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.
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.
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);
}
});
}
Private Sub button1_Click(e As EventArgs)
Application.StartTask(Sub()
' Change the text of button 1 from 1 to 10 in the background.'
For i = 0 To 10
Me.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)
Next
End Sub)
End Sub
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.
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.
// 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();
});
' this has no effect when the client and server are connected using WebSocket.'
Application.StartPolling(1000)
Application.StartTask(Sub()
For i = 0 To 100
Me.label1.Text = "Counting..." + i
Thread.Sleep(1000)
Next
' 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()
End Sub)
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.
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;
}
}
Module Program
Sub New()
' Start the service.'
BackgroundService.Start()
End Sub
...
End Mdule
Module BackgroundService
Private _thread As Thread
Private _stop As Boolean
Public Event Message As BackgroundServiceEventHandler
public Sub Start()
If _thread = null Then
_thread = new Thread(ThreadStart)
End If
_stop = false
_thread.Start()
End Sub
Private Sub ThreadStart()
Dim i as Int32
While Not _stop
Thread.Sleep(1000)
i++
If Not Message Is Nothing Then
Message(Nothing, New BackgroundServiceEventArgs("Hello #" + i))
End If
End While
End Sub
Public Sub Stop()
_stop = true
End Sub
End Module
Handling Events from the Background Service
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;
});
}
Private Sub button1_Click(sender As Object, e As EventArgs)
' Any number of clients can attach to the event from the Background service.'
If _subscribed Then
RemoveHander BackgroundService.Message, AddressOf BackgroundService_Message
Else
AddHandler BackgroundService.Message, AddressOf BackgroundService_Message
End If
_subscribed = Not _subscribed
Me.button1.Text = If (_subscribed, "Unsubscribe", "Subscribe")
End Sub
Private _subscribed as Boolean
' The event handler is called by the Background service thread, so the thread'
' does not 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 Sub BackgroundService_Message(sender As Object, e As BackgroundServiceEventArgs)
' Application.Update() can optionally call a code block in context before updating the UI.'
Application.Update(Me, Sub()
Me.label1.Text = e.Message
End Sub)
End Sub
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.
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 and pollingInterval in .
