Android实现异步加载图片 ListView

引用

http://blog.jteam.nl/2009/09/17/exploring-the-world-of-android-part-2

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：

package com.dragon.example;  
  
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： 
package com.dragon.example;

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;  
        }  
}  


package com.dragon.example;    

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 
package com.dragon.example; 

import java.util.List;  
  
import com.dragon.example.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>