Android实现ListView异步加载图片

转自：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 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：

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;
	    }
}

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 

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 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>