ListView异步加载图片实现思路

public class AsyncImageLoader {
//Cache for image(Type String is the URL of image,the second parameter is soft reference)
private HashMap<String, SoftReference<Bitmap>> imageCache = null;
private Activity context;
public AsyncImageLoader(Activity context){
this.context = context;
imageCache = new HashMap<String, SoftReference<Bitmap>>();
}
public Bitmap loadImage(final ImageView imageView,final String imageURL,final ImageCallBack imageCallBack){
//If the cache contains the reference of bitmap then return
if (imageCache.containsKey(imageURL)) {
SoftReference<Bitmap> bitmapReference = imageCache.get(imageURL);
Bitmap bitmap = bitmapReference.get();
if (bitmap != null) {
return bitmap;
}
}
//Second cache,search local SD card
else {
String fileName = StringUtil.namePicture(imageURL);//获取文件名
boolean isExist = SystemUtils.findPhotoFromSDCard(Constant.INFO_PATH, fileName);
if (isExist) {//是否在SD卡存在图片
Bitmap bitmap = SystemUtils.getPhotoFromSDCard(Constant.INFO_PATH, fileName);
return bitmap;
}
}
final Handler myHandler = new Handler(){
@Override
public void handleMessage(Message msg)
{
imageCallBack.setImage(imageView, (Bitmap)msg.obj);
}
};
//If the bitmap not exists in cache or SD card,then get it from net
new Thread(){
@Override
public void run() {
// TODO Auto-generated method stub
boolean isNetwork = SystemUtils.checkNetwork(context);
if (isNetwork) {
InputStream photoStream = HttpRequest.getImageStream(imageURL);//这里是我自己写的一个类，目的是通过URL地址从服务器获取图片输入流
Bitmap bitmap;
try {
bitmap = ImageTools.getResizeBitmap(photoStream, 128, 128);
if (bitmap != null) {
String fileName = StringUtil.namePicture(imageURL);
//Save image to SD card
SystemUtils.savePhotoToSDCard(bitmap, fileName, Constant.INFO_PATH);
//Put soft reference to cache
imageCache.put(imageURL, new SoftReference<Bitmap>(bitmap));
//Send message to update UI
Message message = myHandler.obtainMessage(0, bitmap);
myHandler.sendMessage(message);
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}.start();
return null;
}
/**
* Interface for load image
* @author Ryan
*
*/
public interface ImageCallBack{
//Set image for imageview through bitmap
public void setImage(ImageView imageView,Bitmap bitmap);
}
}

Bitmap bitmap1 = asyncImageLoader.loadImage(viewHolder.imageView1, url1, new ImageCallBack() {
@Override
public void setImage(ImageView imageView, Bitmap bitmap) {
// TODO Auto-generated method stub
imageView.setImageBitmap(bitmap);
}
});
if (bitmap1 != null) {
viewHolder.imageView1.setImageBitmap(bitmap1);
}else {
viewHolder.imageView1.setImageResource(R.drawable.image_bg);
}