Android实现ListView异步加载图片\AsyncListImage

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 {

    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"?>

    <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"/>