To get an idea how timers work, I'll create an example alarm clock, called Timers on the CD-ROM. You can see this example at work in Figure 8.7. Here, the user can click the "Start clock" button to start the clock, which is displayed in a label control. (I've set the font of the label at design time to use a large font face, as shown in Figure 8.7.) The clock's display is updated once a second in the timer in this example, Timer1. I've set the timer's Interval property to 1000 milliseconds, or one second, which means its Tick event occurs every second. I update the label's text like this in the Tick event handler: Label1.Text = TimeOfDay (the TimeOfDay property returns a DateTime object with the current time).
The user also can enter a time for the alarm to go off in three text boxes (using 24-hour format; for example 13:00:00 for 1:00:00 P.M.), and click the "Alarm on" radio button to "arm" the alarm clock. When the current time equals or exceeds the alarm time, the clock will beep once a second until the user clicks the "Alarm off" radio button. (These two radio buttons, "Alarm on" and "Alarm off", actually set the state of an internal Boolean variable, blnAlarm, which is True when the alarm is armed and False otherwise.)
Here's the whole code. Note that I'm using three handy properties here—TimeOfDay, which returns a DateTime object holding the current time of day; Today, which returns a DateTime object holding today's date; and Now, which returns a DateTime object that holds both today's time and date:
Public Class Form1 Inherits System.Windows.Forms.Form 'Windows Form Designer generated code Dim blnAlarm As Boolean = False Private Sub Timer1_Tick(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Timer1.Tick Label1.Text = TimeOfDay If TextBox1.Text <> "" And TextBox2.Text <> "" And _ TextBox3.Text <> "" Then Dim AlarmTime = New DateTime(Today.Year, Today.Month, _ Today.Day, CInt(TextBox1.Text), CInt(TextBox2.Text), _ CInt(TextBox3.Text)) If Now > AlarmTime And blnAlarm Then Beep() End If End If End Sub Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click Timer1.Enabled = True End Sub Private Sub RadioButton1_CheckedChanged(ByVal sender As _ System.Object, ByVal e As System.EventArgs) _ Handles RadioButton1.CheckedChanged If RadioButton1.Checked Then blnAlarm = True End If End Sub Private Sub RadioButton2_CheckedChanged(ByVal sender As _ System.Object, ByVal e As System.EventArgs) _ Handles RadioButton2.CheckedChanged If RadioButton1.Checked Then blnAlarm = False End If End Sub End Class
And that's all it takes—you can see the results in Figure 8.7. Now we've created a working alarm clock, using timers.