Question

I am currently working on an app that runs an AsyncTask in the background and updates the UI with a progressbar. The progress bar works fine when the app is running, however, when the user exits the app and re-enters, the AsyncTask is still running in the background, but the progress bar won’t update. It’s as if the AsyncTask has detached itself from the activity. Does anyone have any ideas what might be causing this such as general rules involved with AsyncTasks. I can provide code if needed, but it is rather lengthy, so just let me know what parts you would need to see. I should also note that the AsyncTask does complete, I can tell this because it uploads a database to the server when it finishes.

Here is the code:

public class BackgroundAsyncTask extends AsyncTask {
int myProgress;

    @Override
    protected void onPostExecute(Void result) {
        ((TextView) findViewById(R.id.tv1)).setText("");
        Cursor cur = sql3.query("videohashes", null, null, null, null,
                null, null);
        cur.moveToFirst();
        while (!cur.isAfterLast()) {

            Cursor curFrame = sql3.query("table_" + cur.getString(2), null,
                    null, null, null, null, null);
            curFrame.moveToFirst();

            ((TextView) findViewById(R.id.tv1)).append("\nPath: "
                    + cur.getString(1) + "\nHash: " + cur.getString(2)
                    + "\nDate:" + cur.getString(3) + "\nSize: "
                    + cur.getString(4) + " bytes\nResolution"
                    + cur.getString(5) + "\nFormat: " + cur.getString(6)
                    + "\nCodec: " + cur.getString(7) + "\nFPS: "
                    + cur.getString(8) + "\n\nFirst Frame Info:\nType: "
                    + curFrame.getString(1) + "\ncp_num: "
                    + curFrame.getString(2) + "\ndp_num: "
                    + curFrame.getString(3) + "\npts: "
                    + curFrame.getString(4) + "\nqstride: "
                    + curFrame.getString(5) + "\nsize: "
                    + curFrame.getString(6) + "\nqp_stddev: "
                    + curFrame.getString(7) + "\ncount: "
                    + curFrame.getString(8) + "\nqp_avg: "
                    + curFrame.getString(9) + "\n\n");

            cur.moveToNext();
        }
        cur.close();
        ((Button) findViewById(R.id.btnSend)).setEnabled(true);
        ((Button) findViewById(R.id.btnStart)).setEnabled(true);
        sql3.close();
        sharedPreferences.edit().putString("lastVideoInfo", ((TextView) findViewById(R.id.tv1)).getText().toString()).commit();
        sharedPreferences.edit().putBoolean("asyncTaskRunning", false).commit();
        dateNow = new Date();
        super.onPostExecute(result);
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        totProgress = 0;
        currVid = 0;
        curProgress = 0;
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        progress.setProgress(values[0]);
    }

    @Override
    protected Void doInBackground(Void... arg0) {

        // Calculate total size of all files
        for (String path : myFiles) {
            totProgress += getFileSize(path);
        }

        progress.setMax(totProgress);
        String strDB3File = getFilesDir().getPath() + "/VideoHashes.db3";
        sql3 = SQLiteDatabase.openDatabase(strDB3File, null,
                SQLiteDatabase.CREATE_IF_NECESSARY);

        try {
            String mysql = "CREATE TABLE IF NOT EXISTS videohashes (id INTEGER  NOT NULL PRIMARY KEY AUTOINCREMENT, path TEXT NOT NULL, hash TEXT NOT NULL, date TEXT NOT NULL, size INTEGER, resolution TEXT NOT NULL, codec TEXT NOT NULL, format TEXT NOT NULL, fps TEXT NOT NULL)";
            sql3.execSQL(mysql);
        } catch (SQLiteException e) {
            // TODO: handle exception
        }

        for (String path : myFiles) {
            try {

                String hash = getMD5Checksum(path);

                Cursor curFrame = sql3.query("videohashes",
                        new String[] { "hash" }, "hash=?",
                        new String[] { hash }, null, null, null);

                if (!curFrame.moveToFirst()) {
                    ContentValues myInsertData = new ContentValues();
                    myInsertData.put("path", path);
                    myInsertData.put("hash", hash);
                    Date date = new Date();
                    myInsertData.put("date", dateFormat.format(date));
                    myInsertData.put("size", getFileSize(path));

