Using Timers

The Timer is a built-in Visual Basic control that allows you to execute code after a specific amount of time has elapsed from the moment at which the timer is enabled, or repeatedly at specific time intervals. Once enabled, the timer will generate a Tick event at predetermined intervals.

The time that elapses between ticks is specified by the Timer object's Interval property, which can be set at design time or programmatically. The length of the interval is specified in milliseconds (a millisecond is one thousandth of a second, or 0.001 seconds). The default interval is 100 milliseconds, but the range of possible values is 1 to 2,147,483,647.

A timer is typically used in a Visual Basic program by creating an event handler for its Tick event (the event handler contains code that will be executed when a Tick event occurs). The event handler's code will only execute if the Timer object is enabled. The timer is enabled by setting its Enabled property to True, either programmatically or at design time.

When a Timer object is first added to a form, the Enabled property is set to False by default. The Timer object has a Start() method that sets its Enabled property to True, and a Stop() method that sets the Enabled property to False.

The Timer object does not actually appear on the form itself, either in design mode or when debugging / executing the program. It is only visible in the Component Tray (the grey area at the bottom of the form design window). We will create a simple clock-calendar program to demonstrate how the timer may be used.

1. Create a new project called "Clock-Calendar", and create an interface that looks something like the illustration below. Note that we have used labels rather than text boxes for the actual time and date - they will be used only to display information, not to accept user input.

2. In the toolbox, find the Timer object (under the Components tab) and double click on it to place a Timer control in your application. You will see it appear in the form design window's Component Tray.
3. Set the control names as shown in the table below.

Clock-Calendar Form Controls

Control	Name
Form	frmClockCalendar
Top label	lblCurrentTime
Bottom label	lblCurrentDate
Button	cmdExit
Timer	tmrClock

4. Set the application's Startup Object property to Clock-Calendar.frmClockCalendar.
5. In the form design window, click once on the Timer object in the Component Tray to access its properties window, then change the Interval property to "1000" and the Enabled property to "True".
6. Double-click on the Exit button to switch to the code window, and add the following command to the event handler that has been created for you:

End

7. Return to the form design window and double click somewhere within the body of the form to create the form's Load event handler, then enter the code below at the cursor (the Now command retrieves the current date and time information, while the Format() function puts the information into the required format).

lblCurrentTime.Text = Format(Now, "Long Time")
lblCurrentDate.Text = Format(Now, "Long Date")

8. The application now displays the correct date and time when we run it, but the clock does not update itself. To make this happen, return to the form design window and double click on the Timer object in the Component Tray to create the timer's Tick event handler, then enter the following code at the cursor:

lblCurrentTime.Text = Format(Now, "Long Time")
lblCurrentDate.Text = Format(Now, "Long Date")

Run the application and you should see the correct time and date (see the illustration below). The code we added to the Tick event will cause the time and date information to be updated once every second.

The Clock-Calendar application is a somewhat trivial example of how a timer is used in an application, but there are many examples in our everyday lives of real-world situations in which timers perform a crucial role. The wash-cycle of a washing machine is one example of a process that is controlled by a timer. Automatic doors in buildings are also controlled by timers (if their proximity sensors do not detect a person in close proximity to the doors, they will automatically close themselves after a predetermined interval).

Perhaps one of the most ubiquitous examples of how timers impact on our daily life, whether we use a vehicle to get from place to place or travel on foot, is the traffic light. The timing circuits on a set of traffic lights must function perfectly, or there is a good chance of an accident occurring! We will now create a simulation to demonstrate how timers might be used to control a set of four traffic lights at a small crossroads. One of the two intersecting roads runs from North to South, while the other runs from East to West.

1. Create a new project called "TrafficLights" and create an interface that includes four PictureBox objects, each 61 pixels wide and 180 pixels high, as shown below.

2. Set the control names as shown in the table below.

Traffic Light Form Controls

Control	Name
Form	frmTrafficLights
Button1	cmdStart
Button2	cmdPause
Button3	cmdReset
Button4	cmdExit
PictureBox1	picNorth
PictureBox2	picSouth
PictureBox3	picEast
PictureBox4	picWest

3. Set the application's Startup Object property to TrafficLights.frmTrafficLights.
4. Save your work (if you have not already done so) to create the project folder for the "TrafficLights" application.
5. Each traffic light will be represented at any given time by one of four images, each of which represents one possible state for the light, i.e. Red, Red and Amber, Green, or Amber. Download the images for this application from the link below and unzip the images into your project folder.

Download the images here.

6. In the Solution Explorer window, right click on the project name ("TrafficLights") and select Properties from the drop-down window.
7. Click on the Resources tab, then click on "Create or open assembly resources". Your screen should now look something like the illustration below.

8. Click on the drop-down arrow next to Add a Resource and select Add Existing file... (you may see a warning message the first time you attempt to add a resource to the project, but you can safely ignore it in this instance).
9. Browse to the project folder, and select the image file traffic_lights_01.bmp. You should now see a screen like the one shown below.

10. Repeat the above procedure for the remaining three image files.
11. Return to the form design window and double click on the form to create the form's Load event handler.
12. At the cursor, insert the following code:

picNorth.Image = My.Resources.traffic_lights_01
picSouth.Image = My.Resources.traffic_lights_01
picEast.Image = My.Resources.traffic_lights_01
picWest.Image = My.Resources.traffic_lights_01
cmdPause.Enabled = False
cmdReset.Enabled = False

13. Run the program to see the result of your efforts so far. You should see a screen like the one shown below.

14. Add two Timer objects to your form and rename them tmrNS and tmrEW.
15. Set the Interval property for both timers to "1000".

