Tuesday, November 1, 2011

Android AsyncTask Example

 When an asynchronous task is executed, the task goes through 4 steps:
  1. onPreExecute(), invoked on the UI thread immediately after 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.

Source code (main.xml)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
<Button android:id="@+id/bnt1"

Source code (TestActivity.java)
public class TestActivity extends Activity
Button btn;
public void onCreate(Bundle icicle) {
btn.setOnClickListener(new OnClickListener() {

public void onClick(View v) {
new Async().execute();
public class Async extends AsyncTask<String, Integer, String> {
ProgressDialog dialog;
protected void onPreExecute()
dialog=new ProgressDialog(TestActivity.this);
protected String doInBackground(String... params) {
// TODO Auto-generated method stub
   // perform desired task in this doInBackground Block.
for(int i=0;i<20;i++)
try {
} catch (InterruptedException e) {
// TODO Auto-generated catch block

return "";
protected void onProgressUpdate(Integer... values) {
// TODO Auto-generated method stub

protected void onPostExecute(String result)
AlertDialog.Builder a=new Builder(TestActivity.this);
a.setMessage("Successfully Done");

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