Android实现ListView异步加载图片

ListView异步加载图片是非常实用的方法，凡是是要通过网络获取图片资源一般使用这种方法比较好，用户体验好，下面就说实现方法，先贴上主方法的代码：

1. packagecn.wangmeng.test;
3. importjava.io.IOException;
4. importjava.io.InputStream;
5. importjava.lang.ref.SoftReference;
6. importjava.net.MalformedURLException;
7. importjava.net.URL;
8. importjava.util.HashMap;
10. importandroid.graphics.drawable.Drawable;
11. importandroid.os.Handler;
12. importandroid.os.Message;
14. publicclassAsyncImageLoader{
16. privateHashMap<String,SoftReference<Drawable>>imageCache;
18. publicAsyncImageLoader(){
19. imageCache=newHashMap<String,SoftReference<Drawable>>();
20. }
22. publicDrawableloadDrawable(finalStringimageUrl,finalImageCallbackimageCallback){
23. if(imageCache.containsKey(imageUrl)){
24. SoftReference<Drawable>softReference=imageCache.get(imageUrl);
25. Drawabledrawable=softReference.get();
26. if(drawable!=null){
27. returndrawable;
28. }
29. }
30. finalHandlerhandler=newHandler(){
31. publicvoidhandleMessage(Messagemessage){
32. imageCallback.imageLoaded((Drawable)message.obj,imageUrl);
33. }
34. };
35. newThread(){
36. @Override
37. publicvoidrun(){
38. Drawabledrawable=loadImageFromUrl(imageUrl);
39. imageCache.put(imageUrl,newSoftReference<Drawable>(drawable));
40. Messagemessage=handler.obtainMessage(0,drawable);
41. handler.sendMessage(message);
42. }
43. }.start();
44. returnnull;
45. }
47. publicstaticDrawableloadImageFromUrl(Stringurl){
48. URLm;
49. InputStreami=null;
50. try{
51. m=newURL(url);
52. i=(InputStream)m.getContent();
53. }catch(MalformedURLExceptione1){
54. e1.printStackTrace();
55. }catch(IOExceptione){
56. e.printStackTrace();
57. }
58. Drawabled=Drawable.createFromStream(i,"src");
59. returnd;
60. }
62. publicinterfaceImageCallback{
63. publicvoidimageLoaded(DrawableimageDrawable,StringimageUrl);
64. }
66. }
    

以上代码是实现异步获取图片的主方法，SoftReference是软引用，是为了更好的为了系统回收变量，重复的URL直接返回已有的资源，实现回调函数，让数据成功后，更新到UI线程。
几个辅助类文件：

1. packagecn.wangmeng.test;
3. publicclassImageAndText{
4. privateStringimageUrl;
5. privateStringtext;
7. publicImageAndText(StringimageUrl,Stringtext){
8. this.imageUrl=imageUrl;
9. this.text=text;
10. }
11. publicStringgetImageUrl(){
12. returnimageUrl;
13. }
14. publicStringgetText(){
15. returntext;
16. }
17. }
    

1. packagecn.wangmeng.test;
3. importandroid.view.View;
4. importandroid.widget.ImageView;
5. importandroid.widget.TextView;
7. publicclassViewCache{
9. privateViewbaseView;
10. privateTextViewtextView;
11. privateImageViewimageView;
13. publicViewCache(ViewbaseView){
14. this.baseView=baseView;
15. }
17. publicTextViewgetTextView(){
18. if(textView==null){
19. textView=(TextView)baseView.findViewById(R.id.text);
20. }
21. returntextView;
22. }
24. publicImageViewgetImageView(){
25. if(imageView==null){
26. imageView=(ImageView)baseView.findViewById(R.id.image);
27. }
28. returnimageView;
29. }
31. }
    

ViewCache是辅助获取adapter的子元素布局

1. packagecn.wangmeng.test;
3. importjava.util.List;
5. importcn.wangmeng.test.AsyncImageLoader.ImageCallback;
7. importandroid.app.Activity;
8. importandroid.graphics.drawable.Drawable;
9. importandroid.view.LayoutInflater;
10. importandroid.view.View;
11. importandroid.view.ViewGroup;
12. importandroid.widget.ArrayAdapter;
13. importandroid.widget.ImageView;
14. importandroid.widget.ListView;
15. importandroid.widget.TextView;
17. publicclassImageAndTextListAdapterextendsArrayAdapter<ImageAndText>{
19. privateListViewlistView;
20. privateAsyncImageLoaderasyncImageLoader;
22. publicImageAndTextListAdapter(Activityactivity,List<ImageAndText>imageAndTexts,ListViewlistView){
23. super(activity,0,imageAndTexts);
24. this.listView=listView;
25. asyncImageLoader=newAsyncImageLoader();
26. }
28. publicViewgetView(intposition,ViewconvertView,ViewGroupparent){
29. Activityactivity=(Activity)getContext();
31. //InflatetheviewsfromXML
32. ViewrowView=convertView;
33. ViewCacheviewCache;
34. if(rowView==null){
35. LayoutInflaterinflater=activity.getLayoutInflater();
36. rowView=inflater.inflate(R.layout.image_and_text_row,null);
37. viewCache=newViewCache(rowView);
38. rowView.setTag(viewCache);
39. }else{
40. viewCache=(ViewCache)rowView.getTag();
41. }
42. ImageAndTextimageAndText=getItem(position);
44. //LoadtheimageandsetitontheImageView
45. StringimageUrl=imageAndText.getImageUrl();
46. ImageViewimageView=viewCache.getImageView();
47. imageView.setTag(imageUrl);
48. DrawablecachedImage=asyncImageLoader.loadDrawable(imageUrl,newImageCallback(){
49. publicvoidimageLoaded(DrawableimageDrawable,StringimageUrl){
50. ImageViewimageViewByTag=(ImageView)listView.findViewWithTag(imageUrl);
51. if(imageViewByTag!=null){
52. imageViewByTag.setImageDrawable(imageDrawable);
53. }
54. }
55. });
56. if(cachedImage==null){
57. imageView.setImageResource(R.drawable.default_image);
58. }else{
59. imageView.setImageDrawable(cachedImage);
60. }
61. //SetthetextontheTextView
62. TextViewtextView=viewCache.getTextView();
63. textView.setText(imageAndText.getText());
65. returnrowView;
66. }
68. }
    

ImageAndTextListAdapter是实现ListView的Adapter，里面有个技巧就是imageView.setTag(imageUrl)，setTag是存储数据的，这样是为了保证在回调函数时，listview去更新自己对应item,大家仔细阅读就知道了。
最后贴出布局文件：

1. <?xmlversion="1.0"encoding="utf-8"?>
2. <LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"
3. android:orientation="horizontal"
4. android:layout_width="fill_parent"
5. android:layout_height="wrap_content">
7. <ImageViewandroid:id="@+id/image"
8. android:layout_width="wrap_content"
9. android:layout_height="wrap_content"
10. />
12. <TextViewandroid:id="@+id/text"
13. android:layout_width="wrap_content"
14. android:layout_height="wrap_content"/>
16. </LinearLayout>