public abstract class

AsyncTask

extends Object
java.lang.Object
   ↳ android.os.AsyncTask<Params, Progress, Result>

Class Overview

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 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 begin, doInBackground, processProgress and end.

Usage

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

Here is an example of subclassing:

 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 < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
         }
         return totalSize;
     }

     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

Once created, a task is executed very simply:

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

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 am asynchronous task. To mark a type as unused, simply use the type Void:

 private class MyTask extends AsyncTask

 
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. 
     
  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. 
     
  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. 
     
  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.
    5. 
 


 
Threading rules

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

Summary

Nested Classes
public abstract class AsyncTask.Status Indicates the current status of the task. 
Public Constructors
AsyncTask()
Creates a new asynchronous task.
Public Methods
final boolean cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
final AsyncTask<Params, Progress, Result> execute(Params... params)
Executes the task with the specified parameters.
final Result get()
Waits if necessary for the computation to complete, and then retrieves its result.
final Result get(long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.
final AsyncTask.Status getStatus()
Returns the current status of this task.
final boolean isCancelled()
Returns true if this task was cancelled before it completed normally.
Protected Methods
abstract Result doInBackground(Params... params)
Override this method to perform a computation on a background thread.
void onCancelled()
Runs on the UI thread after cancel(boolean) is invoked.
void onPostExecute(Result result)
Runs on the UI thread after doInBackground(Params...).
void onPreExecute()
Runs on the UI thread before doInBackground(Params...).
void onProgressUpdate(Progress... values)
Runs on the UI thread after publishProgress(Progress...) is invoked.
final void publishProgress(Progress... values)
This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running.
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

public AsyncTask ()

Creates a new asynchronous task. This constructor must be invoked on the UI thread.

Public Methods

public final boolean cancel (boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

Parameters
mayInterruptIfRunning true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.
Returns
  • false if the task could not be cancelled, typically because it has already completed normally; true otherwise
See Also

public final AsyncTask<Params, Progress, Result> execute (Params... params)

Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it. This method must be invoked on the UI thread.

Parameters
params The parameters of the task.
Returns
  • This instance of AsyncTask.
Throws
IllegalStateException If getStatus() returns either RUNNING or FINISHED.

public final Result get ()

Waits if necessary for the computation to complete, and then retrieves its result.

Returns
  • The computed result.
Throws
CancellationException If the computation was cancelled.
ExecutionException If the computation threw an exception.
InterruptedException If the current thread was interrupted while waiting.

public final Result get (long timeout, TimeUnit unit)

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.

Parameters
timeout Time to wait before cancelling the operation.
unit The time unit for the timeout.
Returns
  • The computed result.
Throws
CancellationException If the computation was cancelled.
ExecutionException If the computation threw an exception.
InterruptedException If the current thread was interrupted while waiting.
TimeoutException If the wait timed out.

public final AsyncTask.Status getStatus ()

Returns the current status of this task.

Returns
  • The current status.

public final boolean isCancelled ()

Returns true if this task was cancelled before it completed normally.

Returns
  • true if task was cancelled before it completed
See Also

Protected Methods

protected abstract Result doInBackground (Params... params)

Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute(Params...) by the caller of this task. This method can call publishProgress(Progress...) to publish updates on the UI thread.

Parameters
params The parameters of the task.
Returns
  • A result, defined by the subclass of this task.
See Also

protected void onCancelled ()

Runs on the UI thread after cancel(boolean) is invoked.

See Also

protected void onPostExecute (Result result)

Runs on the UI thread after doInBackground(Params...). The specified result is the value returned by doInBackground(Params...) or null if the task was cancelled or an exception occured.

Parameters
result The result of the operation computed by doInBackground(Params...).
See Also

protected void onPreExecute ()

Runs on the UI thread before doInBackground(Params...).

See Also

protected void onProgressUpdate (Progress... values)

Runs on the UI thread after publishProgress(Progress...) is invoked. The specified values are the values passed to publishProgress(Progress...).

Parameters
values The values indicating progress.
See Also

protected final void publishProgress (Progress... values)

This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running. Each call to this method will trigger the execution of onProgressUpdate(Progress...) on the UI thread.

Parameters
values The progress values to update the UI with.
See Also