Example
Using parameters passed by reference
An event may define a ByRef parameter meant to be returned to the caller:
Public Event BeforeSomething(ByRef cancel As Boolean)
Public Event AfterSomething()
Public Sub DoSomething()
Dim cancel As Boolean
RaiseEvent BeforeSomething(cancel)
If cancel Then Exit Sub
'todo: actually do something
RaiseEvent AfterSomething
End Sub
If the BeforeSomething event has a handler that sets its cancel parameter to True, then when execution returns from the handler, cancel will be True and AfterSomething will never be raised.
Private WithEvents foo As Something
Private Sub foo_BeforeSomething(ByRef cancel As Boolean)
cancel = MsgBox("Cancel?", vbYesNo) = vbYes
End Sub
Private Sub foo_AfterSomething()
MsgBox "Didn't cancel!"
End Sub
Assuming the foo object reference is assigned somewhere, when foo.DoSomething runs, a message box prompts whether to cancel, and a second message box says "didn't cancel" only when No was selected.
Using mutable objects
You could also pass a copy of a mutable object ByVal, and let handlers modify that object's properties; the caller can then read the modified property values and act accordingly.
'class module ReturnBoolean
Option Explicit
Private encapsulated As Boolean
Public Property Get ReturnValue() As Boolean
'Attribute ReturnValue.VB_UserMemId = 0
ReturnValue = encapsulated
End Property
Public Property Let ReturnValue(ByVal value As Boolean)
encapsulated = value
End Property
Combined with the Variant type, this can be used to create rather non-obvious ways to return a value to the caller:
Public Event SomeEvent(ByVal foo As Variant)
Public Sub DoSomething()
Dim result As ReturnBoolean
result = New ReturnBoolean
RaiseEvent SomeEvent(result)
If result Then ' If result.ReturnValue Then
'handler changed the value to True
Else
'handler didn't modify the value
End If
End Sub
The handler would look like this:
Private Sub source_SomeEvent(ByVal foo As Variant) 'foo is actually a ReturnBoolean object
foo = True 'True is actually assigned to foo.ReturnValue, the class' default member
End Sub