                    naInit(path);
                    Log.i("VPMA", "After naInit");
                    int[] prVideoRes = naGetVideoResolution();
                    myInsertData.put("resolution", prVideoRes[0] + "x"
                            + prVideoRes[1]);
                    String prVideoCodecName = naGetVideoCodecName();
                    myInsertData.put("codec", prVideoCodecName);
                    String prVideoFormatName = naGetVideoFormatName();
                    myInsertData.put("format", prVideoFormatName);
                    double prFps = naGetVideoFPS();
                    Log.i("VPMA", "fps: " + prFps);
                    myInsertData.put("fps", prFps);
                    Object[] prObjArray = naGetArray();
                    Log.i("VPMA", (String) prObjArray[0]);
                    String[] prStrArray = Arrays.copyOf(prObjArray,
                            prObjArray.length, String[].class);
                    Log.i("VPMA", "before frames");
                    try {
                        String mysql = "CREATE TABLE table_"
                                + hash
                                + " (id INTEGER  NOT  NULL PRIMARY KEY AUTOINCREMENT, type TEXT NOT NULL, cp_num TEXT NOT NULL, dp_num TEXT NOT NULL, pts TEXT NOT NULL, qstride TEXT NOT NULL, size TEXT NOT NULL, qp_stddev TEXT NOT NULL, count TEXT NOT NULL, qp_avg TEXT NOT NULL)";
                        sql3.execSQL(mysql);
                    } catch (SQLiteException e) {
                        // TODO: handle exception
                    }
                    for (String str : prStrArray) {
                        ContentValues myFrameInsertData = new ContentValues();
                        String[] strArr = str.split(",");
                        if (strArr.length == 9) {
                            String stddev = "", strCount = "", strQp_avg = "";
                            double sd, qp_avg, count = 0, sum = 0, sqrSum = 0;
                            try {
                                count = Integer.parseInt(strArr[6].trim());
                                sum = Integer.parseInt(strArr[7].trim());
                                sqrSum = Integer.parseInt(strArr[8].trim());

                                //sd = (sum * sum / count);
                                sd = (sqrSum - (sum*sum/count))/(count-1);//(sqrSum - sd) / (count - 1);
                                stddev = String.valueOf(sd);
                                qp_avg = sum / count;
                                strCount = String.valueOf(count);
                                strQp_avg = String.valueOf(qp_avg);
                            } catch (Exception e) {
                                Log.i("Error: ", "error converting values");
                            }
                            //Log.i("Java Code: ", "Sum: " + sum + " SqrSum: " + sqrSum + " Count: " + count);
                            //Log.i("Java Code: ", "StdDev: " + stddev + " qp_avg: " + strQp_avg);
                            myFrameInsertData.put("type", strArr[0]);
                            myFrameInsertData.put("cp_num", strArr[1]);
                            myFrameInsertData.put("dp_num", strArr[2]);
                            myFrameInsertData.put("pts", strArr[3]);
                            myFrameInsertData.put("qstride", strArr[4]);
                            myFrameInsertData.put("size", strArr[5]);
                            myFrameInsertData.put("qp_stddev", stddev);
                            myFrameInsertData.put("count", strCount);
                            myFrameInsertData.put("qp_avg", strQp_avg);

                            sql3.insert("table_" + hash, null,
                                    myFrameInsertData);
                        }
                    }
                    sql3.insert("videohashes", null, myInsertData);
                    naClose();
                }
                curFrame.close();
                currVid++;
                curProgress += getFileSize(path);
                publishProgress(curProgress);
                Log.i("Progress", "CurrVid:" + currVid + "  Max:"
                        + progress.getMax());
            } catch (Exception e) {
                Log.i("File", "File not Found");
            }
        }
        return null;
    }
}
    }
    if (sharedPreferences.getBoolean("asyncTaskRunning", false) == false)
    {
        ((Button) findViewById(R.id.btnStart)).setEnabled(false);
        progress = (ProgressBar) findViewById(R.id.progressBar1);
        text = (TextView) findViewById(R.id.tv1);
        if (sharedPreferences.contains("lastVideoInfo"))
        {
            text.setText("Last Video Information Parsed " + "(" + dateFormat.format(dateNow) + "):\n\n" +  sharedPreferences.getString("lastVideoInfo", ""));
            ((Button) findViewById(R.id.btnSend)).setEnabled(true);
        }
        else
        {
            text.setText("");
            ((Button) findViewById(R.id.btnSend)).setEnabled(false);
        }
        progress.setProgress(0);

        myFiles = new ArrayList<String>();
        new StartAsyncTask().execute();
    }
}

Answer 1

When the Activity is destroyed, it loses its reference to the AsyncTask, as when the AsyncTask is created it is passed in a reference to the instance of the Activity that creates it. When the instance dies, the reference to the Activity becomes useless.

A better approach would be put the AsyncTask into a Service, set up the Service and set the AsyncTask running in the Service and bind your Activity to the Service.

Then when a new instance of the Activity is created (ie when the user re-enters the app), it can bind to the same Service instance that’s already running and pass in a reference to its self to receive progress info.

Another advantage of this approach is that your Service can put a notification icon in the notification bar, which greatly reduces it chances of being killed by the system, the user could view progress at a glance, and even be notified of when the process is complete.

Allowing an AsyncTask to be cut loose from its owner (is the Activity) and trusting that it will complete what its doing is a pretty bad idea, and will probably have some unexpected results, lead to potential memory leaks etc.