Android实现ListView异步加载图

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：
Java代码 
package cn.wangmeng.test; 
 
import java.io.IOException; 
import java.io.InputStream; 
import java.lang.ref.SoftReference; 
import java.net.MalformedURLException; 
import java.net.URL; 
import java.util.HashMap; 
 
import android.graphics.drawable.Drawable; 
import android.os.Handler; 
import android.os.Message; 
 
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() { 
                 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) { 
            URL m; 
            InputStream i = null; 
            try { 
                m = new URL(url); 
                i = (InputStream) m.getContent(); 
            } catch (MalformedURLException e1) { 
                e1.printStackTrace(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            Drawable d = Drawable.createFromStream(i, "src"); 
            return d; 
        } 
       
         public interface ImageCallback { 
             public void imageLoaded(Drawable imageDrawable, String imageUrl); 
         } 
 
} 

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：
Java代码 
package cn.wangmeng.test; 
 
public class ImageAndText { 
        private String imageUrl; 
        private String text; 
 
        public ImageAndText(String imageUrl, String text) { 
            this.imageUrl = imageUrl; 
            this.text = text; 
        } 
        public String getImageUrl() { 
            return imageUrl; 
        } 
        public String getText() { 
            return text; 
        } 
} 
Java代码 
package cn.wangmeng.test; 
 
import android.view.View; 
import android.widget.ImageView; 
import android.widget.TextView; 
 
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 textView; 
        } 
 
        public ImageView getImageView() { 
            if (imageView == null) { 
                imageView = (ImageView) baseView.findViewById(R.id.image); 
            } 
            return imageView; 
        } 
 
} 

ViewCache是辅助获取adapter的子元素布局
Java代码 
package cn.wangmeng.test; 
 
import java.util.List; 
 
import cn.wangmeng.test.AsyncImageLoader.ImageCallback; 
 
import android.app.Activity; 
import android.graphics.drawable.Drawable; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ArrayAdapter; 
import android.widget.ImageView; 
import android.widget.ListView; 
import android.widget.TextView; 
 
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(); 
        } 
 
        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); 
                    } 
                } 
            }); 
            if (cachedImage == null) { 
                imageView.setImageResource(R.drawable.default_image); 
            }else{ 
                imageView.setImageDrawable(cachedImage); 
            } 
            // Set the text on the TextView 
            TextView textView = viewCache.getTextView(); 
            textView.setText(imageAndText.getText()); 
 
            return rowView; 
        } 
 
} 

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：
Xml代码 
<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
              android:orientation="horizontal" 
              android:layout_width="fill_parent" 
              android:layout_height="wrap_content"> 
 
        <ImageView android:id="@+id/image" 
                   android:layout_width="wrap_content" 
                   android:layout_height="wrap_content" 
                   /> 
 
        <TextView android:id="@+id/text" 
                  android:layout_width="wrap_content" 
                  android:layout_height="wrap_content"/> 
 
</LinearLayout>