HOWTO: Use SetWaitableTimer With Visual Basic

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
• Microsoft Visual Basic Professional Edition, 32-bit only, for Windows, version 4.0

SUMMARY

Many programmers use the Sleep API function to pause the execution of their programs. One of the disadvantages of using Sleep is that it pauses the thread that the application is running in, and that any window that the application has open will not repaint properly. This may present an unattractive user interface to the user. An alternative to Sleep is to use SetWaitableTimer, which will allow the screen to repaint, receive DDE messages, and so forth.

MORE INFORMATION

The following code sample illustrates the use of the SetWaitableTimer API.

Step-by-Step Example

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.



2. Place two CommandButtons on Form1.



3. Paste the following code into the code module of Form1:




Option Explicit

' Sleep API is declared in the form to keep the
' SetWaitableTimer code in its own re-usable module.
Private Declare Sub Sleep Lib "kernel32" ( _
    ByVal dwMilliseconds As Long)

Private Sub Command1_Click()
    Command1.Enabled = False
    Wait 10 ' seconds
    Command1.Enabled = True
End Sub

Private Sub Command2_Click()
    Command2.Enabled = False
    Sleep 10000 ' milliseconds
    Command2.Enabled = True
End Sub

Private Sub Form_Load()
    With Form1
        .Height = 1400
        .Width = 2400
    End With
    With Command1
        .Move 100, 100, 2000, 300
        .Caption = "SetWaitableTimer"
    End With
    With Command2
        .Move 100, 500, 2000, 300
        .Caption = "Sleep"
    End With
End Sub 

4. From the Project menu, click Add Module to add a new module (Module1) to the project.



5. Paste the following code into Module1:




Option Explicit

Private Type FILETIME
    dwLowDateTime As Long
    dwHighDateTime As Long
End Type

Private Const WAIT_ABANDONED& = &H80&
Private Const WAIT_ABANDONED_0& = &H80&
Private Const WAIT_FAILED& = -1&
Private Const WAIT_IO_COMPLETION& = &HC0&
Private Const WAIT_OBJECT_0& = 0
Private Const WAIT_OBJECT_1& = 1
Private Const WAIT_TIMEOUT& = &H102&

Private Const INFINITE = &HFFFF
Private Const ERROR_ALREADY_EXISTS = 183&

Private Const QS_HOTKEY& = &H80
Private Const QS_KEY& = &H1
Private Const QS_MOUSEBUTTON& = &H4
Private Const QS_MOUSEMOVE& = &H2
Private Const QS_PAINT& = &H20
Private Const QS_POSTMESSAGE& = &H8
Private Const QS_SENDMESSAGE& = &H40
Private Const QS_TIMER& = &H10
Private Const QS_MOUSE& = (QS_MOUSEMOVE _
                            Or QS_MOUSEBUTTON)
Private Const QS_INPUT& = (QS_MOUSE _
                            Or QS_KEY)
Private Const QS_ALLEVENTS& = (QS_INPUT _
                            Or QS_POSTMESSAGE _
                            Or QS_TIMER _
                            Or QS_PAINT _
                            Or QS_HOTKEY)
Private Const QS_ALLINPUT& = (QS_SENDMESSAGE _
                            Or QS_PAINT _
                            Or QS_TIMER _
                            Or QS_POSTMESSAGE _
                            Or QS_MOUSEBUTTON _
                            Or QS_MOUSEMOVE _
                            Or QS_HOTKEY _
                            Or QS_KEY)

Private Declare Function CreateWaitableTimer Lib "kernel32" _
    Alias "CreateWaitableTimerA" ( _
    ByVal lpSemaphoreAttributes As Long, _
    ByVal bManualReset As Long, _
    ByVal lpName As String) As Long
    
Private Declare Function OpenWaitableTimer Lib "kernel32" _
    Alias "OpenWaitableTimerA" ( _
    ByVal dwDesiredAccess As Long, _
    ByVal bInheritHandle As Long, _
    ByVal lpName As String) As Long
    
