacturer = (TextView) view.findViewById(R.id.manufacturer);
viewHolder.targetName = (TextView) view.findViewById(R.id.targetName);
viewHolder.targetThumbnail = (ImageView) view.findViewById(R.id.targetThumbnail);

view.setTag(viewHolder);
} else
{
viewHolder = (ListViewHolder) convertView.getTag();
}

TargetDescriptor targetDescriptor = this.selectedTargets.get(position);

viewHolder.manufacturer.setText(targetDescriptor.manufacturer);
viewHolder.targetName.setText(targetDescriptor.targetName);

// At this point I pass the image view reference to my background task to load the image
LoadImageViewAsynchTask loadImageTask = new LoadImageViewAsynchTask(viewHolder.targetThumbnail, targetDescriptor);
loadImageTask.execute(new Integer[]
{ 64, 64 });

return view;
}


EDIT: Those that use the eBay Android App can see the effect I am looking for if that helps.

Answer

Can't believe how stupid I have been, this is a really easy one liner to solve!

Basically in my adapter if the view is being re-cycled, I simply need to null the ImageView bitmap before the view is re-used. That means the image in the list view item is blank before the next image is loaded. I no longer have the issue of the old image being there whilst the new image is being loaded in the background.

I have also made a change in the adapter that cancels any current AsynchTask bound to the view that might still be loading a previous image that is no longer needed. This was very prevalent when I scroll very fast through the list view, with often two or more image loads backing up, so the image would change several times before settling on the correct image.

Here is the new code, with the changes commented so you can see what is happening:

@Override
public View getView(int position, View convertView, ViewGroup parent)
{
    ListViewHolder viewHolder;

    // if this is not a recycled view then create a new view for the data...
    if (convertView == null)
    {
        convertView = this.inflater.inflate(R.layout.target_list_view_layout, null, true);

        viewHolder = new ListViewHolder();

        viewHolder.manufacturer = (TextView) convertView.findViewById(R.id.manufacturer);
        viewHolder.targetName = (TextView) convertView.findViewById(R.id.targetName);
        viewHolder.targetThumbnail = (ImageView) convertView.findViewById(R.id.targetThumbnail);

        convertView.setTag(viewHolder);
    } else
    {
        viewHolder = (ListViewHolder) convertView.getTag();

        // Cancel the previous attempt to load an image as this is going to be superceded by the next image
        viewHolder.loadImageViewAsynchTask.cancel(true);

        // Clear down the old image so when this view is displayed, the user does not see the old image before the
        // new image has a chance to load in the background
        viewHolder.targetThumbnail.setImageBitmap(null);
    }

    TargetDescriptor targetDescriptor = this.selectedTargets.get(position);

    viewHolder.manufacturer.setText(targetDescriptor.manufacturer);
    viewHolder.targetName.setText(targetDescriptor.targetName);

    LoadImageViewAsynchTask loadImageViewAsynchTask = new LoadImageViewAsynchTask(viewHolder.targetThumbnail);
    loadImageViewAsynchTask.setTargetDescriptor(targetDescriptor);
    loadImageViewAsynchTask.execute(new Integer[]
    { 64, 64 });

    // Keep a reference to the task so we can cancel it if the view is recycled next time round to prevent
    // un-neccessary image loads that are out of date
    viewHolder.loadImageViewAsynchTask = loadImageViewAsynchTask;

    return convertView;
}