Android实现ListView异步加载图片

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，不用让用户等待下去，下面就说实现方法，先贴上主方法的代码： 

01. package cn.wangmeng.test;

02.  

03. import java.io.IOException;

04. import java.io.InputStream;

05. import java.lang.ref.SoftReference;

06. import java.net.MalformedURLException;

07. import java.net.URL;

08. import java.util.HashMap;

09.  

10. import android.graphics.drawable.Drawable;

11. import android.os.Handler;

12. import android.os.Message;

13.  

14. public class AsyncImageLoader {

15.  

16.      private HashMap<String, SoftReference<Drawable>> imageCache;

17.        

18.          public AsyncImageLoader() {

19.              imageCache = new HashMap<String, SoftReference<Drawable>>();

20.          }

21.        

22.          public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {

23.              if (imageCache.containsKey(imageUrl)) {

24.                  SoftReference<Drawable> softReference = imageCache.get(imageUrl);

25.                  Drawable drawable = softReference.get();

26.                  if (drawable != null) {

27.                      return drawable;

28.                  }

29.              }

30.              final Handler handler = new Handler() {

31.                  public void handleMessage(Message message) {

32.                      imageCallback.imageLoaded((Drawable) message.obj, imageUrl);

33.                  }

34.              };

35.              new Thread() {

36.                  @Override

37.                  public void run() {

38.                      Drawable drawable = loadImageFromUrl(imageUrl);

39.                      imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));

40.                      Message message = handler.obtainMessage(0, drawable);

41.                      handler.sendMessage(message);

42.                  }

43.              }.start();

44.              return null;

45.          }

46.        

47.         public static Drawable loadImageFromUrl(String url) {

48.             URL m;

49.             InputStream i = null;

50.             try {

51.                 m = new URL(url);

52.                 i = (InputStream) m.getContent();

53.             } catch (MalformedURLException e1) {

54.                 e1.printStackTrace();

55.             } catch (IOException e) {

56.                 e.printStackTrace();

57.             }

58.             Drawable d = Drawable.createFromStream(i, "src");

59.             return d;

60.         }

61.        

62.          public interface ImageCallback {

63.              public void imageLoaded(Drawable imageDrawable, String imageUrl);

64.          }

65.  

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：

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01. package cn.wangmeng.test;

02.  

03. public class ImageAndText {

04.         private String imageUrl;

05.         private String text;

06.  

07.         public ImageAndText(String imageUrl, String text) {

08.             this.imageUrl = imageUrl;

09.             this.text = text;

10.         }

11.         public String getImageUrl() {

12.             return imageUrl;

13.         }

14.         public String getText() {

15.             return text;

16.         }

17. }

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01. package cn.wangmeng.test;

02.  

03. import android.view.View;

04. import android.widget.ImageView;

05. import android.widget.TextView;

06.  

07. public class ViewCache {

08.  

09.         private View baseView;

10.         private TextView textView;

11.         private ImageView imageView;

12.  

13.         public ViewCache(View baseView) {

14.             this.baseView = baseView;

15.         }

16.  

17.         public TextView getTextView() {

18.             if (textView == null) {

19.                 textView = (TextView) baseView.findViewById(R.id.text);

20.             }

21.             return textView;

22.         }

23.  

24.         public ImageView getImageView() {

25.             if (imageView == null) {

26.                 imageView = (ImageView) baseView.findViewById(R.id.image);

27.             }

28.             return imageView;

29.         }

30.  

31. }

ViewCache是辅助获取adapter的子元素布局 

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01. package cn.wangmeng.test;

02.  

03. import java.util.List;

04.  

05. import cn.wangmeng.test.AsyncImageLoader.ImageCallback;

06.  

07. import android.app.Activity;

08. import android.graphics.drawable.Drawable;

09. import android.view.LayoutInflater;

10. import android.view.View;

11. import android.view.ViewGroup;

12. import android.widget.ArrayAdapter;

13. import android.widget.ImageView;

14. import android.widget.ListView;

15. import android.widget.TextView;

16.  

17. public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {

18.  

19.         private ListView listView;

20.         private AsyncImageLoader asyncImageLoader;

21.  

22.         public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {

23.             super(activity, 0, imageAndTexts);

24.             this.listView = listView;

25.             asyncImageLoader = new AsyncImageLoader();

26.         }

27.  

28.         public View getView(int position, View convertView, ViewGroup parent) {

29.             Activity activity = (Activity) getContext();

30.  

31.             // Inflate the views from XML

32.             View rowView = convertView;

33.             ViewCache viewCache;

34.             if (rowView == null) {

35.                 LayoutInflater inflater = activity.getLayoutInflater();

36.                 rowView = inflater.inflate(R.layout.image_and_text_row, null);

37.                 viewCache = new ViewCache(rowView);

38.                 rowView.setTag(viewCache);

39.             } else {

40.                 viewCache = (ViewCache) rowView.getTag();

41.             }

42.             ImageAndText imageAndText = getItem(position);

43.  

44.             // Load the image and set it on the ImageView

45.             String imageUrl = imageAndText.getImageUrl();

46.             ImageView imageView = viewCache.getImageView();

47.             imageView.setTag(imageUrl);

48.             Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {

49.                 public void imageLoaded(Drawable imageDrawable, String imageUrl) {

50.                     ImageView imageViewByTag = (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.             // Set the text on the TextView

62.             TextView textView = viewCache.getTextView();

63.             textView.setText(imageAndText.getText());

64.  

65.             return rowView;

66.         }

67.  

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：

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01. <?xml version="1.0" encoding="utf-8"?>

02. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

03.               android:orientation="horizontal"

04.               android:layout_width="fill_parent"

05.               android:layout_height="wrap_content">

06.  

07.         <ImageView android:id="@+id/image"

08.                    android:layout_width="wrap_content"

09.                    android:layout_height="wrap_content"

10.                    />

11.  

12.         <TextView android:id="@+id/text"

13.                   android:layout_width="wrap_content"

14.                   android:layout_height="wrap_content"/>

15.  

16. </LinearLayout>