Android实现ListView异步加载图片

当listview需要从网络上加载图片的时候，我们就需要考虑用异步加载。我们可以用AsyncTask来解决问题，AsyncTask专门被设计来处理这种情况，但是AsyncTask的缺陷是最多只能同时有10个线程，而listview一次需要显示的图片可能不止10个。因此我们考虑为每一张图片开启一个加载线程，主方法的代码如下：

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>

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

AsyncListImage.zip (50.5 KB)