These events fire toward the end of a form's life. Their event
procedures are therefore good places to put "cleanup" code. Once again, the exact
circumstances and timing of these events can differ.
The DeActivate Event
The DeActivate event fires when
a form or a control on the form loses focus
to another object in the application outside
the form. The DeActivate event does not fire
when the user navigates to a completely different
application, but only when the user navigates
elsewhere in the current application. The DeActivate
event does not fire when the application closes
or when the current form unloads.
NOTE : Be Careful With the End Statement.
You can quickly terminate your application by putting a one-word statement, "End",
in your code. However you should be aware that doing so will abruptly stop any
further processing by the application.
In particular this means that loaded forms' QueryUnload, Unload, and Terminate
events will not run. If you've put code in these event procedures to perform cleanup
on the environment or save pending data, this code will not run when you call
End.
Pressing the VCR stop button on the VB menu in order to stop your designtime application
is equivalent to calling the End statement.
The QueryUnload Event
The QueryUnload event fires just
before a form unloads from memory. The QueryUnload
event fires just before the Unload event. Its
main purpose is to let you detect why the form
is being unloaded and to programmatically halt
unloading if necessary. The QueryUnload event
procedure takes two parameters:
1. Cancel This is a True/False
value which is False by default. When Cancel
is False, it means that the unloading will continue.
You can set it to True to stop the form from
unloading.
2. UnloadMode This parameter can take several values, corresponding
to how the QueryUnload event was triggered. You can compare UnloadMode's value
with one of the following VB internal constants:
-
vbformControlMenu.
The form's QueryUnload event was triggered
because the user is closing the form.
-
vbformCode.
The form's QueryUnload event was triggered
by code that programmatically closes the
form.
-
vbAppWindows.
The QueryUnload event was triggered because
the Windows session is ending.
-
vbAppTaskManager.
The QueryUnload event was triggered because
the Windows Task Manager is closing your
application.
-
vbformMDIform.
The form is an MDI Child, and the MDI Parent
is closing.
A common use of the QueryUnload event is to prompt the user to
save changes (see Figure 6.2).
FIGURE 6.2 A typical use of the QueryUnload event is to give users a chance
to change their minds.
You give the user the option to
cancel the unload. If the user chooses to cancel,
you can set the Cancel parameter to True, as
in Listing 6.1.
LISTING 6.1
SETTING THE CANCEL PARAMETER IN THE QUERYUNLOAD EVENT PROCEDURE
Private Sub frmData_QueryUnload(Cancel
as Integer, _
UnloadMode As Integer)
Dim intUserChoice As Integer
intUserChoice = MsgBox("Save Changes?"
, _
vbYesNoCancel)
If intUserChoice = vbYes Then
Call SaveData
ElseIf intUserChoice = vbCancel Then
Cancel = True
End If
End Sub
The Unload Event
The Unload event procedure is
where programmers usually put cleanup code.
The Unload event fires after the QueryUnload
event. The Unload event procedure takes a single
parameter, the Cancel parameter. Unload's
Cancel parameter works the same as QueryUnload's
Cancel parameter.
It is possible to stop the form
from unloading in the Unload event procedure
by setting Cancel to True. However, since the
Unload event doesn't receive the QueryUnload
event's UnloadMode parameter, your Unload
event procedure has less information about why
the form is being unloaded than the QueryUnload
event procedure has.
For forms which are not MDI Child
forms, Unload always happens immediately after
QueryUnload (unless, of course, the unloading
was just cancelled in the QueryUnload by setting
the Cancel parameter to True).
NOTE : Timing
of QueryUnload and Unload in MDI Applications.
In an MDI application, MDI Child forms have
a slightly different timing for QueryUnload
and Unload events. See the section in this chapter
titled "Using the Unload and QueryUnload
Events in an MDI Application" for more
information
The Terminate Event
The Terminate event happens when
the instance of the form has been completely
unloaded and all the form's variables
have been set to Nothing. The Terminate event
happens only when you explicitly set the form
to Nothing in your code during or after the
Unload event procedure. For example, you could
use the statement:
Set Form1 = Nothing
after calling the Unload statement
for Form1.
You might use the Terminate event to perform final cleanup operations
for your form.
