ListView 的图片下载，优化造成的跳变

public class MainActivity extends Activity {
HashMap<Integer,Bitmap> map = new HashMap<Integer,Bitmap>();
private ListView listview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listview = (ListView)findViewById(R.id.listView1);
MyApapter myApapter = new MyApapter();
listview.setAdapter(myApapter);
}
class User
{
ImageView imageview;
ProgressBar pb;
TextView tv;
Button button;
}
class MyApapter extends BaseAdapter
{
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view = null;
User user = new User();
LayoutInflater inflater = getLayoutInflater();
if(convertView==null)
{
view = inflater.inflate(R.layout.itmp, null);
user.imageview = (ImageView)view.findViewById(R.id.imageView1);
user.pb = (ProgressBar)view.findViewById(R.id.progressBar1);
user.button = (Button)view.findViewById(R.id.button1);
user.tv = (TextView)view.findViewById(R.id.textView1);
view.setTag(user);
}else
{
view = convertView;
user = (User)view.getTag();
}
//下载图片
//设置图片还未开始下载时的状态
user.imageview.setImageResource(R.drawable.ic_launcher);
Bitmap bitmap = map.get(position);
if (bitmap==null)
{
MyAsyncTask myimageview = new MyAsyncTask(user.imageview,position,user.pb);
myimageview.execute("http://192.168.56.1:8080/service/qq"+position+".png");
}else
{
user.imageview.setImageBitmap(bitmap);
}
return view;
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return 10;
}

@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return null;
}

@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return 0;
}
}
class MyAsyncTask extends AsyncTask<String, Void, Bitmap>
{
int position;
ImageView imageview;
ProgressBar pb;
public MyAsyncTask(ImageView imageview,int position,ProgressBar pb) {
this.imageview = imageview;
this.position = position;
this.pb = pb;
}
@Override
protected Bitmap doInBackground(String... params) {
Bitmap bitmap = null;
try {
URL url = new URL(params[0]);
URLConnection connection = url.openConnection();
InputStream is = connection.getInputStream();
bitmap = BitmapFactory.decodeStream(is);
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap result) {
//得到当前屏幕可见第一行
int fvb = listview.getFirstVisiblePosition();
int lvp = listview.getLastVisiblePosition();
if (position>=fvb&&position<=lvp)
{
imageview.setImageBitmap(result);
}
map.put(position, result);
super.onPostExecute(result);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}