In their initial state, both pairs of traffic lights (North-South and East-West) are set to Red. This is the default condition and means that no traffic will be moving initially. The normal sequence for traffic lights is Red (stop), Red and Amber (get ready), Green (go), Amber (prepare to stop) and Red (stop) once more.

While one pair of lights goes through this sequence, the other pair must remain on Red to ensure that traffic flows across the intersection from North to South and vice versa, or from East to West and vice versa, but not both at the same time. In real life, the timing used can vary from one set of traffic lights to another depending on the amount of traffic that goes through the intersection on a daily basis, on the time of day (since traffic flows can vary considerably throughout the day), and on the day of the week (weekdays are often busier than weekends).

For the purposes of this application we will assume that there is a uniform flow of traffic through the intersection in all four directions. Each pair of traffic lights will be sequenced as follows:

1. Red - 3 seconds
2. Red and Amber - 3 seconds
3. Green - 10 seconds
4. Amber - 3 seconds

Although the order is not particularly important, we will start proceedings with the North-South pair of lights. When this pair of lights has completed one cycle, during which the East-West pair will remain in the Red condition, it will return itself to the Red condition. The East-West pair of lights will then commence its own cycle, during which the North-South pair of lights will remain in the Red condition. The entire sequence is then repeated.

The simulation is achieved by changing the images displayed by each PictureBox object as required in order to reflect the current status of each pair of lights. Since a timer simply "ticks" at pre-determined intervals and does not actually keep track of how many times it has ticked, we need to create variables to store this information.

16. Add the following line of code, immediately following the frmTrafficLights class declaration:

Dim NSCount, EWCount As Integer

17. Add the following code at the start of the form's Load procedure:

NSCount = 0
EWCount = 0

18. Return to the form design window and double-click on the Start button to create the event handler for the button's Click event.
19. At the cursor, enter the following code:

If NSCount = 0 And EWCount = 0 Then
  tmrNS.Start()
ElseIf NSCount > 0 Then
  tmrNS.Start()
Else
  tmrEW.Start()
End If
cmdPause.Enabled = True
cmdReset.Enabled = True
cmdStart.Enabled = False

20. Return to the form design window and double-click on the tmrNS timer object in the Component Tray to create the event handler for the timer's Tick event.
21. At the cursor, enter the following code:

NSCount += 1
If NSCount = 3 Then
  picNorth.Image = My.Resources.traffic_lights_02
  picSouth.Image = My.Resources.traffic_lights_02
ElseIf NSCount = 6 Then
  picNorth.Image = My.Resources.traffic_lights_03
  picSouth.Image = My.Resources.traffic_lights_03
ElseIf NSCount = 16 Then
  picNorth.Image = My.Resources.traffic_lights_04
  picSouth.Image = My.Resources.traffic_lights_04
ElseIf NSCount = 19 Then
  picNorth.Image = My.Resources.traffic_lights_01
  picSouth.Image = My.Resources.traffic_lights_01
  NSCount = 0
  tmrNS.Stop ()
  tmrEW.Start ()
End If

Clicking the Start button sets the Enabled property of tmrNS or tmrEW to True. Which timer is started depends on whether this is a new start, or whether the program is already running. If the program is running but has been paused, the code must determine which timer was last active.

When a timer is enabled, the Tick event handler will execute once for each time interval of 1000 milliseconds (1 second). It also enables the Pause and Reset buttons, and disables the Start button. The variable NSCount is incremented by one each time this code is executed. When the value of NSCount reaches 3 (a condition checked for by the first If statement), the existing images for picNorth and picSouth (showing the traffic lights in the Red condition) are swapped for copies of images showing the traffic lights in the Red and Amber condition.

The code within the second If statement is triggered when NSCount reaches 6, replacing the Red and Amber traffic lights with Green traffic lights. The third If statement is activated when NSCount reaches 16 and changes the traffic light images to Amber.

The last If statement is activated when NSCount reaches 19, and restores the Red traffic light images. It also resets NSCount to 0, disables tmrNS (by calling its stop() procedure), and enables tmrEW (by calling its start() procedure) .

22. Return to the form design window and double-click on the tmrEW timer object in the Component Tray to create the event handler for the timer's Tick event (this will be very similar to the code for tmrNS).
23. At the cursor, enter the following code:

EWCount += 1
If EWCount = 3 Then
  picEast.Image = My.Resources.traffic_lights_02
  picWest.Image = My.Resources.traffic_lights_02
ElseIf EWCount = 6 Then
  picEast.Image = My.Resources.traffic_lights_03
  picWest.Image = My.Resources.traffic_lights_03
ElseIf EWCount = 16 Then
  picEast.Image = My.Resources.traffic_lights_04
  picWest.Image = My.Resources.traffic_lights_04
ElseIf EWCount = 19 Then
  picEast.Image = My.Resources.traffic_lights_01
  picWest.Image = My.Resources.traffic_lights_01
  EWCount = 0
  tmrEW.Stop ()
  tmrNS.Start ()
End If

24. Add the following code to the program's Pause button:

If NSCount > 0 Then
  tmrNS.Stop ()
  cmdPause.Enabled = False
  cmdStart.Enabled = True
ElseIf EWCount > 0 Then
  tmrEW.Stop ()
  cmdPause.Enabled = False
  cmdStart.Enabled = True
End If

25. Finally, add the code below to the program's Reset button and run the program to check its operation.

picNorth.Image = My.Resources.traffic_lights_01
picSouth.Image = My.Resources.traffic_lights_01
picEast.Image = My.Resources.traffic_lights_01
picWest.Image = My.Resources.traffic_lights_01
tmrNS.Enabled = False
tmrEW.Enabled = False
NSCount = 0
EWCount = 0
cmdPause.Enabled = False
cmdReset.Enabled = False
cmdStart.Enabled = True