AsyncTask Cancel is not working

Question

I am learning how to cancel asyncTask so there is no uses on the code below.

I tried to called the asyncTask and execute it then cancel it and execute it.

        MyAsyncTask asyncTask = new MyAsyncTask();
        Log.i("cancel","cancel 1");     
        asyncTask.execute();
        Log.i("cancel","cancel 2");
        asyncTask.onCancelled();
        Log.i("cancel","cancel 3");
        asyncTask.execute();
        Log.i("cancel","cancel done");

"cancel 1" and "cancel 2" is executed finely as it shown on logcat but ActivityThread.performLaunchActivity error is thrown when "cancel 3" is trying to execute. (cancel 3 is not showing on logcat) Anything wrong with my asyncTask code?

private class MyAsyncTask extends AsyncTask<String,Integer,Void>{

    private ArrayList<Map<String, String>> list;
    private ProgressBar progressBar;
    @Override
    protected Void doInBackground(String... arg0) {

        progressBar = (ProgressBar)findViewById(R.id.year_expense_progressbar);
        progressBar.setVisibility(View.VISIBLE);
        getListView().setVisibility(View.GONE);
        textView.setVisibility(View.GONE);

        list = new ArrayList<Map<String, String>>();

        String time = "";
        String category = "";
        String expense = "";
        String day = "";
        String month = "";


        totalExpense = 0;       


        Cursor c = SQLLiteAdapter.fetchAllItems();

        while(c.moveToNext() != false){

            if(isCancelled()){
                Log.e("cancel","cancel inside background");
                break;
            }


            // if there is nothing is input, don't execute verifyLevel
            if(tokenizedResult.isEmpty()) break;


            category = c.getString(c.getColumnIndex(SQLLiteAdapter.col_category));
            expense = c.getString(c.getColumnIndex(SQLLiteAdapter.col_price));
            time = c.getString(c.getColumnIndex(SQLLiteAdapter.col_time));
            day = c.getString(c.getColumnIndex(SQLLiteAdapter.col_day));
            month = c.getString(c.getColumnIndex(SQLLiteAdapter.col_month));


            VerifyLevel a = new VerifyLevel(tokenizedResult,category,expense,time,day,month); 
            if(!a.isEmpty()){
                list.add(a.addToList());
            }           

            totalExpense += a.totalExpense;          
        }



        return null;
    }

    @Override
    protected void onPostExecute(Void result)
    {
        progressBar.setVisibility(View.GONE);
        getListView().setVisibility(View.VISIBLE);
        textView.setVisibility(View.VISIBLE);
        fillData(list);
        textView.setText("Total Expense is "+convertNumeric(totalExpense));         
        Log.i("yearExpense","buildList is finished");

    }

    @Override
    protected void onCancelled(){
        super.onCancelled();
        list.clear();
        progressBar.setVisibility(View.GONE);
        totalExpense = 0;
        Log.i("yearEx","Cancel asyncTask");
        Toast.makeText(getApplicationContext(), "cancelled", Toast.LENGTH_SHORT).show();

    }       
}

a good post on how to cancel an asynctask http://www.quicktips.in/correct-way-to-cancel-an-asynctask-in-android/ — Deepak Swami, Jun 29 '16 at 17:21

score 14 · Accepted Answer · answered Apr 11 '13 at 11:36

My approach is slightly different and perhaps, a little lengthy. But it has always worked without any issues.

This piece of code goes in the doInBackground(). If you have a for... loop in it, then use the break; statement. If you do not use a for.... loop, remove it.

// CHECK IF THE USER HAS CANCELLED THE TASK
if (isCancelled())  {
    cancelTask = true; // (OPTIONAL) THIS IS AN ADDITIONAL CHECK I HAVE USED. THINK OF IT AS A FAIL SAFE.
    break; // REMOVE IF NOT USED IN A FOR LOOP
}

You already have an Asynctask declared: MyAsyncTask asyncTask = new MyAsyncTask();. Along with that, also create two instances of boolean. Call them, for example:

boolean blMyAsyncTask;
boolean cancelTask;

Now, in the onPreExecute(), toggle the status of the blMyAsyncTask instance:

blMyAsyncTask = true;

And in the onPostExecute():

blMyAsyncTask = false;

And, in the same onPostExecute(), I also do the remainder functions after checking the state of the cancelTask boolean. For example:

if (cancelTask == false)    {
    // THE NORMAL CODE YOU HAVE IN YOUR onPostExecute()
}

Finally, in the onDestroy() (I use this, but I suspect the onPause() could work too. Never done it in the onPause() in all honesty), check the status of the boolean blMyAsyncTask

if (blMyAsyncTask== true)   {
    asyncTask.cancel(true);
}

As I said at the start, it is lengthy, perhaps even complicated, but it has never failed. I also think of this as a little modular if you would. If I have more Asycntasks added to the Activity, I can add another check in the onDestroy().

@edisonTHK: Thank you. Glad I could be of help. By the way, does it work now? — Siddharth Lele, Apr 11 '13 at 11:47

score 3 · Answer 2 · answered Apr 11 '13 at 11:13

3

you should not call asyncTask.onCancelled(); directly. You can call cancel(true) and inside your doInBackground() you check for isCancelled().

answered Apr 11 '13 at 11:13

Budius

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score 0 · Answer 3 · answered Apr 11 '13 at 11:13

0

To cancel a AsyncTask call the below line.

asyncTask.cancel();

onCancelled() is a override method that is executed when ever cancel is called.

answered Apr 11 '13 at 11:13

Shankar Agarwal

34,573
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score 0 · Answer 4 · answered Apr 11 '13 at 11:18

0

You can use either

asynctask.cancel(true);

or

asyncTask.wait();

"true " if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

answered Apr 11 '13 at 11:18

Muhamed Riyas M

5,055
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AsyncTask Cancel is not working

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