当用户在listview中滚动到底部时，我通过api从web加载数据，但当我在listview上滚动一次时，它在我的活动中多次调用asynctask，这会导致listview中的数据重复，在出现异常的情况下，活动中会有很多对话框和toast，当我滚动到底部并只需触摸滚动上的listview时，方法get fires调用异步任务多次出现，请告诉我如何防止这种情况。
这是我的onscroll方法代码。
listView.setOnScrollListener(new OnScrollListener() {
// boolean mIsScrollingUp;
// int mLastFirstVisibleItem;
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
// TODO Auto-generated method stub
if(scrollState == OnScrollListener.SCROLL_STATE_TOUCH_SCROLL)
userscrolled=true;
}
@Override
public void onScroll(AbsListView view, int firstVisibleItem,
int visibleItemCount, int totalItemCount) {
// TODO Auto-generated method stub
int lasinscren = firstVisibleItem + visibleItemCount;
if(userscrolled&&(lasinscren==totalItemCount)&&!lodinmore&&(visibleItemCount<totalItemCount)&&havedata)
{
progress = true;
if(Search_API==false){
if(connectionDetector.isConnectintoInternet()){
System.out.println("inside listview on scroll function");
darList_task = new DAR_list_task(DAR_Activity.this).execute();
lodinmore = true;
}else
{
Toast.makeText(getApplicationContext(), "No internet connection", Toast.LENGTH_SHORT).show();
}
}else
{
new search_task(DAR_Activity.this).execute();
}
}
}
});
这是我的异步任务
public class DAR_list_task extends AsyncTask<Void, Void, Void>
{
Context activity;
public DAR_list_task(Context activity) {
// TODO Auto-generated constructor stub
this.activity = activity;
}
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
if(progress==true){
if(limit==0)
{
dialog = new Dialog(DAR_Activity.this);
dialog.getWindow().requestFeature(Window.FEATURE_NO_TITLE);
dialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
LinearLayout linearLayout = (LinearLayout)getLayoutInflater().inflate(R.layout.dialog_progress, null);
dialog.setContentView(linearLayout);
ProgressBar progressBar1 = (ProgressBar)findViewById(R.id.progress_dialog);
dialog.setCancelable(false);
dialog.getWindow().clearFlags(WindowManager.LayoutParams.FLAG_DIM_BEHIND);
dialog.setCanceledOnTouchOutside(false);
dialog.show();
dialog.getWindow().setGravity(Gravity.CENTER);
}else
{
progressBar.setVisibility(View.VISIBLE);
}
}
}
@Override
protected Void doInBackground(Void... params) {
// TODO Auto-generated method stub
response=null;
lodinmore = true;
URL Url;
try {
byte[] data;
data = login_id.getBytes("UTF-8");
String login_base_64 = Base64.encode(data);
data = gcm_id.getBytes("UTF-8");
String gcm_base64= Base64.encode(data);
if(url.contentEquals(""))
{
Url = new URL(getResources().getString(R.string.dar_list_view));
}else
{
Url = new URL("http://"+url+"/smart_oms/dar_app/view_dar.php");
}
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
nameValuePairs.add(new BasicNameValuePair("login_id", login_base_64));
nameValuePairs.add(new BasicNameValuePair("gcm_id", gcm_base64));
nameValuePairs.add(new BasicNameValuePair("limit", String.valueOf(limit)));
HttpURLConnection httpURLConnection = (HttpURLConnection)Url.openConnection();
httpURLConnection.setRequestMethod("POST");
httpURLConnection.setUseCaches(false);
httpURLConnection.setDoOutput(true);
OutputStream os = httpURLConnection.getOutputStream();
BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
bufferedWriter.write(getQueryString(nameValuePairs));
bufferedWriter.flush();
bufferedWriter.close();
os.close();
httpURLConnection.connect();
InputStream inputStream = httpURLConnection.getInputStream();
response = BufferReaderMaker.readContentFromIS(inputStream);
}catch( final UnknownHostException ex)
