Ideal way to cancel an executing AsyncTask

Question

I am running remote audio-file-fetching and audio file playback operations in a background thread using AsyncTask  A Cancellable progress bar is shown for the time the fetch operation runs    I want to cancel abort the AsyncTask run when the user cancels  decides against  the operation  What is the ideal way to handle such a case

User · Answer

I don t like to force interrupt my async tasks with cancel true  unnecessarily because they may have resources to be freed  such as closing sockets or file streams  writing data to the local database etc  On the other hand  I have faced situations in which the async task refuses to finish itself part of the time  for example sometimes when the main activity is being closed and I request the async task to finish from inside the activity s onPause   method  So it s not a matter of simply calling running   false  I have to go for a mixed solution  both call running   false  then giving the async task a few milliseconds to finish  and then call either cancel false  or cancel true    if  backgroundTask    null        backgroundTask requestTermination        try           Thread sleep  int  0 5   1000          catch  InterruptedException e            e printStackTrace              if  backgroundTask getStatus      AsyncTask Status FINISHED            backgroundTask cancel false             backgroundTask   null      As a side result  after doInBackground   finishes  sometimes the onCancelled   method is called  and sometimes onPostExecute    But at least the async task termination is guaranteed

User · Answer

Simple  don t use an AsyncTask  AsyncTask is designed for short operations that end quickly  tens of seconds  and therefore do not need to be canceled   Audio file playback  does not qualify  You don t even need a background thread for ordinary audio file playback

User · Answer

Just discovered that AlertDialogs s boolean cancel       I ve been using everywhere actually does nothing  Great  So       public class MyTask extends AsyncTask lt Void  Void  Void gt         private volatile boolean running   true      private final ProgressDialog progressDialog       public MyTask Context ctx            progressDialog   gimmeOne ctx            progressDialog setCancelable true           progressDialog setOnCancelListener new OnCancelListener                  Override             public void onCancel DialogInterface dialog                       actually could set running   false  right here  but I ll                    stick to contract                  cancel true                                          Override     protected void onPreExecute             progressDialog show                Override     protected void onCancelled             running   false              Override     protected Void doInBackground Void    params             while  running                   does the hard work                   return null

User · Answer

This is how I write my AsyncTask the key point is add Thread sleep 1        Override   protected Integer doInBackground String    params             Log d TAG  PRE    url     params 0            Log d TAG  PRE    file name     params 1            downloadPath   params 1            int returnCode   SUCCESS          FileOutputStream fos   null          try               URL url   new URL params 0                File file   new File params 1                fos   new FileOutputStream file                long startTime   System currentTimeMillis                URLConnection ucon   url openConnection                InputStream is   ucon getInputStream                BufferedInputStream bis   new BufferedInputStream is                byte   data   new byte 10240                int nFinishSize   0              while  bis read data  0  10240     -1                   fos write data  0  10240                   nFinishSize    10240                    Thread sleep  1       this make cancel method work                   this publishProgress nFinishSize                                           data   null                  Log d TAG   download ready in                        System currentTimeMillis   - startTime    1000                        sec               catch  IOException e                    Log d TAG  PRE    Error      e                   returnCode   FAIL            catch  Exception e                    e printStackTrace                         finally              try                   if fos    null                      fos close                  catch  IOException e                    Log d TAG  PRE    Error      e                   e printStackTrace                                     return returnCode

User · Answer

The only way to do it is by checking the value of the isCancelled   method and stopping playback when it returns true

User · Answer

If you re doing computations    You have to check isCancelled   periodically    If you re doing a HTTP request    Save the instance of your HttpGet or HttpPost somewhere  eg  a public field   After calling cancel  call request abort    This will cause IOException be thrown inside your doInBackground    In my case  I had a connector class which I used in various AsyncTasks  To keep it simple  I added a new abortAllRequests method to that class and called this method directly after calling cancel

User · Answer

With reference to Yanchenko s answer on 29 April  10  Using a  while running   approach is neat when your code under  doInBackground  has to be executed multiple times during every execution of the AsyncTask  If your code under  doInBackground  has to be executed only once per execution of the AsyncTask  wrapping all your code under  doInBackground  in a  while running   loop will not stop the background code  background thread  from running when the AsyncTask itself is cancelled  because the  while running   condition will only be evaluated once all the code inside the while loop has been executed at least once  You should thus either   a   break up your code under  doInBackground  into multiple  while running   blocks or   b   perform numerous  isCancelled  checks throughout your  doInBackground  code  as explained under  Cancelling a task  at https   developer android com reference android os AsyncTask html   For option  a   one can thus modify Yanchenko s answer as follows   public class MyTask extends AsyncTask lt Void  Void  Void gt     private volatile boolean running   true           Override protected void onCancelled         running   false      Override protected Void doInBackground Void    params            does the hard work      while  running               part 1 of the hard work            while  running               part 2 of the hard work                        while  running               part x of the hard work           return null              For option  b   your code in  doInBackground  will look something like this   public class MyTask extends AsyncTask lt Void  Void  Void gt             Override protected Void doInBackground Void    params            part 1 of the hard work                if  isCancelled     return null           part 2 of the hard work                if  isCancelled     return null                       part x of the hard work                if  isCancelled     return null

User · Answer

Our global AsyncTask class variable       LongOperation LongOperationOdeme   new LongOperation      And KEYCODE BACK action which interrupt AsyncTask      Override     public boolean onKeyDown int keyCode  KeyEvent event            if  keyCode    KeyEvent KEYCODE BACK                LongOperationOdeme cancel true                     return super onKeyDown keyCode  event           It works for me

User · Answer

The thing is that AsyncTask cancel   call only calls the onCancel function in your task  This is where you want to handle the cancel request   Here is a small task I use to trigger an update method  private class UpdateTask extends AsyncTask lt Void  Void  Void gt             private boolean running   true            Override         protected void onCancelled                 running   false                      Override         protected void onProgressUpdate Void    values                super onProgressUpdate values               onUpdate                        Override         protected Void doInBackground Void    params                 while running                     publishProgress                                return null

[android] Ideal way to cancel an executing AsyncTask

Examples related to android

Examples related to android-asynctask