Why Context can't be used in doInBackground() of AsyncTask

Question

I am just finding what is the fact behind not using Context inside doInBackground(). In-fact we can\'t update the UI inside doInBackground() di

Answer 1

Q Why Context can't be used in doInBackground() of AsyncTask?

Context 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 (activity, package/application)

Where as doInBackground() has no relation/(context) or is not related to the present Activity when its in doInBackground(). You have no access to the main thread. Its completely a new activity which will perform its task even if your Main Thread/Activity has stopped working.

AsyncTask do not run on the main thread or the main UI thread which you used to call it.It runs on a separate thread to perform the task given to it

To be able to do changes to the UI after the AsycTask completes, you have to call

  protected void onPostExecute(String string) 
    { 
         Toast c=Toast.makeText(getApplicationContext(), string,  Toast.LENGTH_LONG);
            c.show();

          }

All the UI changes can be done in protected void onPostExecute()

In short doInBackground() Cannot do any changes to your Main Thread and Is not Dependent on your Main UI/Thread so there is no question of using context in doInBackground()

Q Why we can use context in onPreExecute() and onPostExecute()?

A AsycTask has

1.onPreExecute() Pre/Before creating a new Thread (Can do changes to Main Thread )

onPreExecute() has context to the main thread / AsycTask Still in Main thread

2.doInBackground() Has created a New thread of its own to perform the task given. Once in this State/Thread You cannot do anything until its complete.

doInBackground() No Context . New thread is created of its own.Once new thread is created it will complete its given task irrespective of the main thread getting Killed/Stopped./ AsycTask in new thread.

3.onPostExecute()- After doInBackground() completes its task onPostExecute() is called So that results of the computation can be Used by the main thread

onPostExecute() has context to the main thread /AsycTask back in Main Thread.

Answer 2

We can use context in doinbackground method but only to get value from ui component. We can not use contexr to set value in ui component

Answer 3

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.

AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent pacakge such as Executor, ThreadPoolExecutor and FutureTask.

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 Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.

For more information about using tasks and threads, read the Processes and Threads developer guide.