{
runOnUiThread(new Runnable() {
public void run() {
Toast.makeText(getApplicationContext(), ex.getMessage(), Toast.LENGTH_SHORT).show();
}
});
}catch (final ConnectTimeoutException e) {
// TODO: handle exception
runOnUiThread(new Runnable() {
public void run() {
Toast.makeText(getApplicationContext(), e.getMessage(), Toast.LENGTH_SHORT).show();
}
});
}
catch (Exception e) {
// TODO: handle exception
}
return null;
}
@Override
protected void onPostExecute(Void result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
Object json = null;
try {
if(response!=null)
{
if(progress==false)
{
dar_List_Items.clear();
dar_Aadapter = new DAR_Aadapter(DAR_Activity.this, dar_List_Items);
listView.setAdapter(dar_Aadapter);
dar_Aadapter.notifyDataSetChanged();
}
try {
json = new JSONTokener(response).nextValue();
} catch (JSONException e1) {
// TODO Auto-generated catch block
ErrorDialog errorDialog = new ErrorDialog();
errorDialog.Dialog(activity, statusCode, response, login_id, "", new DAR_list_task(activity));
}
if(json instanceof JSONArray)
{
havedata=false;
Toast.makeText(getApplicationContext(), "No More DAR", 0).show();
}else if(json instanceof JSONObject)
{
JSONObject jsonObject = new JSONObject(response);
JSONArray jsonArray = jsonObject.getJSONArray("view_dar_detail");
for(int i =0 ;i<jsonArray.length();)
{
JSONObject json_data = jsonArray.getJSONObject(i);
String dar_id = json_data.optString("customer_id");
String Customer_name = json_data.optString("customer_name");
String Contacted= json_data.optString("contact_person_name");
String product = json_data.optString("product");
String status = json_data.optString("dar_status");
String Contact_type = json_data.optString("contact_type");
String created_date = json_data.optString("dar_created_date");
String request_date = json_data.optString("requeste_date");
DAR_List_Item dar_List_Item = new DAR_List_Item(dar_id,Customer_name, Contacted, Contact_type, product, status, created_date, request_date,"abcd");
dar_List_Items.add(dar_List_Item);
i++;
}
int index = listView.getFirstVisiblePosition();
View v= listView.getChildAt(0);
int top = (v == null) ? 0 : v.getTop();
listView.setAdapter(null);
DAR_Aadapter aadapter = new DAR_Aadapter(DAR_Activity.this, dar_List_Items);
listView.setAdapter(aadapter);
aadapter.notifyDataSetChanged();
listView.setSelectionFromTop(index, top);
}else
{
ErrorDialog errorDialog = new ErrorDialog();
errorDialog.Dialog(activity, statusCode, response, login_id, "", new DAR_list_task(activity));
}
lodinmore = false;
}
lodinmore = false;
} catch (JSONException e) {
// TODO Auto-generated catch block
ErrorDialog errorDialog = new ErrorDialog();
errorDialog.Dialog(activity, statusCode, response, login_id, "", new DAR_list_task(activity));
}
if(progress==true){
if(limit==0)
{
dialog.dismiss();
}else{
progressBar.setVisibility(View.GONE);
}
}
if(json instanceof JSONObject)
limit = limit +10;
}
}
p请帮助我，我已经尝试了很多方法，比如使用标志值等等，但都不适合我。
如何防止多次调用异步任务？当我滚动到底部并只触摸列表时，如何防止asynctask被触发？
有一个解决办法：
final int lastItem = firstVisibleItem + visibleItemCount;
if(lastItem == totalItemCount) {
if(lastLastitem !=lastItem){ //to avoid multiple calls for last item, declare it as a field in your Activity
lastLastitem = lastItem;
// Your async task here
}
一个简单的方法是定义一个布尔参数：
if(view.getLastVisiblePosition() == totalItemCount-1 && isLoadMore){
isLoadMore = false;
pageNum++;
loadMoreData(searchStr, donViID, pageNum, Constant.RECORD_PER_PAGE);
}