Using the Timer Control

Timer controls respond to the passage of time. They are independent of the user, and you can program them to take actions at regular intervals. A typical response is checking the system clock to see if it is time to perform some task. Timers also are useful for other kinds of background processing.

Figure 7.46   The timer control

Each timer control has an Interval property that specifies the number of milliseconds that pass between one timer event to the next. Unless it is disabled, a timer continues to receive an event (appropriately named the Timer event) at roughly equal intervals of time.

The Interval property has a few limitations to consider when you're programming a timer control:

• If your application or another application is making heavy demands on the system — such as long loops, intensive calculations, or drive, network, or port access — your application may not get timer events as often as the Interval property specifies.
  
• The interval can be between 0 and 64,767, inclusive, which means that even the longest interval can't be much longer than one minute (about 64.8 seconds).
  
• The interval is not guaranteed to elapse exactly on time. To ensure accuracy, the timer should check the system clock when it needs to, rather than try to keep track of accumulated time internally.
  
• The system generates 18 clock ticks per second — so even though the Interval property is measured in milliseconds, the true precision of an interval is no more than one-eighteenth of a second.

Every timer control must be associated with a form. Therefore, to create a timer application, you must create at least one form (though you don't have to make the form visible if you don't need it for any other purpose).

Note   The word "timer" is used in several ways in Visual Basic, each closely related to the workings of the timer control. In addition to the control name and control type, "timer" is used in the Timer event and the Timer function.

Sample Application: Alarm.vbp

Techniques for working with the timer control are included in the Alarm sample application (Alarm.vbp) which is listed in the Samples directory.

Placing a Timer Control on a Form

Placing a timer control on a form is like drawing any other control: Click the timer button in the toolbox and drag it onto a form.

The timer appears on the form at design time only so you can select it, view its properties, and write an event procedure for it. At run time, a timer is invisible and its position and size are irrelevant.

Initializing a Timer Control

A timer control has two key properties.

Note that the Enabled property for the timer is different from the Enabled property for other objects. With most objects, the Enabled property determines whether the object can respond to an event caused by the user. With the Timer control, setting Enabled to False suspends timer operation.

Remember that the Timer event is periodic. The Interval property doesn't determine "how long" as much as it determines "how often." The length of the interval should depend on how much precision you want. Because there is some built-in potential for error, make the interval one-half the desired amount of precision.

Note   The more often a timer event is generated, the more processor time is used in responding to the event. This can slow down overall performance. Don't set a particularly small interval unless you need it.