AsyncTask in Android

Hello Friends,

Today I’ll discuss about “AsyncTask”. It stands for Asynchronous Task, which is one of the most important concept used in android development. It’s more or less related to thread concept used in various languages.

Definition : An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread.

Let’s understand this in this way, whenever we download anything using our browser, the downloading process is done by the browser in the background while we still surf the other web pages. It means, the downloading process doesn’t affect the working of browser. In this example we can relate the term UI with browser and downloading process with AsyncTask(background thread). It is highly recommended that we should not use AsyncTask for long operations.AsyncTask is an abstract class which is defined by 3 generic types, called Params, Progress and Result.  I personally recommend you to brush-up the concepts of Generic Types, Abstract Class and preferably File Handling before getting into this. 

In order to use AsyncTask class, we must subclass it.  

private class DownloadingTask extends AsyncTask<URL, Integer, Long> 

This AsyncTask class provides us the following methods to override and we must override at least one.  
1. OnPreExecute()
2. doInBackground(Params...)         [most often we override this method]
3. onProgressUpdate(Progress...)
4. onPostExecute(Result)
 

Note : Do not call any of the above mentioned methods manually. 

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

1. onPreExecute(), As the method name suggests, it is invoked on the UI thread before the task is executed. In simple term we can understand this like, arranging all the required documents before going for an interview.
2. doInBackground(Params...), This method is 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:
    https://developer.android.com/reference/android/os/AsyncTask.html