Question

basically i am reading image bytes from file i have a loop which has to read almost 20 images from file and in this loop i have making the object of my class which extends ASyncTask class and i am reading bytes and drawing on canvas in doItBackground() method and setting my image on onPostExecute() method. so basically Asynchronously 20 threads starts execution in background. now problem is it is not showing any image.

anybody have any idea how to handle multiple AsyncTask class? here is my code.

    @Override
    protected Void doInBackground(Void... params) 
    {
//      Log.i("start background thread", "start background thread");
        Set<Integer> keys;
        keys = iTileListOffsets.keySet();
        boolean found = false;
        Object key[] = keys.toArray();
        int k = 0;

        for (int i=0; i < key.length && found==false ; i++)
        {               
            k = (Integer) key[i];
            MapTileHeader tileHeader = iTileListOffsets.get(k);
            if (tileHeader.equalsWith(mapTile.getiMapTileHeader()) )
            {                   
                found = true; 
            }
        }

        if(found)
        {           
            try 
            {   
                synchronized (MapMaker.getSingletonObject()) 
                {
                    fc.position(0);                     
                    input.skipBytes(k);                         
                    int tSizeOfTile = input.readInt();              
                    mapTile.loadTileWithFileHeader(input, tSizeOfTile, headerBytesArray);
                }

                mapMaker.getBufferedTilesDataList().add(mapTile);                       
            }
            catch (Exception e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            byte imageTile[] = mapTile.getImageTilesBytes();             //bufferedTile.getImageTilesBytes();                   

            if(mapTile.getImageTilesBytes() != null)
            {                       
                try
                {
                    Bitmap tileBitmap = BitmapFactory.decodeByteArray(imageTile, 0, imageTile.length);
                    canvas.drawBitmap(tileBitmap, rect.left, rect.top, null);
                    tileBitmap = null;
                }
                catch(Exception e)
                {
                    e.printStackTrace();
                }                   
            }
        }
//      Log.i("ENDDDD background thread", "ENDDD background thread");
        return null;
    }

    @Override
    protected void onPostExecute(Void result)
    {
        // TODO Auto-generated method stub
        super.onPostExecute(result);        
        Log.i("start POSTECEXCUTE thread", "start POSTECEXCUTE thread");

        MyRect mapRect = aBufferArea.GetScreenRectangle(buffer, aScrnArea); 
        Bitmap smallBitmap = Bitmap.createBitmap(bitmap, mapRect.left, mapRect.top, mapRect.width, mapRect.height);         
        image.setImageBitmap(smallBitmap);

        Log.i("ENDDDDD POSTECEXCUTE thread", "ENDDDD POSTECEXCUTE thread");
    }

thanks

Answer 1

I dont think you can update the UI directly from doInBackground, as the code in this method is run in a backgound thread and does not have direct access to the IU thread. You can update the UI either from onPostExecute or from onProgressUpdate. You can call the latter of these from doInBackground by calling publishProgress.

There is a simple example in the documentation for AsyncTask, and this one is a little more complex.