我的应用程序中有一个自定义列表视图。我使用 http 请求以 JSON 格式获取数据。有一个异步任务来加载列表视图的整个数据（图像除外）。
在这个异步任务的 onpostexecute() 中，我调用了另一个异步任务来获取列表视图的图像。
现在我正在尝试使用 textchangedlistener 在列表视图上实现搜索功能
问题是图像在后台加载，我经常遇到空指针异常。
谁能建议我摆脱这种情况的方法。我目前正在做的是在加载图像之前我不显示搜索框编辑文本。但这会让用户等待很多，尤其是对于长列表。
整个代码太长，无法发布。所以，我只是在这里粘贴它的简要概述。如果您需要其他任何东西，请告诉我。我也会这样做。
public class abcd extends Activity {
ListView todlistview;
List<HashMap<String, Object>> cnt = null;
ArrayList<HashMap<String, Object>> searchResults; /
.
.
/** AsyncTask to parse json data and load ListView */
private class ListViewLoaderTask extends AsyncTask<String, Void, SimpleAdapter>{
@Override
protected SimpleAdapter doInBackground(String... strJson) {
.
.
.
adapter = new SimpleAdapter(getBaseContext(), cnt, com.example.promoapp.R.layout.textviewfield, from, to);
return adapter;
}
@Override
protected void onPostExecute(SimpleAdapter adapter) {
searchResults=new ArrayList<HashMap<String, Object>> (cnt);
// Setting adapter for the listview
todlistview.setAdapter(adapter);
for(int i=0;i<adapter.getCount();i++){
HashMap<String, Object> hm = (HashMap<String, Object>) adapter.getItem(i);
.
.
ImageLoaderTask imageLoaderTask = new ImageLoaderTask();
// Starting ImageLoaderTask to download and populate image in the listview
imageLoaderTask.execute(hm);
}
}
}
/** AsyncTask to download and load an image in ListView */
private class ImageLoaderTask extends AsyncTask<HashMap<String, Object>, Void, HashMap<String, Object>>{
Dialog dil;
@Override
protected HashMap<String, Object> doInBackground(HashMap<String, Object>... hm) {
InputStream iStream=null;
.
// Create a hashmap object to store image path and its position in the listview
// Storing the path to the temporary image file
// Storing the position of the image in the listview
// Returning the HashMap object containing the image path and position
}
@Override
protected void onPostExecute(HashMap<String, Object> result) {
}
}
private TextWatcher filterTextWatcher = new TextWatcher() {
@Override
public void onTextChanged(CharSequence s, int start, int before,
int count) {
String searchString=Search.getText().toString();
int textLength=searchString.length();
searchResults.clear();
for(int i=0;i<cnt.size();i++)
{
String placeName=cnt.get(i).get("country").toString();
if(textLength<=placeName.length()){
//compare the String in EditText with Names in the ArrayList
if(searchString.equalsIgnoreCase(placeName.substring(0,textLength))){
//Toast.makeText(getApplicationContext(),placeName,1).show();
searchResults.add(cnt.get(i));
}
}
}
String[] from = { "country","flag","details"};
int[] to = { com.example.promoapp.R.id.tv_country,com.example.promoapp.R.id.iv_flag,com.example.promoapp.R.id.tv_country_details};
adapter = new SimpleAdapter(getBaseContext(), searchResults, com.example.promoapp.R.layout.textviewfield, from, to);
todlistview.setAdapter(adapter);
adapter.notifyDataSetChanged();
}
};
}