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(, 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, , 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