Como utilizar SetWaitableTimer com Visual Basic

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Exemplo passo a passo

1. Inicie um novo projecto EXE padrão no Visual Basic. É criado o Form1 por predefinição.
2. Coloque dois CommandButtons no Form1.
3. Cole o código seguinte no módulo de código do 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. No menu Project , clique em Add Module para adicionar um novo módulo (Módulo1) ao projecto.
5. Cole o código seguinte no 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
           ft.dwHighDateTime = ft.dwHighDateTime + 1 
       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. Execute o projecto.
7. Clique no botão de suspensão . Enquanto a aplicação está no modo de suspensão, repare que o ecrã não redesenhar (arraste outra janela de início da aplicação e, em seguida, arraste-o fora do escritório). Em seguida, tente o mesmo teste com o botão SetWaitableTimer .

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