Private Declare Function SetWaitableTimer Lib "kernel32" ( _
    ByVal hTimer As Long, _
    lpDueTime As FILETIME, _
    ByVal lPeriod As Long, _
    ByVal pfnCompletionRoutine As Long, _
    ByVal lpArgToCompletionRoutine As Long, _
    ByVal fResume As Long) As Long
    
Private Declare Function CancelWaitableTimer Lib "kernel32" ( _
    ByVal hTimer As Long)
    
Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long
    
Private Declare Function WaitForSingleObject Lib "kernel32" ( _
    ByVal hHandle As Long, _
    ByVal dwMilliseconds As Long) As Long
    
Private Declare Function MsgWaitForMultipleObjects Lib "user32" ( _
    ByVal nCount As Long, _
    pHandles As Long, _
    ByVal fWaitAll As Long, _
    ByVal dwMilliseconds As Long, _
    ByVal dwWakeMask As Long) As Long

Public Sub Wait(lNumberOfSeconds As Long)
    Dim ft As FILETIME
    Dim lBusy As Long
    Dim lRet As Long
    Dim dblDelay As Double
    Dim dblDelayLow As Double
    Dim dblUnits As Double
    Dim hTimer As Long
    
    hTimer = CreateWaitableTimer(0, True, App.EXEName & "Timer")
    
    If Err.LastDllError = ERROR_ALREADY_EXISTS Then
        ' If the timer already exists, it does not hurt to open it
        ' as long as the person who is trying to open it has the
        ' proper access rights.
    Else
        ft.dwLowDateTime = -1
        ft.dwHighDateTime = -1
        lRet = SetWaitableTimer(hTimer, ft, 0, 0, 0, 0)
    End If
    
    ' Convert the Units to nanoseconds.
    dblUnits = CDbl(&H10000) * CDbl(&H10000)
    dblDelay = CDbl(lNumberOfSeconds) * 1000 * 10000
    
    ' By setting the high/low time to a negative number, it tells
    ' the Wait (in SetWaitableTimer) to use an offset time as
    ' opposed to a hardcoded time. If it were positive, it would
    ' try to convert the value to GMT.
    ft.dwHighDateTime = -CLng(dblDelay / dblUnits) - 1
    dblDelayLow = -dblUnits * (dblDelay / dblUnits - _
        Fix(dblDelay / dblUnits))
    
    If dblDelayLow < CDbl(&H80000000) Then
        ' &H80000000 is MAX_LONG, so you are just making sure
        ' that you don't overflow when you try to stick it into
        ' the FILETIME structure.
        dblDelayLow = dblUnits + dblDelayLow
    End If
    
    ft.dwLowDateTime = CLng(dblDelayLow)
    lRet = SetWaitableTimer(hTimer, ft, 0, 0, 0, False)
    
    Do
        ' QS_ALLINPUT means that MsgWaitForMultipleObjects will
        ' return every time the thread in which it is running gets
        ' a message. If you wanted to handle messages in here you could,
        ' but by calling Doevents you are letting DefWindowProc
        ' do its normal windows message handling---Like DDE, etc.
        lBusy = MsgWaitForMultipleObjects(1, hTimer, False, _
            INFINITE, QS_ALLINPUT&)
        DoEvents
    Loop Until lBusy = WAIT_OBJECT_0
    
    ' Close the handles when you are done with them.
    CloseHandle hTimer

End Sub 

6. Run the project.



7. Click the Sleep button. While the application is sleeping, notice that the screen does not repaint (drag another window over top of the application, and then drag it away). Then, try the same test with the SetWaitableTimer button.

REFERENCES

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

Q184796 HOWTO: Use a Waitable Timer with an Asynchronous Procedure Call

Additional query words:

Keywords : kbAPI kbSDKWin32 kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpVB kbDSupport
Version : WINDOWS:4.0,5.0,6.0
Platform : WINDOWS
Issue type : kbhowto