Event is a mechanism of providing a notification to external listeners when certain action occurs. For example your object might need to tell its listeners that it is loaded, updated or removed. In Microsoft Office application example of the event would be: opening of the document, entering text in MS word, recalculating the value of cell in Excel, etc.
When developing automation software events are handled to invoke custom functionality, e.g. add log entry when cell value is changed in Excel.
It is possible to consume (handle) events of existing object or declare and raise your own events in custom classes.
If object exposes events it is required to declare this object as class variable and use WithEvents keyword
Dim WithEvents myObj As CustomObjectWithEvents
Visual Basic Editor will show all available events in the drop-down as shown below, and it is possible to select the required event from the list to subscribe to the notification.
Event handler function should be named as _ and match the signature of the event
Function myObj_SomeEvent(someParam As String) As Integer 'Handling 'SomeEvent' of 'myObj' variable myObj_SomeEvent = 0 End Function
Events can be declared in the class using the Event keyword. Signature of the event must be specified by providing the parameter names and their types in the parenthesis.
The following example demonstrates how to raise and handle the event.
The project tree looks the following:
EventsRaiser Class Module
This class raises the event named Completed with a string parameter of timeStamp. Event is raised when the DoWork method is called.
Public Event Completed(timeStamp As String) Public Sub DoWork() 'Do work RaiseEvent Completed(Now) End Sub
EventsHandler Class Module
This class handles the event and displays the message box.
Dim WithEvents myEventRaiser As EventsRaiser Private Sub Class_Initialize() Set myEventRaiser = New EventsRaiser myEventRaiser.DoWork End Sub Private Sub myEventRaiser_Completed(timeStamp As String) MsgBox "Completed: " & timeStamp End Sub
This module is an entry point which instantiates an instance of the events handler and keeps it in the memory.
Dim swEventsHandler As EventsHandler Sub main() Set swEventsHandler = New EventsHandler End Sub