Working with AsyncTask

What is AsyncTask?

AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.

AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. 

When to Use?

AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent pacakge such as Executor, ThreadPoolExecutor and FutureTask.

How to Use?

AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground(Params...)), and most often will override a second one (onPostExecute(Result).)

AsyncTask's generic types

The three types used by an asynchronous task are the following:

1. Params, the type of the parameters sent to the task upon execution.
2. Progress, the type of the progress units published during the background computation.
3. Result, the type of the result of the background computation.

Not all types are always used by an asynchronous task. 

To mark a type as unused, simply use the type Void:

private class MyTask extends AsyncTask<Void, Void, Void> { ... }

Steps of AsyncTask

When an asynchronous task is executed, the task goes through 4 steps:

1. onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.
2. doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.
3. onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.
4. onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Working Example

Here I am showing an example of downloading an image from URL through AsyncTask.

// imageView to load the image

ImageView imageView1 = (ImageView) v.findViewById(R.id.image1);

// another loading image will be showed until image from URL is loaded

imageView1.setImageResource(R.drawable.loading);

try {

   String URL = "my-image-url";

   imageView1.setTag(URL);

   // execute the task

   new DownloadImageTask1().execute(imageView1);

} catch (Exception e) {

   e.printStackTrace();

}

// ... 

// AsyncTask must be subclassed to be used. 

public class DownloadImageTask1 extends AsyncTask<ImageView, Void, Bitmap> {

ImageView imageView = null;

@Override

protected Bitmap doInBackground(ImageView... imageViews) {

    this.imageView = imageViews[0];

    return download_Image((String)imageView.getTag());

}

@Override

protected void onPostExecute(Bitmap result) {

    imageView.setImageBitmap(result);

}

@Override

protected void onPreExecute() {

    super.onPreExecute();

}

private Bitmap download_Image(String url) {

      Bitmap bmp =null;

        try{

            URL ulrn = new URL(url);

            HttpURLConnection con = (HttpURLConnection)ulrn.openConnection();

            InputStream is = con.getInputStream();

            bmp = BitmapFactory.decodeStream(is);

            if (null != bmp)

                return bmp;

            }catch(Exception e){}

        return bmp;

}

}