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.)




Comments

Post a Comment

Popular posts from this blog

All about Context

Context Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system.  It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc. As the name suggests, its the context of current state of the application/object. It lets newly created objects understand what has been going on. Typically you call it to get information regarding another part of your program. Context is used for many operations but mostly to load and access resources. This is why all the widgets receive a Context parameter in their constructor. In a regular Android application, you usually have two kinds of Context, Activity and Application. It’s usually the first one that the developer passes to classes and methods that need a Context. A Context is a handle to the system; it provides services l
Read more

How to use LocalBroadcastManager?

                LocalBroadcastManager allows to register and send broadcasts to local objects within your application. Advantages over sending Global Broadcasts: ·          No need to worry about leaking of data as the data you are broadcasting never leave your app. ·          It is more efficient than global broadcast. ·          Not possible for other applications to send these broadcasts to your applications. LocalBroadcastManger is introduced in android 3.0, so you have to use android support library v4 for prior releases. Create receiver.  private BroadcastReceiver mReceiver = new BroadcastReceiver() {             @Override             public void onReceive(Context context, Intent intent) {                   // your code             }       } Register receiver LocalBroadcastManager. getInstance (getActivity()).registerReceiver( mReceiver , new IntentFilter("loc_receiver")); Send Broadcast Intent intent = new Int
Read more