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
1. public class
2. private
3.
4. public
5. new
6. }
7.
8. public Drawable loadDrawable(final String imageUrl, final
9. if
10. SoftReference<Drawable> softReference = imageCache.get(imageUrl);
11. Drawable drawable = softReference.get();
12. if (drawable != null) {
13. return
14. }
15. }
16. final Handler handler = new
17. @Override
18. public void
19. imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
20. }
21. };
22. new
23. @Override
24. public void
25. Drawable drawable = loadImageFromUrl(imageUrl);
26. new
27. 0, drawable);
28. handler.sendMessage(message);
29. }
30. }.start();
31. return null;
32. }
33.
34. public static
35. // ...
36. }
37.
38. public interface
39. public void
40. }
41. } 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
1. public class ImageAndTextListAdapter extends
2.
3. private
4. private
5.
6. public
7. super(activity, 0, imageAndTexts);
8. this.listView = listView;
9. new
10. }
11.
12. @Override
13. public View getView(int
14. Activity activity = (Activity) getContext();
15.
16. // Inflate the views from XML
17. View rowView = convertView;
18. ViewCache viewCache;
19. if (rowView == null) {
20. LayoutInflater inflater = activity.getLayoutInflater();
21. null);
22. new
23. rowView.setTag(viewCache);
24. else
25. viewCache = (ViewCache) rowView.getTag();
26. }
27. ImageAndText imageAndText = getItem(position);
28.
29. // Load the image and set it on the ImageView
30. String imageUrl = imageAndText.getImageUrl();
31. ImageView imageView = viewCache.getImageView();
32. imageView.setTag(imageUrl);
33. new
34. public void
35. ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
36. if (imageViewByTag != null) {
37. imageViewByTag.setImageDrawable(imageDrawable);
38. }
39. }
40. });
41. imageView.setImageDrawable(cachedImage);
42.
43. // Set the text on the TextView
44. TextView textView = viewCache.getTextView();
45. textView.setText(imageAndText.getText());
46.
47. return
48. }
49. } 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
1. ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
2. if (imageViewByTag != null) {
3. imageViewByTag.setImageDrawable(imageDrawable);
4. }
这里通过ViewCatch来减少了 findViewById的使用
1. public class
2.
3. private
4. private
5. private
6.
7. public
8. this.baseView = baseView;
9. }
10.
11. public
12. if (textView == null) {
13. textView = (TextView) baseView.findViewById(R.id.text);
14. }
15. return
16. }
17.
18. public
19. if (imageView == null) {
20. imageView = (ImageView) baseView.findViewById(R.id.image);
21. }
22. return
23. }
24. }
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
