Android Cancel Async Task

Question

I use an async task to upload an image and get some results   While uploading the image I see a progress dialog  written in onPreExecute   method like this       protected void onPreExecute               uploadingDialog   new ProgressDialog MyActivity this             uploadingDialog setMessage  uploading              uploadingDialog setCancelable true            uploadingDialog show            Ok when I press the back button  obviously the dialog disappears because of the setCancelable true    But  obviously  the async task doesn t stop   So how can I fix this  I want to cancel both dialog and async task when I press the back button  Any ideas   EDIT  FOUND THE SOLUTION  SEE MY ANSWER BELOW

User · Answer

I had a similar problem - essentially I was getting a NPE in an async task after the user had destroyed the activity. After researching the problem on Stack Overflow, I adopted the following solution:

volatile boolean running;

public void onActivityCreated (Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    running=true;
    ...
    }


public void onDestroy() {
    super.onDestroy();

    running=false;
    ...
}

Then, I check "if running" periodically in my async code. I have stress tested this and I am now unable to "break" my activity. This works perfectly and has the advantage of being simpler than some of the solutions I have seen on SO.

User · Answer

I would like to improve the code  When you canel the aSyncTask the onCancelled    callback method of aSyncTask  gets automatically called  and there you can hide your progressBarDialog   You can include this code as well   public class information extends AsyncTask lt String  String  String gt                 Override         protected void onPreExecute                 super onPreExecute                        Override         protected String doInBackground String    arg0                return null                      Override         protected void onPostExecute String result                super onPostExecute result               this cancel true                       Override         protected void onProgressUpdate String    values                super onProgressUpdate values                       Override         protected void onCancelled                 Toast makeText getApplicationContext     asynctack cancelled        Toast LENGTH SHORT  show                dialog hide      hide the progressbar dialog here                  super onCancelled

User · Answer

Most of the time that I use AsyncTask my business logic is on a separated business class instead of being on the UI  In that case  I couldn t have a loop at doInBackground    An example would be a synchronization process that consumes services and persist data one after another   I end up handing on my task to the business object so it can handle cancelation  My setup is like this   public abstract class MyActivity extends Activity        private Task mTask      private Business mBusiness       public void startTask             if  mTask    null                mTask cancel true                     mTask   new mTask            mTask execute             protected class Task extends AsyncTask lt Void  Void  Boolean gt         Override     protected void onCancelled             super onCancelled             mTask cancel true               ask if user wants to try again             Override     protected Boolean doInBackground Void    params            return mBusiness synchronize this               Override     protected void onPostExecute Boolean result            super onPostExecute result            mTask   null           if  result                   done                    else                  ask if user wants to try again                    public class Business       public boolean synchronize AsyncTask lt         gt  task            boolean response   false          response   loadStuff task            if  response              response   loadMoreStuff task            return response             private boolean loadStuff AsyncTask lt         gt  task            if  task    null  amp  amp  task isCancelled    return false              load stuff          return true

User · Answer

create some member variables in your activity like   YourAsyncTask mTask  Dialog mDialog    use these for your dialog and task   in onPause    simply call   if mTask  null  mTask cancel     if mDialog  null  mDialog dismiss

User · Answer

FOUND THE SOLUTION  I added an action listener before uploadingDialog show   like this       uploadingDialog setOnCancelListener new DialogInterface OnCancelListener              public void onCancel DialogInterface dialog                  myTask cancel true                   finish                          That way when I press the back button  the above OnCancelListener cancels both dialog and task  Also you can add finish   if you want to finish the whole activity on back pressed  Remember to declare your async task as a variable like this       MyAsyncTask myTask null    and execute your async task like this       myTask   new MyAsyncTask        myTask execute

User · Answer

How to cancel AsyncTask  Full answer is here - Android AsyncTask Example  AsyncTask provides a better cancellation strategy  to terminate currently running task   cancel boolean mayInterruptIfitRunning   myTask cancel false - It makes isCancelled returns true  Helps to cancel the task   myTask cancel true      It also makes isCancelled   returns true  interrupt the background thread and relieves resources    It is considered as an arrogant way  If there is any thread sleep   method performing in background thread  cancel true  will interrupt background thread at that time  But cancel false  will wait for it and cancel task when that method completes   If you invoke cancel   and doInBackground   hasn   t begun execute yet  onCancelled   will invoke   After invoking cancel      you should check value returned by isCancelled   on doInbackground   periodically  just like shown below   protected Object doInBackground Params    params       while  condition         if  isCancelled     break    return null

User · Answer

From SDK      Cancelling a task      A task can be cancelled at any time by invoking cancel boolean      Invoking this method will cause subsequent calls to isCancelled      to return true      After invoking this method  onCancelled Object   instead of    onPostExecute Object  will be invoked after doInBackground Object    returns     To ensure that a task is cancelled as quickly as possible     you should always check the return value of isCancelled   periodically from    doInBackground Object     if possible  inside a loop for instance     So your code is right for dialog listener   uploadingDialog setOnCancelListener new DialogInterface OnCancelListener         public void onCancel DialogInterface dialog            myTask cancel true             finish                Now  as I have mentioned earlier from SDK  you have to check whether the task is cancelled or not  for that you have to check isCancelled   inside the onPreExecute   method   For example   if  isCancelled         break  else         do your work here

User · Answer

You can just ask for cancellation but not really terminate it  See this answer

User · Answer

I spent a while figuring this out  all I wanted was a simple example of how to do it  so I thought I d post how I did it  This is some code that updates a library and has a progress dialog showing how many books have been updated and cancels when a user dismisses the dialog   private class UpdateLibrary extends AsyncTask lt Void  Integer  Boolean gt       private ProgressDialog dialog   new ProgressDialog Library this       private int total   Library instance appState getAvailableText   length      private int count   0         Used as handler to cancel task if back button is pressed     private AsyncTask lt Void  Integer  Boolean gt  updateTask   null        Override     protected void onPreExecute            updateTask   this          dialog setProgressStyle ProgressDialog STYLE HORIZONTAL           dialog setOnDismissListener new OnDismissListener                                 Override             public void onDismiss DialogInterface dialog                    updateTask cancel true                                     dialog setMessage  Updating Library               dialog setMax total           dialog show                Override     protected Boolean doInBackground Void    arg0                for  int i   0  i  lt  appState getAvailableText   length i                     if isCancelled                         break                                      Do your updating stuff here                               Override     protected void onProgressUpdate Integer    progress           count    progress 0           dialog setProgress count               Override     protected void onPostExecute Boolean finished           dialog dismiss            if  finished              DialogHelper showMessage Str TEXT UPDATELIBRARY  Str TEXT UPDATECOMPLETED  Library instance           else              DialogHelper showMessage Str TEXT UPDATELIBRARY Str TEXT NOUPDATE   Library instance

[android] Android: Cancel Async Task

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