TimerCallback Delegate

Parameters

• state
  Type: System.Object
  An object that contains application-specific information that is relevant to the method invoked by this delegate.
  -or-
  A null reference (Nothing in Visual Basic).

Imports System.Threading

' The following Imports are not required for the timer. They merely simplify 
' the code.
Imports System.Windows.Controls
Imports System.Windows.Input

' The Example class holds a reference to the timer, and contains the 
' event handler for the MouseLeftButtonUp events that control the demo.
'
Public Class Example

   ' The Shared Demo method sets the starting message and creates an 
   ' instance of Example, which hooks up the handler for the MouseLeftButtonUp
   ' event.
   Public Shared Sub Demo(ByVal outputBlock As TextBlock)

      outputBlock.Text &= "Click to create the timer." & vbLf
      Dim dummy As New Example(outputBlock)

   End Sub


   ' Instance data for the demo.
   Private phase As Integer = 0
   Private t As Timer

   Public Sub New(ByVal outputBlock As TextBlock)

      ' Hook up the mouse event when a new Example object is created. Note
      ' that this keeps garbage collection from reclaiming the Example 
      ' object.
      AddHandler outputBlock.MouseLeftButtonUp, AddressOf Me.MouseUp

   End Sub

   Private Sub MouseUp(ByVal sender As Object, ByVal e As MouseButtonEventArgs)

      Dim outputBlock As TextBlock = CType(sender, TextBlock)

      If phase = 0 Then
         ' On the first click, create the timer.
         outputBlock.Text &= vbLf & "Creating the timer at " & _
            DateTime.Now.ToString("h:mm:ss.fff") & _
            ", to start in 1 second with a half-second interval." & vbLf & _
            "Click to change the interval from 1/2 second to 1 second." & vbLf & vbLf

         ' Create a timer that invokes the callback method after one second 
         ' (1000 milliseconds) and every 1/2 second thereafter. The TextBlock
         ' that is used for output is passed as the state object. Visual Basic 
         ' infers the delegate type, as if you had typed the following:
         '     New TimerCallback(AddressOf MyTimerCallback)
         ' 
         t = New Timer(AddressOf MyTimerCallback, outputBlock, 1000, 500)

      ElseIf phase = 1 Then
         ' On the next click, change the timer interval to every second.
         outputBlock.Text &= vbLf & "Changing the interval to one second." & vbLf & _
            "Click to destroy the timer." & vbLf & vbLf

         t.Change(0, 1000)

      Else
         ' On the last click, destroy the timer and shut down the demo.
         outputBlock.Text &= vbLf & "Destroying the timer." & vbLf & _
            "Refresh the page to run the demo again."
         RemoveHandler outputBlock.MouseLeftButtonUp, AddressOf Me.MouseUp

         t.Dispose()

      End If

      phase += 1

   End Sub


   ' The shared callback method is invoked on a ThreadPool thread by the Timer. 
   ' The state object is passed to the callback method on each invocation. In this
   ' example, the state object is the TextBlock that displays output. In order to
   ' update the TextBlock object, which is on the UI thread, you must make the  
   ' cross-thread call by using the Dispatcher object that is associated with the 
   ' TextBlock.
   Private Shared Sub MyTimerCallback(ByVal state As Object)

      Dim outputBlock As TextBlock = CType(state, TextBlock)
      Dim msg As String = DateTime.Now.ToString("h:mm:ss.fff") & _
                          " MyTimerCallback was called." & vbLf

      outputBlock.Dispatcher.BeginInvoke(displayHelper, outputBlock, msg)

   End Sub

   ' The DisplayOutput helper method and its delegate, displayHelper, are used by
   ' the BeginInvoke method of the Dispatcher object.
   Private Shared displayHelper _
                    As New Action(Of TextBlock, String)(AddressOf DisplayOutput)
   Private Shared Sub DisplayOutput(ByVal tb As TextBlock, ByVal msg As String)
      tb.Text &= msg
   End Sub

