listView 图片加载性能优化

ListView 是一种可以显示一系列项目并能进行滚动显示的 View，每一行的Item可能包含复杂的结构，可能会从网络上获取icon等的一些图标信息，就现在的网络速度要想保持ListView运行的很好滚动流畅是做不到的



所以这里就需要把这些信息利用多线程实现异步加载



实现这样功能的类





[java] view plaincopyprint?public class AsyncImageLoader { 
    private HashMap<String, SoftReference<Drawable>> imageCache; 
  
    public AsyncImageLoader() { 
        imageCache = new HashMap<String, SoftReference<Drawable>>(); 
    } 
  
    public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) { 
        if (imageCache.containsKey(imageUrl)) { 
            SoftReference<Drawable> softReference = imageCache.get(imageUrl); 
            Drawable drawable = softReference.get(); 
            if (drawable != null) { 
                return drawable; 
            } 
        } 
        final Handler handler = new Handler() { 
            @Override 
            public void handleMessage(Message message) { 
                imageCallback.imageLoaded((Drawable) message.obj, imageUrl); 
            } 
        }; 
        new Thread() { 
            @Override 
            public void run() { 
                Drawable drawable = loadImageFromUrl(imageUrl); 
                imageCache.put(imageUrl, new SoftReference<Drawable>(drawable)); 
                Message message = handler.obtainMessage(0, drawable); 
                handler.sendMessage(message); 
            } 
        }.start(); 
        return null; 
    } 
  
    public static Drawable loadImageFromUrl(String url) { 
        // ...  
    } 
  
    public interface ImageCallback { 
        public void imageLoaded(Drawable imageDrawable, String imageUrl); 
    } 
} 
public class AsyncImageLoader {
    private HashMap<String, SoftReference<Drawable>> imageCache;

    public AsyncImageLoader() {
        imageCache = new HashMap<String, SoftReference<Drawable>>();
    }

    public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {
        if (imageCache.containsKey(imageUrl)) {
            SoftReference<Drawable> softReference = imageCache.get(imageUrl);
            Drawable drawable = softReference.get();
            if (drawable != null) {
                return drawable;
            }
        }
        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message message) {
                imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
            }
        };
        new Thread() {
            @Override
            public void run() {
                Drawable drawable = loadImageFromUrl(imageUrl);
                imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
                Message message = handler.obtainMessage(0, drawable);
                handler.sendMessage(message);
            }
        }.start();
        return null;
    }

    public static Drawable loadImageFromUrl(String url) {
        // ...
    }

    public interface ImageCallback {
        public void imageLoaded(Drawable imageDrawable, String imageUrl);
    }
}




注意这里使用了 SoftReference来缓存图片，允许 GC在需要的时候可以对缓存中的图片进行清理。它这样工作：

·         调用 loadDrawable(ImageUrl, imageCallback)，传入一个匿名实现的 ImageCallback接口

·         如果图片在缓存中不存在的话，图片将从单一的线程中下载并在下载结束时通过 ImageCallback回调

·         如果图片确实存在于缓存中，就会马上返回，不会回调 ImageCallback



        然后我们还可以根据09google I/0开发者大会提到的方式来继续优化Adapter 使用ViewHolder来减少一些比较费时的操作，譬如inflate XML 和 findViewById()等操作

       [java] view plaincopyprint?public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> { 
  
    private ListView listView; 
    private AsyncImageLoader asyncImageLoader; 
  
    public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) { 
        super(activity, 0, imageAndTexts); 
        this.listView = listView; 
        asyncImageLoader = new AsyncImageLoader(); 
    } 
  
    @Override 
    public View getView(int position, View convertView, ViewGroup parent) { 
        Activity activity = (Activity) getContext(); 
  
        // Inflate the views from XML  
        View rowView = convertView; 
        ViewCache viewCache; 
        if (rowView == null) { 
            LayoutInflater inflater = activity.getLayoutInflater(); 
            rowView = inflater.inflate(R.layout.image_and_text_row, null); 
            viewCache = new ViewCache(rowView); 
            rowView.setTag(viewCache); 
        } else { 
            viewCache = (ViewCache) rowView.getTag(); 
        } 
        ImageAndText imageAndText = getItem(position); 
  
