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Making ListView Scrolling Smooth

The key to a smoothly scrolling ListView is to keep the application’s main thread (the UI thread) free from heavy processing. Ensure you do any disk access, network access, or SQL access in a separate thread. To test the status of your app, you can enable StrictMode.

Use a Background Thread

Using a background thread ("worker thread") removes strain from the main thread so it can focus on drawing the UI. In many cases, using AsyncTask provides a simple way to perform your work outside the main thread. AsyncTask automatically queues up all the execute() requests and performs them serially. This behavior is global to a particular process and means you don’t need to worry about creating your own thread pool.

In the sample code below, an AsyncTask is used to load images in a background thread, then apply them to the UI once finished. It also shows a progress spinner in place of the images while they are loading.

// Using an AsyncTask to load the slow images in a background thread
new AsyncTask<ViewHolder, Void, Bitmap>() {
    private ViewHolder v;

    @Override
    protected Bitmap doInBackground(ViewHolder... params) {
        v = params[0];
        return mFakeImageLoader.getImage();
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
        if (v.position == position) {
            // If this item hasn't been recycled already, hide the
            // progress and set and show the image
            v.progress.setVisibility(View.GONE);
            v.icon.setVisibility(View.VISIBLE);
            v.icon.setImageBitmap(result);
        }
    }
}.execute(holder);

Beginning with Android 3.0 (API level 11), an extra feature is available in AsyncTask so you can enable it to run across multiple processor cores. Instead of calling execute() you can specify executeOnExecutor() and multiple requests can be executed at the same time depending on the number of cores available.

Hold View Objects in a View Holder

Your code might call findViewById() frequently during the scrolling of ListView, which can slow down performance. Even when the Adapter returns an inflated view for recycling, you still need to look up the elements and update them. A way around repeated use of findViewById() is to use the "view holder" design pattern.

A ViewHolder object stores each of the component views inside the tag field of the Layout, so you can immediately access them without the need to look them up repeatedly. First, you need to create a class to hold your exact set of views. For example:

static class ViewHolder {
  TextView text;
  TextView timestamp;
  ImageView icon;
  ProgressBar progress;
  int position;
}

Then populate the ViewHolder and store it inside the layout.

ViewHolder holder = new ViewHolder();
holder.icon = (ImageView) convertView.findViewById(R.id.listitem_image);
holder.text = (TextView) convertView.findViewById(R.id.listitem_text);
holder.timestamp = (TextView) convertView.findViewById(R.id.listitem_timestamp);
holder.progress = (ProgressBar) convertView.findViewById(R.id.progress_spinner);
convertView.setTag(holder);

Now you can easily access each view without the need for the look-up, saving valuable processor cycles.

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