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).

No problem:
1st step: we declare a class with the members:
      1 child (age)  event
      2 adult (age)
      3 senior(age)
      4 calcAge (...)
         this procedure accepts the dateofbirth and calculates the age.
        It checks the age an decides 3 stages
              1 child: raiseevent child(age)
              2 adult : raiseevent adult (age)
              3 senior: raiseevent senior(age)
2nd step: we use the class by creating an object objOfClass with the following members
      1 objOfClass_Child   event  it occurs when age was in a stage to raise child event
      2 objOfClass_Adult   event  it occurs when age was in a stage to raise adult event
      3 objOfClass_Senior event  it occurs when age was in a stage to raise senior tevent

     Having three events we create 3 sub procedures to handle these events.
    Private Sub objOfClass_Child ( ......
    Private Sub objOfClass_Adult (......
    Private Sub objOfClass_Senior (......

finally: what is the advantage?
"This way coding is simplified,
If rules change for age grouping, you need to change the relevant code in the class.
Otherwise the code of the event handlers, the three procedures, need to be modified.

Many people are using access without custom events, but they are there, just if you decide to use them."

Yes, having a class Called Person with properties and events like a person is Child, Adult, or Senior. A function in this class accepts an age as years groups the person to proper age group.

To use the class we create an Object variable called Driver As Person.
Compare this with Dim myTextBox As TextBox

Cehck the attached database for the Class Person, and Object Driver.
Try for number of years 17, 33, 77

If you want to change age groups you change it in the class definition and the program works normally.

Practical use,
In each event you may ask for other info relevant to the age.
If you want to add another age group called Infant, all you have to to is add this to the class.
Add the relevant sub for Driver_Infant.
You have a structured program, where less mistakes can creep in.
Good luck!

I would expect in normal terms that this use of an event would come into play when for instance the child event is raised and if for example it was a credit application then the event would reject it accordingly.  i.e. it is simply activities that relate to the specific condition and could as said be coded as a direct check on age and calling a sub.  The advantage of an event in this circumstance is as for Object oriented code in that it hides all the functionality making the main code easier.

You observe that the process of following such code can be harder than a linear sequence BUT it is in my view easier since it seperates out specific functionality, and therefore you can deal with specific code branches in isolation.  Like all code however you can create excessive structures that make it harder but that is poor design rather than the mechanism.  

Chris

First off recall that this use of class is from HNASR, I commented on the more classical use of classes for application level events.

However again to use my best efforts:

Consider a 'box', this box is called Person and everywhere I want to do something on some data for person I push the data into the box and out comes my answer.  Everywhere I use person I do exactly the same but one day I need to modify person.  I might add a new capability in which case it is not immediately critical but if I change an existing capability to for example to extend the valid range then in this use of a class I make the change in one location - Person.  In classical coding I would have to change every use of a code block doing this moding.

Classical coding - calculation is distributed and repetitive
Subroutine based - coding calculation is centralised but related data operates independently
Event oriented - coding calculation is centralised with related data all contained and working together

CHris

In Visual Basic, two types of events:
1 automatically happening (raised) events.
      When you click a button then, whether you like it or not, a Button.CLick event occurs.
      In fact many events happened before clicking the button, like mouse Enter, hover, Exit of any control the mouse crossed on its way to the Button.
     To handle any of these events, you have to write a procedure and add to it Handles Object.Event (Ex: Private Sub EvH (...) Handles Button1.Click)
2 not automatically raised events
     These are created by the user
     You need to specify the event (in automatically raised events this is defined in the Button class by Visual Basic),
     You need to tell visual basic when the event happens (this is defined in automatic events by Visual Basic)
     You need to specify the code to handle the event, and this you have to do it for both

Why use Custom event?
You develop a program and you want (See, you want) to record info about the use of the program. You want to check if a user changed the price of a merchandize. You add an event in the class, and why not in the code?  chris_bottomley commented on this to localize changes in one place. I add that a class hides data from the user. In the class you specify when to raise the event, may be you want to raise the event if certain users do that. You don't want the users to know (Security measures). This is called Data Hiding in Object Oriented Programming.
In the code you only incorporate the Event Handler, which may save the info passed to it into database. As Visual Basic knows when click event occurred, you alone know when a salary raises event occurs.
If you want to change the conditions for raising the salary raises event, you only do that in one place, in the class. You may be referring to salary rise in many places of your program and you do not want to hunt for the code to modify.