Question : When To Use Custom Events

I have been programming Office applications with VBA for several years now and have read many books on the subject.  One thing I have never quite been able to grasp is when and why to use custom events.  Events for application objects make sense because they can start code execution with parameters when the the application is not already executing code.  But in order for custom events to fire, code has to be already executing as a result of some other trigger.  So what is the purpose of doing the extra coding just to make something an event plus handler, instead of an ordinary subroutine that passes parameters around?  In fact, it seems that if you do use events and handlers, you're actually making the call chain harder to follow in the code.  For example, one book provided an example where a user form has a public WithEvents object variable that is used to instantiate a text file class.  The text file class has an OpenFile routine that accesses the text file reads it line by line.  Each time a line is read, the ReadLine event fires.  The listener then shifts execution back to the user form to handle MyTextFile_ReadLine(LineText as String), which drops the line into a textbox and inserts a carriage return.  Why would this make more sense than just having a public FileText Property in the class or a GetText Procedure?

Mike

Answer : When To Use Custom Events

This is a class definition from the uploaded database:  Class
'------------------------------------------------------
Option Compare Database
Private l As Single
Private w As Single
Public Event AllowedAreaExceeded(ByVal area As Single)
Public Function calcArea(ByVal l As Single, ByVal w As Single)
    Dim a As Single
    a = l * w
    If a > 100 Then
        RaiseEvent AllowedAreaExceeded(a)
    End If
    calcArea = a
End Function
'------------------------------------------------------------
This class has two members:
Public function  calcArea
Public event  AllowedAreaExceeded
When the function is executed, it calculates the area of a rectangle, then  compares the result with 100, if greater then you want an event to happen .  You achive this by issuing  the command
RaiseEvent AllowedAreaExceeded(a)
It is raised, allowed to happen, when a>100
Compare it to a Command1_click event, which occurs when we click a button.

This is the code behind the form:
Option Compare Database
Private WithEvents myClass1 As Class1

Private Sub Command3_Click()
Dim a As Single
     a = myClass1.calcArea(10, 11)
End Sub

Private Sub Form_Load()
    Set myClass1 = New Class1
End Sub
Private Sub myClass1_AllowedAreaExceeded(ByVal a As Single)
    MsgBox ("Area " & a & " exceeds allowed")
End Sub

Here are some helping notes which I taught today for a Visual Basic class. Command1, Command2, and so on are the default names access gives for command buttons when newly created.
1)      Access helps us to create event-driven  applications
2)      Event driven means the application is idle and is waiting for an event to drive it (let it work)
3)      The form has few controls with many events (actions) like button clicked, textbox changed, form clicked, form changed size, and mouse is hovering over an image.
4)      Of all these events we select the suitable ones, when happen, to drive the application (event driven application)
5)      As an example we select the click event of a command button (Command1_Click)
6)      We need a procedure which will execute when the event occurs: In access a procedure that starts like,
Private Sub Command1_Click (&..) will execute when Command1 control is clicked.
This procedure is called an event handler.
This procedure is a must to drive the application
Code has to be inserted in this procedure to do useful work.
One statement may be to call a sub procedure to calculate the average of 3 numbers
Another statement may assign the result of a function to a variable
7)      Other procedures should be included to cater for the previous commands
8)      Command1_Click is an event handler, and the other procedure is "I call it an order or command  handler,  it handles the order or the command to do something"
9)      There are situations when you need your own event to occur, like the area exceeds 100 units square.
10)      You declare this type of event in a Class,  and it will occur (when you issue RaiseEvent statement) in the procedures when the area > 100.
11)      Command buttons, textboxes, and other controls are defined by access as classes. When you click on a button icon and draw a button on a form, access creates an object of the class called Command1 as default. The class has events and occur when we click or hover  or change size and so on)
12)      When you select click event of a button and write code it will handle the click event.
13)      For our defined event, we write the procedure using the name of the event to handle the event when it occurs (Raised).