Background Tasks

Background Tasks are one of the most important features of Wisej.NET. No other Web development system allows developers to launch background tasks (threads) on the server and update the client asynchronously with the same level of simplicity and robustness.

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What is a Background Task?

A Background Task runs on the server in the background after the client request has completed, allowing users to continue working while the task executes.

Background tasks operate independently from the session and user that initiated them. They run as separate threads without any session knowledge. You can even start separate threads when an application first loads, before any session or user request exists.

To run code when an application first loads, create a static constructor on any class. The most appropriate would be Program.cs or the first page or window created for new sessions.

What can I do in a Background Task?

Anything - connect to other servers, query databases, use the file system, access remote services, etc.

How do I terminate a Background Task?

Simply return from the call. A background task (like any thread) starts by calling your start method and runs until you exit it. Typically it's a loop that checks conditions and uses Thread.Sleep(ms) periodically, exiting when complete.

Update the UI

Wisej.NET provides a straightforward way to launch a Background Task bound to a specific session, allowing code to interact with the user's UI in real-time.

C#

void button1_Click(EventArgs e)
{
  Application.StartTask(() =>
  {
    // Change the text of button 1 from 1 to 10 in the background.
    for (int i = 0; i < 10; i++)
    {
      this.button1.Text = i.ToString();

      // We are running out-of-bound and we need to push the updates
      // to the client using Application.Update(). It can be called at any time
      // and interval. It simply flushes the changed back to the client.
      Application.Update(this);

      Thread.Sleep(1000);
    }
  });
}

VB.NET

The example above updates the specific button1 in the session that started the task because the thread is started using Application.StartTask().

Application.StartTask() starts a new thread and updates the context to match the calling session. If using Thread.Start() instead, the thread method couldn't access session objects unless wrapped in Application.RunInContext().

When code runs in context (either in-bound or out-of-bound for background tasks), you can call Application.Update() anytime to push UI updates to the client.

If the client or server doesn't support WebSocket, Application.Update() has no effect since the connection doesn't exist. Wisej.NET provides alternative client updates through automatic polling. See Real Time Web Applications and pollingInterval in Configuration.

Systems Without WebSocket

Older clients or servers may not support WebSocket connections, preventing out-of-bound UI updates from background tasks reaching the client browser.

Wisej.NET provides alternative client updates using Application.StartPolling(milliseconds) and Application.EndPolling() to enable periodic server polling. These methods are ignored when using WebSocket connections.

C#

// this has no effect when the client and server are connected using WebSocket.
Application.StartPolling(1000);

Application.StartTask(() => {

    for (int i = 0; i < 100; i++) {
        this.label1.Text = "Counting..." + i;
        Thread.Sleep(1000);
    }

    // this has no effect when the client and server are NOT connected using WebSocket.
    Application.Update(this);

    // this has no effect when the client and server are connected using WebSocket.
    Application.EndPolling();
});

VB.NET

Check with Microsoft and other technology providers to determine which platforms support WebSocket connections

Background Task with Multiple Users

Since Background Tasks aren't bound to specific sessions, they can perform global tasks serving multiple users simultaneously. While the previous example showed a single-session task, we can create session-independent tasks.

Examples include chat servers, stock market watchers, or any task periodically checking resources. The recommended approach is firing events from the service (background task) and letting user sessions subscribe to these events.

Below is a simplified service firing events every second with a subscribing client.

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C#

static class Program
{
  static Program()
  {
    // Start the service.
    BackgroundService.Start();
   }
   ...
}

public static class BackgroundService
{
  private static Thread _thread;
  private static bool _stop = false;

  public static event BackgroundServiceEventHandler Message;

  public static void Start()
  {
    if (_thread == null)
      _thread = new Thread(ThreadStart);

    _stop = false;
    _thread.Start();
  }

  private static void ThreadStart()
  {
    var i = 0;
    while (!_stop)
    {
      Thread.Sleep(1000);

      i++;
      if (Message != null)
        Message(null, new BackgroundServiceEventArgs("Hello #" + i));
    }
  }

  public static void Stop()
  {
    _stop = true;
  }
}

VB.NET

Handling Events from the Background Service

C#

private void button1_Click(object sender, EventArgs e)
{
  // Any number of clients can attach to the event from the Background service.
  if (_subscribed)
    BackgroundService.Message -= BackgroundService_Message;
  else
    BackgroundService.Message += BackgroundService_Message;

  _subscribed = !_subscribed;
  this.button1.Text = _subscribed ? "Unsubscribe" : "Subscribe";
}
private bool _subscribed;

// The event handler is called by the Background service thread, so the thread
// doesn't have any knowledge of the session, but "this" refers to this specific
// object, which allows Wisej.NET to restore the context while executing the code below.

private void BackgroundService_Message(object sender, BackgroundServiceEventArgs e)
{
  // Application.Update() can optionally call a code block in context before updating the UI.
  Application.Update(this, () =>
  {
    this.label1.Text = e.Message;
  });
}

VB.NET

The power and simplicity of background tasks in Web applications through the Wisej.NET framework allows handling the browser from the server as if it were a screen directly connected to the server.