End Class

' This example produces output similar to the following:
'
'Click to create the timer.
'
'Creating the timer at 3:17:36.980, to start in 1 second with a half-second interval.
'Click to change the interval from 1/2 second to 1 second.
'
'3:17:38.072 MyTimerCallback was called.
'3:17:38.586 MyTimerCallback was called.
'3:17:39.101 MyTimerCallback was called.
'3:17:39.580 MyTimerCallback was called.
'
'Changing the interval to one second.
'Click to destroy the timer.
'
'3:17:39.656 MyTimerCallback was called.
'3:17:40.689 MyTimerCallback was called.
'
'Destroying the timer.
'Refresh the page to run the demo again.

using System;
using System.Threading;

// The following Imports are not required for the timer. They merely simplify 
// the code.
using System.Windows.Controls;
using System.Windows.Input;

// The Example class holds a reference to the timer, and contains the 
// event handler for the MouseLeftButtonUp events that control the demo.
//
public class Example
{
   // The static Demo method sets the starting message and creates an 
   // instance of Example, which hooks up the handler for the MouseLeftButtonUp
   // event.
   public static void Demo(TextBlock outputBlock)
   {
      outputBlock.Text += "Click to create the timer.\n";
      Example dummy = new Example(outputBlock);
   }


   // Instance data for the demo.
   private int phase = 0;
   private Timer t;

   public Example(TextBlock outputBlock)
   {
      // Hook up the mouse event when a new Example object is created. Note
      // that this keeps garbage collection from reclaiming the Example 
      // object.
      outputBlock.MouseLeftButtonUp += new MouseButtonEventHandler(this.MouseUp);
   }

   private void MouseUp(object sender, MouseButtonEventArgs e)
   {
      TextBlock outputBlock = (TextBlock) sender;

      if (phase==0)
      {
         // On the first click, create the timer.
         outputBlock.Text += "\nCreating the timer at " + 
               DateTime.Now.ToString("h:mm:ss.fff") + 
               ", to start in 1 second with a half-second interval.\n" +
               "Click to change the interval from 1/2 second to 1 second.\n\n";

         // Create a timer that invokes the callback method after one second 
         // (1000 milliseconds) and every 1/2 second thereafter. The TextBlock
         // that is used for output is passed as the state object. C# infers the 
         // delegate type, as if you had typed the following:
         //     new TimerCallback(MyTimerCallback)
         // 
         t = new Timer(MyTimerCallback, outputBlock, 1000, 500);

      }
      else if (phase==1)
      {
         outputBlock.Text += "\nChanging the interval to one second.\n" +
                             "Click to destroy the timer.\n\n";
         t.Change(0, 1000);
      }
      else
      {
         // On the last click, destroy the timer and shut down the demo.
         outputBlock.Text += "\nDestroying the timer.\n" + 
                             "Refresh the page to run the demo again.";
         outputBlock.MouseLeftButtonUp -= new MouseButtonEventHandler(this.MouseUp);

         t.Dispose();
      }

      phase += 1;
   }


   // The static callback method is invoked on a ThreadPool thread by the Timer. 
   // The state object is passed to the callback method on each invocation. In this
   // example, the state object is the TextBlock that displays output. In order to
   // update the TextBlock object, which is on the UI thread, you must make the  
   // cross-thread call by using the Dispatcher object that is associated with the 
   // TextBlock.
   private static void MyTimerCallback(object state)
   {
      TextBlock outputBlock = (TextBlock) state;
      string msg = DateTime.Now.ToString("h:mm:ss.fff") + " MyTimerCallback was called.\n";

      outputBlock.Dispatcher.BeginInvoke(delegate () { outputBlock.Text += msg; });
   }
}

/* This example produces output similar to the following:

Click to create the timer.

Creating the timer at 3:40:17.712, to start in 1 second with a half-second interval.
Click to change the interval from 1/2 second to 1 second.

3:40:18.820 MyTimerCallback was called.
3:40:19.335 MyTimerCallback was called.
3:40:19.849 MyTimerCallback was called.

Changing the interval to one second.
Click to destroy the timer.

3:40:20.317 MyTimerCallback was called.
3:40:21.331 MyTimerCallback was called.

Destroying the timer.
Refresh the page to run the demo again.
 */