7.4 Multitask Your Access Basic Code

10.10 Set a Maximum Locking Interval for a Record

10.10.1 Problem

You've employed pessimistic locking on your application's forms to prevent two users from making changes to the same record at the same time. Sometimes, a user will lock a record for an excessive period of time; for example, he might start to edit a record and then get a long phone call or leave for lunch without saving or canceling his edits. Is there any way to limit how long a user can lock a record and time out the user when the locking time limit has been exceeded?

10.10.2 Solution

There's no built-in database or form option for "maximum record lock interval," but you can create your own record lock timeout feature by making use of the form's Timer event. This solution shows you how to create such a facility using an event procedure attached to the form's Timer event.

To add a record lock timeout feature to your own application, follow these steps for each form for which you wish to enable this feature:

1. Open the form in design mode, and add to the form an unbound text box named txtMessage that will be used to display the countdown message. This control should be at least 3.45" wide and 0.1667" high. On the sample form, we have placed txtMessage in the form's footer, but you can place it anywhere you'd like.

2. Change the form's TimerInterval property to 1000. This will cause any code attached to the form's Timer event to be executed every 1,000 ms (or 1 second).

3. Create an event procedure attached to the form's Timer event. Figure 10-26 shows how the properties sheet for the form should look after completing these steps.

   Figure 10-26. The event procedure attached to the Timer event will execute once every second

   

4. Add the following code to the form's event procedure:

   ' Record lock timeout time in seconds Private Const conMaxLockSeconds = 60 Private Sub Form_Timer(  )     Dim intElapsed As Integer     Dim strMsg As String     Dim ctlmsg As Control          Static sfDirty As Boolean     Static sdatTimerStart As Date          Set ctlmsg = Me!txtMessage          If Me.NewRecord Then       Exit Sub     End If     Select Case Me.Dirty     ' Record has been modified since last save.     Case True         If sfDirty Then             ' Elapsed time may be over one minute, so grab both             ' the minutes and seconds portions of the elapsed time.             intElapsed = Minute(Now(  ) - sdatTimerStart) * 60 _              + Second(Now(  ) - sdatTimerStart)             If intElapsed < conMaxLockSeconds Then                 ' Update message control with remaining time.                 strMsg = "Edit time remaining: " _                  & (conMaxLockSeconds - intElapsed) & " seconds."                 ctlmsg = strMsg                 If intElapsed > (0.9 * conMaxLockSeconds) Then                     ctlmsg.ForeColor = vbRed                 End If             Else                 ' Time out user and undo changes.                 ctlmsg = ""                 ctlmsg.ForeColor = vbBlack                 ' Undo changes. One of these DoCmd statements                 ' might generate an error, so ignore errors.                 On Error Resume Next                 DoCmd.DoMenuItem acFormBar, acEditMenu, _                  acUndo, 0, acMenuVer70                 DoCmd.DoMenuItem acFormBar, acEditMenu, _                  acUndo, 0, acMenuVer70                 On Error GoTo 0                 sfDirty = False                 MsgBox "You have exceeded the maximum record " _                  & "lock period (" & conMaxLockSeconds _                  & " seconds). " & vbCrLf & vbCrLf _                  & "Your changes have been discarded!", _                  vbCritical + vbOKOnly, "Record Timeout"             End If         Else             ' Start timing the edits.             sdatTimerStart = Now(  )             sfDirty = True         End If          ' Record has not been modified since last save.     Case False         If sfDirty Then             ' User has saved changes, so stop timer.             sfDirty = False             ctlmsg = ""         End If     End Select End Sub

   Alternately, you can import the frmEmployees sample form from 10-10.MDB, open frmEmployees in design mode, pull up the Timer event procedure code, and copy all the lines between Private Sub Form_Timer( ) and End Sub to the clipboard. Close the sample form, open your own form's Timer event procedure, and paste the code from the sample form into your event procedure. Now delete frmEmployees from your database.

5. Save your form, and open and test it.

Now load the 10-10.MDB database. Open the frmEmployees sample form to test out the record lock timeout feature. Make a change to an existing record and leave the record in an unsaved state. After a brief delay, a message appears in the form's footer informing you how many seconds of edit time remain (see Figure 10-27). The number counts down second by second; the message color changes to red when only a few seconds remain.

Figure 10-27. The changes to this record will be timed out unless they are saved

Finally, if you haven't either saved or undone your changes during the specified time interval, your edits will be undone and a confirming dialog will inform you of the event (see Figure 10-28).

Figure 10-28. The changes to the record have timed out

10.10.3 Discussion

The technique in this solution makes use of the form's Timer event, the form's Dirty property, and a couple of static variables to repeatedly check to see if the form has had unsaved changes for an extended period of time.

