Developer Guides

Usage

 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i 

 new DownloadFilesTask().execute(url1, url2, url3);
 

AsyncTask's generic types

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.

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

The 4 steps

1. AsyncTask`3.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. AsyncTask`3.doInBackground(Params...), invoked on the background thread immediately after AsyncTask`3.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 AsyncTask`3.publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the AsyncTask`3.onProgressUpdate(Progress...) step.
3. AsyncTask`3.onProgressUpdate(Progress...), invoked on the UI thread after a call to AsyncTask`3.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. AsyncTask`3.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

Threading rules

• The AsyncTask class must be loaded on the UI thread. This is done automatically as of NoType:android/os/Build$VERSION_CODES;Href=../../../reference/android/os/Build.VERSION_CODES.html#JELLY_BEAN.
• The task instance must be created on the UI thread.
• AsyncTask`3.execute(Params...) must be invoked on the UI thread.
• Do not call AsyncTask`3.onPreExecute(), AsyncTask`3.onPostExecute(Result), AsyncTask`3.doInBackground(Params...), AsyncTask`3.onProgressUpdate(Progress...) manually.
• The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Memory observability

• Set member fields in the constructor or AsyncTask`3.onPreExecute(), and refer to them in AsyncTask`3.doInBackground(Params...).
• Set member fields in AsyncTask`3.doInBackground(Params...), and refer to them in AsyncTask`3.onProgressUpdate(Progress...) and AsyncTask`3.onPostExecute(Result).

Order of execution

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 Java.Lang.Thread and Android.OS.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 package such as Java.Util.Concurrent.IExecutor, Java.Util.Concurrent.ThreadPoolExecutor and Java.Util.Concurrent.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.

AsyncTask must be subclassed to be used. The subclass will override at least one method (AsyncTask`3.doInBackground(Params...)), and most often will override a second one (AsyncTask`3.onPostExecute(Result).)

Here is an example of subclassing:

Once created, a task is executed very simply:

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

Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Java.Lang.Void:

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

A task can be cancelled at any time by invoking AsyncTask`3.cancel(boolean). Invoking this method will cause subsequent calls to AsyncTask`3.isCancelled() to return true. After invoking this method, AsyncTask`3.onCancelled(Result), instead of AsyncTask`3.onPostExecute(Result) will be invoked after AsyncTask`3.doInBackground(Params...) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of AsyncTask`3.isCancelled() periodically from AsyncTask`3.doInBackground(Params...), if possible (inside a loop for instance.)

There are a few threading rules that must be followed for this class to work properly:

AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with NoType:android/os/Build$VERSION_CODES;Href=../../../reference/android/os/Build.VERSION_CODES.html#DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with NoType:android/os/Build$VERSION_CODES;Href=../../../reference/android/os/Build.VERSION_CODES.html#HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invoke AsyncTask`3.executeOnExecutor(java.util.concurrent.Executor, Params...) with AsyncTask`3.THREAD_POOL_EXECUTOR.

[Android Documentation]