Async Task

	1. Async task is a helper class used for doing background work in separate thread and post the result back to the UI thread .
	2. It has 3 generics as input, params, progress,result
	3. 4 methods onPreExecute(), doInBackground(),onProgressUpdate(),onPostExecute()
	4. UI thread should start execute method.
	5. Async task can be cancelled at any point of time. onCancelled () will be called if its cancelled. And we can check isCancelled and exit the doInbackground method()
	6. This is not used for very long running operations.
	7. It can be used for short background operations typpically for few seconds.


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