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
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01.public class AsyncImageLoader {
02. private HashMap<String, SoftReference<Drawable>> imageCache;
03.
04. public AsyncImageLoader() {
05. imageCache = new HashMap<String, SoftReference<Drawable>>();
06. }
07.
08. public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {
09. if (imageCache.containsKey(imageUrl)) {
10. SoftReference<Drawable> softReference = imageCache.get(imageUrl);
11. Drawable drawable = softReference.get();
12. if (drawable != null) {
13. return drawable;
14. }
15. }
16. final Handler handler = new Handler() {
17. @Override
18. public void handleMessage(Message message) {
19. imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
20. }
21. };
22. new Thread() {
23. @Override
24. public void run() {
25. Drawable drawable = loadImageFromUrl(imageUrl);
26. imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
27. Message message = handler.obtainMessage(0, drawable);
28. handler.sendMessage(message);
29. }
30. }.start();
31. return null;
32. }
33.
34. public static Drawable loadImageFromUrl(String url) {
35. // ...
36. }
37.
38. public interface ImageCallback {
39. public void imageLoaded(Drawable imageDrawable, String imageUrl);
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
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01.public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {
02.
03. private ListView listView;
04. private AsyncImageLoader asyncImageLoader;
05.
06. public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {
07. super(activity, 0, imageAndTexts);
08. this.listView = listView;
09. asyncImageLoader = new AsyncImageLoader();
10. }
11.
12. @Override
13. public View getView(int position, View convertView, ViewGroup parent) {
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. rowView = inflater.inflate(R.layout.image_and_text_row, null);
22. viewCache = new ViewCache(rowView);
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. Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {
34. public void imageLoaded(Drawable imageDrawable, String imageUrl) {
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 rowView;
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
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01.ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
02. if (imageViewByTag != null) {
03. imageViewByTag.setImageDrawable(imageDrawable);
04. }
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
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01.public class ViewCache {
02.
03. private View baseView;
04. private TextView textView;
05. private ImageView imageView;
06.
07. public ViewCache(View baseView) {
08. this.baseView = baseView;
09. }
10.
11. public TextView getTextView() {
12. if (textView == null) {
13. textView = (TextView) baseView.findViewById(R.id.text);
14. }
15. return titleView;
16. }
17.
18. public ImageView getImageView() {
19. if (imageView == null) {
20. imageView = (ImageView) baseView.findViewById(R.id.image);
21. }
22. return imageView;
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
本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/wanglong0537/archive/2011/04/19/6334005.aspx