滚动时Android ListView生涩

Question

I'm using XMLParser to get a XML with Date for a Listview. The Parser send the Date to the LazyAdapter und the LazyAdapter build the List.

When I scrolling in my App, the ListView ist jerky and slow. I think the Problem will be the ImageLoader. When I disable the line with the ImageLoader, its work better and not jerky.

The LazyAdapter:

  public View getView(int position, View convertView, ViewGroup parent) {
        View vi=convertView;
        if(convertView==null)


        vi = inflater.inflate(R.layout.list_row, null);

        TextView id = (TextView)vi.findViewById(R.id.id); 
        TextView title = (TextView)vi.findViewById(R.id.title);
        TextView artist = (TextView)vi.findViewById(R.id.artist);
        ImageView thumb_image=(ImageView)vi.findViewById(R.id.list_image); 

        HashMap<String, String> coupon = new HashMap<String, String>();
        coupon = data.get(position);

        id.setText(coupon.get(NewCoupons.id));
        title.setText(title);
        artist.setText(coupon.get(NewCoupons.artist));
        imageLoader.DisplayImage(KEY_THUMN, thumb_image);
        return vi;
    }

The ImageLoader:

public class ImageLoader {

    MemoryCache memoryCache=new MemoryCache();
    FileCache fileCache;
    private Map<ImageView, String> imageViews=Collections.synchronizedMap(new WeakHashMap<ImageView, String>());
    ExecutorService executorService; 

    public ImageLoader(Context context){
        fileCache=new FileCache(context);
        executorService=Executors.newFixedThreadPool(5);
    }

    final int stub_id = R.drawable.no_image;
    public void DisplayImage(String url, ImageView imageView)
    {
        imageViews.put(imageView, url);
        Bitmap bitmap=memoryCache.get(url);
        if(bitmap!=null)
            imageView.setImageBitmap(bitmap);
        else
        {
            queuePhoto(url, imageView);
            imageView.setImageResource(stub_id);
        }
    }

    private void queuePhoto(String url, ImageView imageView)
    {
        PhotoToLoad p=new PhotoToLoad(url, imageView);
        executorService.submit(new PhotosLoader(p));
    }

    private Bitmap getBitmap(String url) 
    {
        File f=fileCache.getFile(url);

        //from SD cache
        Bitmap b = decodeFile(f);
        if(b!=null)
            return b;

        //from web
        try {
            Bitmap bitmap=null;
            URL imageUrl = new URL(url);
            HttpURLConnection conn = (HttpURLConnection)imageUrl.openConnection();
            conn.setConnectTimeout(30000);
            conn.setReadTimeout(30000);
            conn.setInstanceFollowRedirects(true);
            InputStream is=conn.getInputStream();
            OutputStream os = new FileOutputStream(f);
            Utils.CopyStream(is, os);
            os.close();
            bitmap = decodeFile(f);
            return bitmap;
        } catch (Exception ex){
           ex.printStackTrace();
           return null;
        }
    }

    //decodes image and scales it to reduce memory consumption
    private Bitmap decodeFile(File f){
        try {
            //decode image size
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            BitmapFactory.decodeStream(new FileInputStream(f),null,o);

            //Find the correct scale value. It should be the power of 2.
            final int REQUIRED_SIZE=70;
            int width_tmp=o.outWidth, height_tmp=o.outHeight;
            int scale=1;
            while(true){
                if(width_tmp/2<REQUIRED_SIZE || height_tmp/2<REQUIRED_SIZE)
                    break;
                width_tmp/=2;
                height_tmp/=2;
                scale*=2;
            }

            //decode with inSampleSize
            BitmapFactory.Options o2 = new BitmapFactory.Options();
            o2.inSampleSize=scale;
            return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
        } catch (FileNotFoundException e) {}
        return null;
    }

    //Task for the queue
    private class PhotoToLoad
    {
        public String url;
        public ImageView imageView;
        public PhotoToLoad(String u, ImageView i){
            url=u; 
            imageView=i;
        }
    }

    class PhotosLoader implements Runnable {
        PhotoToLoad photoToLoad;
        PhotosLoader(PhotoToLoad photoToLoad){
            this.photoToLoad=photoToLoad;
        }

        @Override
        public void run() {
            if(imageViewReused(photoToLoad))
                return;
            Bitmap bmp=getBitmap(photoToLoad.url);
            memoryCache.put(photoToLoad.url, bmp);
            if(imageViewReused(photoToLoad))
                return;
            BitmapDisplayer bd=new BitmapDisplayer(bmp, photoToLoad);
            Activity a=(Activity)photoToLoad.imageView.getContext();
            a.runOnUiThread(bd);
        }
    }

    boolean imageViewReused(PhotoToLoad photoToLoad){
        String tag=imageViews.get(photoToLoad.imageView);
        if(tag==null || !tag.equals(photoToLoad.url))
            return true;
        return false;
    }

    //Used to display bitmap in the UI thread
    class BitmapDisplayer implements Runnable
    {
        Bitmap bitmap;
        PhotoToLoad photoToLoad;
        public BitmapDisplayer(Bitmap b, PhotoToLoad p){bitmap=b;photoToLoad=p;}
        public void run()
        {
            if(imageViewReused(photoToLoad))
                return;
            if(bitmap!=null)
                photoToLoad.imageView.setImageBitmap(bitmap);
            else
                photoToLoad.imageView.setImageResource(stub_id);
        }
    }

    public void clearCache() {
        memoryCache.clear();
        fileCache.clear();
    }

}

Answer 1

You should pause your image loader during scroll. This is the best way to keep the scrolling very smooth.

To do that you need to add a setPaused(boolean pause) method in your image loader. When paused you image loader should stop processing the queue and wait until the pause is cancelled. This method will be triggered by an implementation of OnScrollListener inside the onScrollStateChanged() , by checking the scrollState parameter.

imageLoader.setPaused(scrollState == OnScrollListener.SCROLL_STATE_FLING);

For more information, my source is a presentation given by Cyril Mottier :

https://speakerdeck.com/cyrilmottier/optimizing-android-ui-pro-tips-for-creating-smooth-and-responsive-apps

See from slide 98.

---

There is also an easier way but less efficient: just set the image loader thread priority to a lower value. Add the following line at the begginning of run() method inside your Runnable implementation :

Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

Answer 2

There is a useful little open source library on GitHub that can handle this.

https://github.com/nostra13/Android-Universal-Image-Loader

It allows for pausing on scrolling as well as others niceties such as allowing the images to fade in and image caching.

Answer 3

1. first, you should use convertview and ViewHolder

    static class ViewHolder { TextView idTestView; TextView titleTestView; TextView artistTestView; } public View getView(int position, View convertView, ViewGroup parent) { ViewHolder holder; if(convertView == null) { convertView = inflater.inflate(R.layout.list_row, null); holder = new ViewHolder(); holder.idTestView = (TextView)convertView.findViewById(R.id.id); holder.titleTestView = (TextView)convertView.findViewById(R.id.title); holder.artistTestView = (TextView)convertView.findViewById(R.id.artist); holder.image = (ImageView)convertView.findViewById(R.id.list_image); convertView.setTag(holder); } else { holder = convertView.getTag(); } // initial viewholder attributes 

   }

2. you should use imageloader to show picture, and cache in memory also cache in dist. you can get it from there: https://github.com/nostra13/Android-Universal-Image-Loader

3. your listview should add setOnScrollListener, when scroll flying, you should not load picture