All about AsyncTask


AsyncTask can be useful in situations when there is a need for doing a long task like calling Web service and then show it in the UI as soon as it is finished. 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. An AsyncTask runs on a background thread and the result is published on the UI. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.

When using AsyncTask make an internal class which extend AsyncTask class. You should override atleast one method of AsyncTask (doInBackground).

private class DownloadTask extends AsyncTask<String, Void, String> {
   // Run on UI thread
   @Override
   protected void onPreExecute() {
      // do initialization 
   }

   // Run on Background thread
   @Override
   protected String doInBackground(String... urls) {
     publishProgress(progress value);
     // task which you want to do in background
     return response;
   }

   // Run on UI thread
   protected void onProgressUpdate(Integer... progress) {
     // do the progress update here
   }

   // Run on UI thread
   @Override
   protected void onPostExecute(String result) {
     // get the response and do what you want
   }
}

Execute it like this: 

new DownloadTask().execute(var1, var2, var3);

You can cancel a task anytime by invoking cancel(boolean)
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> { ... }

The 4 Steps

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.
Rules to execute AsyncTask
  1. The task instance must be created on the UI thread.
  2. execute(Params...) must be invoked on the UI thread.
  3. Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) manually.
  4. The task can be executed only once (an exception will be thrown if a second execution is attempted.)




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