Async Task

What is Async Task?

Async task performs Long running operations,publish results on the UI thread.

AsyncTask : This class allows you to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.

When to Use?

  • 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 package such as ExecutorThreadPoolExecutor and FutureTask.
  • AsyncTask must be subclassed to be used.
  • The subclass will override at least one method (doInBackground(Params...)),
  • most often will override a second one (onPostExecute(Result).)




Parameters Of Async Task:

There are three types of parameters are used for asynchronous task.

The three types used by an asynchronous task are the following:

  1. Params, the type of the parameters sent to the task for an 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.

To mark a type as unused, simply use the type Void

private class MyTask extends AsyncTask<Void, Void, Void> { ... }

Method of Async Task:

  1. onPreExecute(), invoked on the UI thread before 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.

Cancelling a task

  • A task can be cancelled at any time by invoking cancel(boolean).
  • Invoking this method will cause subsequent calls to isCancelled() to return true.
  • After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns.
  • To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically fromdoInBackground(Object[]), if possible (inside a loop for instance.)

Order of execution

  • When first introduced, AsyncTasks were executed serially on a single background thread.
  • Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel.
  • Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

How to  run AsyncTask Parallel?

If you want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.