        // Load the image and set it on the ImageView  
        String imageUrl = imageAndText.getImageUrl(); 
        ImageView imageView = viewCache.getImageView(); 
        imageView.setTag(imageUrl); 
        Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() { 
            public void imageLoaded(Drawable imageDrawable, String imageUrl) { 
                ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl); 
                if (imageViewByTag != null) { 
                    imageViewByTag.setImageDrawable(imageDrawable); 
                } 
            } 
        }); 
        imageView.setImageDrawable(cachedImage); 
  
        // Set the text on the TextView  
        TextView textView = viewCache.getTextView(); 
        textView.setText(imageAndText.getText()); 
  
        return rowView; 
    } 
}  
public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {

    private ListView listView;
    private AsyncImageLoader asyncImageLoader;

    public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {
        super(activity, 0, imageAndTexts);
        this.listView = listView;
        asyncImageLoader = new AsyncImageLoader();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Activity activity = (Activity) getContext();

        // Inflate the views from XML
        View rowView = convertView;
        ViewCache viewCache;
        if (rowView == null) {
            LayoutInflater inflater = activity.getLayoutInflater();
            rowView = inflater.inflate(R.layout.image_and_text_row, null);
            viewCache = new ViewCache(rowView);
            rowView.setTag(viewCache);
        } else {
            viewCache = (ViewCache) rowView.getTag();
        }
        ImageAndText imageAndText = getItem(position);

        // Load the image and set it on the ImageView
        String imageUrl = imageAndText.getImageUrl();
        ImageView imageView = viewCache.getImageView();
        imageView.setTag(imageUrl);
        Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {
            public void imageLoaded(Drawable imageDrawable, String imageUrl) {
                ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
                if (imageViewByTag != null) {
                    imageViewByTag.setImageDrawable(imageDrawable);
                }
            }
        });
        imageView.setImageDrawable(cachedImage);

        // Set the text on the TextView
        TextView textView = viewCache.getTextView();
        textView.setText(imageAndText.getText());

        return rowView;
    }
} 





      这里我们没有加载完iamge之后直接设定到相应的ImageView上 ，而是通过Tag查找，这里我们重用的View 这里有个listView的引用来通过Tag查找 可见 CallBack的实现



      [c-sharp] view plaincopyprint?ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl); 
               if (imageViewByTag != null) { 
                   imageViewByTag.setImageDrawable(imageDrawable); 
               } 
ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
                if (imageViewByTag != null) {
                    imageViewByTag.setImageDrawable(imageDrawable);
                }



      这里通过ViewCatch来减少了 findViewById的使用



     [c-sharp] view plaincopyprint?public class ViewCache { 
  
    private View baseView; 
    private TextView textView; 
    private ImageView imageView; 
  
    public ViewCache(View baseView) { 
        this.baseView = baseView; 
    } 
  
    public TextView getTextView() { 
        if (textView == null) { 
            textView = (TextView) baseView.findViewById(R.id.text); 
        } 
        return titleView; 
    } 
  
    public ImageView getImageView() { 
        if (imageView == null) { 
            imageView = (ImageView) baseView.findViewById(R.id.image); 
        } 
        return imageView; 
    } 
}  
public class ViewCache {

    private View baseView;
    private TextView textView;
    private ImageView imageView;

    public ViewCache(View baseView) {
        this.baseView = baseView;
    }

    public TextView getTextView() {
        if (textView == null) {
            textView = (TextView) baseView.findViewById(R.id.text);
        }
        return titleView;
    }

    public ImageView getImageView() {
        if (imageView == null) {
            imageView = (ImageView) baseView.findViewById(R.id.image);
        }
        return imageView;
    }
} 



     总结 ：这里主要做了三点优化





在单一线程里加载图片
  重用列表中行
缓存行中的 View