The timer procedure begins by declaring several variables, including the following static variables:

sfDirty

Saves a Boolean variable that notes if the form was dirty (i.e., has unsaved changes)

sdatTimerStart

Saves the date/time the record was first dirtied

In addition, the code uses the NewRecord property to determine if the user is working with a new record and exits if this is the case. Since a user adding a new record can't lock the records of other users and likely will need additional time to complete a new record, we decided not to subject record additions to the timeout process. Here's the initial code of the event procedure:

Dim intElapsed As Integer Dim strMsg As String Dim ctlmsg As Control Static sfDirty As Boolean Static sdatTimerStart As Date Set ctlmsg = Me!txtMessage If Me.NewRecord Then   Exit Sub End If

The remainder of the event procedure uses a Select Case...End Select statement to branch on the value of the form's Dirty property and compare it against sfDirty (the value of the form's Dirty property the last time we checked). The process is summarized in Table 10-17.

 

Table 10-17. The state table for the Form_Timer event procedure

Current Dirty value	Value of sfDirty	Action needed
True	True	Form remains dirty. Check if time limit has been exceeded and undo edits if so.
True	False	Form has just been dirtied, so set sfDirty to True and sdatTimerStart to Now( ).
False	True	User has saved changes, so set sfDirty to False.
False	False	No action needed.

 

If the form is currently dirty (Me.Dirty = True) or was previously dirty (sfDirty = True), and the elapsed time is less than conMaxLockSeconds, the following piece of code is executed:

' Elapsed time may be over one minute, so grab both ' the minutes and seconds portions of the elapsed time. intElapsed = Minute(Now(  ) - sdatTimerStart) * 60 _  + Second(Now(  ) - sdatTimerStart) If intElapsed < conMaxLockSeconds Then     ' Update message control with remaining time.     strMsg = "Edit time remaining: " _      & (conMaxLockSeconds - intElapsed) & " seconds."     ctlmsg = strMsg     If intElapsed > (0.9 * conMaxLockSeconds) Then         ctlmsg.ForeColor = vbRed     End If Else     ' ...See below... End If

The code updates the txtMessage control with the countdown message, changing the color of the text to red if the elapsed time is greater than 90% of conMaxLockSeconds to call extra attention to an impending timeout.

If the form is currently dirty (Me.Dirty = True) or was previously dirty (sfDirty = True), and the elapsed time is greater than or equal to conMaxLockSeconds, the following piece of code is executed:

' Time out user and undo changes. ctlmsg = "" ctlmsg.ForeColor = vbBlack ' Undo changes. One of these DoCmd statements ' might generate an error, so ignore errors. On Error Resume Next DoCmd.DoMenuItem acFormBar, acEditMenu, acUndo, 0, acMenuVer70 DoCmd.DoMenuItem acFormBar, acEditMenu, acUndo, 0, acMenuVer70 On Error GoTo 0 sfDirty = False MsgBox "You have exceeded the maximum record lock period (" _  & acbMaxLockSeconds & " seconds). " & vbCrLf & vbCrLf _  & "Your changes have been discarded!", _  vbCritical + vbOKOnly, "Record Timeout"

The edits to the record are undone by using DoCmd.DoMenuItem to simulate the user selecting Edit Undo using the menus. Access has a two-level undo buffer (one for the current control and one for the current record), so you must issue the undo command twice, ignoring any errors that occur if only one undo was necessary. Next, the code puts up a message box to inform the user that he or she has lost the edits.

If the form is currently dirty (Me.Dirty = True) but wasn't previously dirty (sfDirty = False), sfDirty is set to True and the starting time is stored away in sdatTimerStart, as the following code shows:

' Start timing the edits. sdatTimerStart = Now(  ) sfDirty = True

If the form is not currently dirty (Me.Dirty = True) but was previously dirty (sfDirty = True), the code stops the timer by setting sfDirty to False and clearing txtMessage:

' User has saved changes, so stop timer. sfDirty = False ctlmsg = ""

Finally, if the form is not currently dirty (Me.Dirty = True) and wasn't previously dirty (sfDirty = False), nothing needs to be done.

While the code for this solution could have been placed in a global module, we chose not to, since its two static variables must be maintained between calls to the event procedure. Since this code could be used in multiple forms within the application, we chose to encapsulate it within each form's event procedure. You may wish to split the code into two parts: one part that maintains the static variables in the form's Timer event procedure, and a second common component that lives in a global module. To accomplish this, you'd have to pass three variables (by reference) to the common function: a form variable referencing the form, and the two static variables, sfDirty and sdatTimerStart.