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.

Solution 1

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();

Solution 2

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
}

Solution 3

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<Void, Integer, Boolean>{
    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<Void, Integer, Boolean> 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 < 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);
    }
}

Solution 4

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();

Solution 5

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<String, String, String>
    {
        @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();
        }

    }

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