js.lang

Class AsyncTask<Value>

java.lang.Object

js.lang.AsyncTask<Value>

Type Parameters:

Value - task returned value type.

public abstract class AsyncTask<Value>
extends Object

Asynchronous tasks that may return a value. This class is the base class for tasks executed into a separated thread and allowing for a return value. AsyncTask is abstract; concrete subclass should implement execute() abstract method.

There are basically two usage patterns for handling the returned value: first is to override onPostExecute(Object) like in sample code below. Please note that value handling hook is executed in the same thread as task logic.

 AsyncTask<Object> task = new AsyncTask<Object>() {
        protected Object execute() throws Throwable {
                // asynchronous task logic
        }
 
        protected void onPostExecute(Object value) {
                // take some actions on task returned value
                // executed in the same thread as execute()
        }
 };
 task.start();
 

Second is supplying a Callback implementation to asynchronous task constructor. After execute() completes, Callback.handle(Object) is transparently invoked in the same thread on which task was executed.

 AsyncTask<Object> task = new AsyncTask<Object>(callback) {
        protected Object execute() throws Throwable {
                // asynchronous task logic;
        }
 };
 task.start();
 

Because of his nature, asynchronous logic exceptions are not directly accessible to caller thread. Once asynchronous thread launched the caller thread has no chance to catch execution exceptions. To overcome this limitation this class supply onThrowable(Throwable) hook. If exception occur neither onPostExecute(Object) nor Callback.handle(Object) are invoked.

 AsyncTask<Object> task = new AsyncTask<Object>(callback) {
        protected Object execute() throws Throwable {
                // asynchronous task logic;
        }
 
        protected void onThrowable(Throwable throwable) {
                // handle asynchronous execution exception
        }
 };
 task.start();
 

Version:

final

Author:

Iulian Rotaru

Field Summary

Fields

Modifier and Type	Field and Description
protected Callback<Value>	callback Asynchronous task listener.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	AsyncTask() Constructor for tasks that may use onPostExecute(Object) for returned value handling.
protected	AsyncTask(Callback<Value> callback) Constructor for task using callback for returning value handling.

Method Summary

Modifier and Type	Method and Description
protected abstract Value	execute() Execute task logic and return a resulting value.
protected void	onPostExecute(Value value) Task returned value handler.
protected void	onPreExecute() Hook executed before task logic execution.
protected void	onThrowable(Throwable throwable) Hook executed when task logic fails.
void	start() Start asynchronous task execution.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

callback

protected Callback<Value> callback

Asynchronous task listener.

Constructor Detail

AsyncTask

protected AsyncTask()

Constructor for tasks that may use onPostExecute(Object) for returned value handling. See first usage pattern from class description.

AsyncTask

protected AsyncTask(Callback<Value> callback)

Constructor for task using callback for returning value handling. See the second usage pattern from class description.

Parameters:

callback - callback to be invoked on task completion.

Method Detail

execute

protected abstract Value execute()
                          throws Throwable

Execute task logic and return a resulting value.

Returns:

value resulting from task execution.

Throws:

Throwable - any exception task logic may throw.

start

public void start()

Start asynchronous task execution.

onPreExecute

protected void onPreExecute()

Hook executed before task logic execution. This default implementation is just a placeholder, it does nothing.

onPostExecute

protected void onPostExecute(Value value)

Task returned value handler. This hook is executed after task logic execution and its default action is to delegate callback, of course if one supplied to constructor.

Parameters:

value - value returned by task logic.

onThrowable

protected void onThrowable(Throwable throwable)

Hook executed when task logic fails. Concrete subclass may override this method and take recoverable actions. This default implementation does nothing.

Parameters:

throwable - exception generated by failed task logic.