Android runOnUiThread crash when pressing Back button

Question

I am using the DownloadManager to download files from a server, I'm doing it from an adapter, and I'm showing the download progress in the fragment using a runOnUiThread. it's working great, but when I press the back button while downloading, the runOnUiThread throws a NullPointerException. I tried putting a variable in the onPause with an if statement to stop the method but it doesn't work either.

what do you recommend to do to solve this crash?

here's my code:

new Thread(new Runnable() {
                        @Override
                        public void run() {

                            downloading = true;

                            while (downloading) {

                                DownloadManager.Query q = new DownloadManager.Query();
                                q.setFilterById(myDownloadReference);

                                Cursor cursor = downloadManager.query(q);
                                cursor.moveToFirst();
                                int bytes_downloaded = cursor.getInt(cursor
                                        .getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
                                int bytes_total = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));

                                if (cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS)) == DownloadManager.STATUS_SUCCESSFUL) {
                                    downloading = false;
                                    downloadingDocs.remove(doc);

                                    //remove the progressBar when download is complete
                                    fragment.getActivity().runOnUiThread(new Runnable() {
                                        @Override
                                        public void run() {
                                            finalHolder2.downloadProgress.setVisibility(View.INVISIBLE);
                                        }
                                    });

                                    //check if it's the last download and open it.
                                    if (downloadingDocs.size() == 0) {
                                        actionIntentOpenDocs(doc, saveFile);
                                    }
                                }
                                //final double dl_progress = (bytes_downloaded / bytes_total) * 100;
                                final int dl_progress = (int) ((bytes_downloaded * 100l) / bytes_total);

                                //display the download progress
                                fragment.getActivity().runOnUiThread(new Runnable() {
                                    @Override
                                    public void run() {
                                        if(Constants.stopUiThreadWork == null) {
                                            finalHolder2.downloadProgress.setProgress((int) dl_progress);
                                        }
                                    }
                                });
                                cursor.close();
                            }
                        }
                    }).start();

it's crashing on this line :

fragment.getActivity().runOnUiThread(new Runnable() {

Crash stack trace :

FATAL EXCEPTION: Thread-4807
Process: com.appus.app, PID: 6553
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.FragmentActivity.runOnUiThread(java.lang.Runnable)' on a null object reference
        at com.availo.app.UI.adapters.MainFragmentDocumentAdapter$1$1.run(MainFragmentDocumentAdapter.java:265)
        at java.lang.Thread.run(Thread.java:818)

Answer 1

The best way to manage this kind of thing is through a Service. The reason is that a service can better manage background threads, even if the activity isn't running, while things like an AsyncTask will stop working when the activity is done. Take a look at this site on how to set up a Download Service.