滚动时 Android ListView 生涩

Question

我正在使用 XMLParser 为 ListView 获取带有日期的 XML。解析器将日期发送到 LazyAdapter，然后由 LazyAdapter 构建列表。

当我在我的应用程序中滚动时，ListView 不稳定且缓慢。我认为问题将出在 ImageLoader 上。当我使用 ImageLoader 禁用该行时，它工作得更好而且不抖动。

惰性适配器:

  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;
    }

图像加载器:

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

您应该在滚动期间暂停图像加载器。这是保持滚动非常流畅的最佳方式。

为此，您需要在图像加载器中添加 setPaused(boolean pause) 方法。暂停时，图像加载器应停止处理队列并等待暂停被取消。此方法将由 onScrollStateChanged() 中的 OnScrollListener 实现触发，方法是检查 scrollState 参数。

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

有关更多信息，我的来源是 Cyril Mottier 的演讲:

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

见幻灯片 98。

---

还有一种更简单但效率较低的方法:只需将图像加载程序线程的优先级设置为较低的值。在 Runnable 实现中的 run() 方法开头添加以下行:

Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);