Timer class

A Timer is a non-visual Win32 control that raises a Timer event at a programmable interval. Drop one onto a Form (or UserControl) at design time, set Interval to the desired millisecond period, set Enabled to True, and handle the Timer event. Timers are invisible at run time --- they appear only as icons in the designer.

The default property is Enabled and the default event is Timer.

Private Sub Form_Load()
    Timer1.Interval = 1000      ' fire once per second
    Timer1.Enabled  = True
End Sub

Private Sub Timer1_Timer()
    lblClock.Caption = Format$(Now, "hh:mm:ss")
End Sub

Starting and stopping

Two properties between them decide whether the Timer event fires:

• Enabled is the master switch. While it is False, the timer is dormant regardless of Interval.
• Interval is the period between events, in milliseconds. Setting it to 0 stops events from firing even while Enabled remains True.

Either property can be flipped from inside the Timer handler itself --- a one-shot timer disables itself on the first tick:

Private Sub tmrStartup_Timer()
    tmrStartup.Enabled = False      ' fire only once
    LoadInitialData
End Sub

Assigning a negative value to Interval raises run-time error 380 (Invalid property value).

Accuracy and resolution

The control wraps Win32's per-window timer queue, which runs on the standard message pump. Two consequences follow:

• Resolution is coarse. The OS quantises timer ticks to the system clock-tick period --- typically ~15.6 ms on desktop Windows. Intervals shorter than that are silently rounded up. For sub-millisecond pacing use a multimedia timer or QueryPerformanceCounter directly.
• Ticks can be skipped under load. If the message pump is blocked when a tick is due, no events queue up --- the runtime delivers a single Timer event when the pump resumes, not one for each missed period. Long-running work inside the handler therefore lengthens the next interval rather than producing a backlog.

For periodic UI updates (a clock, a progress animation, a poll for external state) the Timer is exactly the right tool. For precise wall-clock pacing, audio scheduling, or anything that must keep up under heavy CPU load, it is not.

Control arrays

A control array of timers lets one handler service several periodic tasks while keeping a single shared code path. The array is declared at design time on the first item; further items are added at run time with Load and removed with Unload, exactly as for a windowed control. Inside the shared Timer handler, Index identifies which timer fired.

Private Sub tmrPoll_Timer(Index As Integer)
    Select Case Index
        Case 0: RefreshStatus
        Case 1: PollPrinterQueue
        Case 2: TrimLogFile
    End Select
End Sub

Index raises run-time error 343 (Object not an array) when read on a timer that is not part of a control array.

Properties

ControlType

A read-only ControlTypeConstants value identifying this control as a timer. Always vbTimer.

Enabled

The master switch for the timer. While False, the timer is dormant and the Timer event does not fire regardless of Interval. Boolean, default True. Default property.

Syntax: object.Enabled [ = boolean ]

Index

When the timer is part of a control array, the Long zero-based index of this instance within the array. Read-only at run time. Raises run-time error 343 (Object not an array) on a timer that is not part of an array.

Interval

The period between Timer events, in milliseconds. Long, default 0.

Syntax: object.Interval [ = value ]

value

A non-negative Long giving the interval in milliseconds. Setting 0 stops events from firing. Negative values raise run-time error 380 (Invalid property value).

The effective resolution is the OS clock-tick period (~15.6 ms on desktop Windows); shorter intervals are silently rounded up. See Accuracy and resolution.

Left

The horizontal position of the timer icon in the designer, in the container's ScaleMode units. Single. The runtime value has no visual effect --- the timer is invisible --- but it is preserved so the designer can place the icon back where it was.

Name

The unique design-time name of the timer on its parent form. String, read-only at run time.

Parent

A reference to the Form (or UserControl) that contains this timer. Read-only.

Tag

A free-form String the application can use to associate custom data with the timer. Ignored by the framework. Inherited from the base control class. Useful for control arrays --- e.g. holding the name of the operation a poll-timer is responsible for.

Top

The vertical position of the timer icon in the designer. Counterpart to Left; like Left, it has no visual effect at run time.

Events

Timer

Raised every Interval milliseconds while Enabled is True and Interval is greater than zero. Default event.

Syntax: object_Timer( )

For a timer that is part of a control array, the handler receives the array Index of the timer that fired:

Syntax: object_Timer( Index As Integer )

The event is delivered through the normal Win32 message pump --- see Accuracy and resolution for the implications.