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Sunday, 15 April 2012

Asynchronous task, Updating UI from background in android

As all we know in android there are two ways to perform background task such as downloading image and other task that include long operation
  • Using Thread 
  • Asynchronous Task                          
Using Thread, there is one dis-advantage that we can not update user interface other wise we will get Looper.loop() exception

So its efficient to use Asynchronous task.
An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called ParamsProgress and Result, and 4 steps, called onPreExecutedoInBackgroundonProgressUpdate and onPostExecute.

Let take one example of downloading images from URL's Array.So body of Asynchronous task 's body will be like this

 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
         return totalSize;

     protected void onProgressUpdate(Integer... progress) {

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
    Like Thread we can create asynchronous task like this

 new DownloadFilesTask().execute(url1, url2, url3);
    Now asynchronous has three generic type.Let have a look on their requirement 

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

    But its not necessary to use all can set like this if you do not want to use 

    private class MyTask extends AsyncTask<Void, Void, Void> { ... }
     Now it has four step -

    1)  onPreexecute() -Before it start executing it call this method .It call before it goes to perform task in background so we can display a ProgressDialog here )doInBackground() - perform long background task here

    3)onProgressUpdate() - Update your progress of dialog to show to user

    4)onPostExecute() - Post or show your result here .Dismiss progress dialog  box here
    There are a few threading rules that must be followed for this class to work properly:
    • The task instance must be created on the UI thread.
    • execute(Params...) must be invoked on the UI thread.
    • Do not call onPreExecute()onPostExecute(Result)doInBackground(Params...)onProgressUpdate(Progress...) manually.
    • The task can be executed only once (an exception will be thrown if a second execution is attempted.)
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