au.com.cybersearch2.classytask

Class UserTask<Progress>

java.lang.Object

au.com.cybersearch2.classytask.Executable

au.com.cybersearch2.classytask.TaskRunner

au.com.cybersearch2.classytask.UserTask<Progress>

public abstract class UserTask<Progress>
extends TaskRunner

UserTask

Author:

Romain Guy 25/04/2014

UserTask 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.

A user task is defined by a computation that runs on a background thread and whose result is published on the UI thread. A user task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called begin, doInBackground, processProgress and end.

Usage

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

Here is an example of subclassing:

 private class DownloadFilesTask extends UserTask<URL, Integer, Long> {
     public File 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));
         }
     }

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

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

Once created, a task is executed very simply:

 new DownloadFilesTask().execute(new URL[] { ... });
 

User task's generic types

The three types used by a user 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 a user task. To mark a type as unused, simply use the type Void:

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

The 4 steps

When a user task is executed, the task goes through 4 steps:

1. BackgroundTask.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. BackgroundTask.doInBackground(), invoked on the background thread immediately after () finishes executing. This step is used to perform background computation that can take a long time. The parameters of the user 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(Object[]) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Object[]) step.
3. (Object[]), invoked on the UI thread after a call to publishProgress(Object[]). 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. BackgroundTask.onPostExecute(Boolean), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Threading rules

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

• The task instance must be created on the UI thread.
• TaskRunner.execute(BackgroundTask) must be invoked on the UI thread.
• Do not call (), BackgroundTask.onPostExecute(Boolean), BackgroundTask.doInBackground(), (Object[]) manually.
• The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Constructor Summary

Constructors

Constructor and Description
UserTask(TaskManager taskManager, InternalHandler internalHandler) Creates a new user task.

Method Summary

Methods

Modifier and Type	Method and Description
void	onProgressUpdate(Object[] values) Runs on the UI thread after publishProgress(Object[]) is invoked.

Methods inherited from class au.com.cybersearch2.classytask.TaskRunner

cancel, execute, getStatus, isCancelled

Methods inherited from class au.com.cybersearch2.classytask.Executable

waitForTask

Methods inherited from class java.lang.Object

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

Constructor Detail

UserTask

public UserTask(TaskManager taskManager,
        InternalHandler internalHandler)

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

Method Detail

onProgressUpdate

public void onProgressUpdate(Object[] values)

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

Parameters:

values - The values indicating progress.

See Also:

publishProgress(Object[]), BackgroundTask.doInBackground()