Friday, February 12, 2016

Event-Driven Programming

This topic may seem to come from out-of-nowhere, but I promised a member at UtterAccess that I will elaborate on this subject from our discussions in the forums.

What is Event-Driven Programming?

In a nutshell, event-driven programming (EDP) is a coding methodology where the flow of the program is determined by "events." It is a different approach than procedural programming where the flow of the program is dictated by the steps specified as written in the program and object-oriented programming (OOP) where the flow of the program relies on the interaction between objects and their properties and methods.
Now, it so happens that the majority of Access VBA programming is an example of EDP (it also has some aspects of OOP mixed in), and that's what this topic is all about. I just wanted to describe how Access uses EDP to help understand why we have to do certain things when writing our VBA procedures.

EDP Components

Event-driven programming requires three components:
  1. Event Handler
  2. Event Binding (or Wiring)
  3. Main Loop
Event Handler
The first step in developing an event-driven program is to write a series of subroutines or procedures that will be executed once an event occurs. Some examples of events that can occur in a Graphical User Interface (GUI) application are clicking on a command button, opening a form, and entering data in a textbox. Access allows three types of code as event handlers as seen in figure 1.

Figure 1. Choose Builder

 Access event  handlers can be a macro (saved or embedded), an expression (usually a function call), or a procedure. As with many programming environments, Access also provides an event template when using the Code Builder so that all you have to do is write the code for the event. Here's an example of an event procedure using the template that Access provided:
Private Sub txtDateReported_BeforeUpdate(Cancel As Integer)
   If Me.txtDateReported > Date Then    
      MsgBox "Employee report date cannot be in the future.", _
         vbInformation, "Invalid!"
      Cancel = True
   End If
End Sub

Event Binding
After writing the code for handling an event in your program, the next step is to bind it to the specific event that the code is supposed to handle. I prefer to call this process as "wiring" the event. The method of wiring an event to its event handler in Access will depend on which code builder you used for creating the event handler. If you didn't use the Code Builder, you can select a name of a Macro from the dropdown in the event property to wire that event to a saved macro. If you used the Macro Builder, the event property that you are wiring (binding) will have "[Embedded Macro]" displayed. If you used the Expression Builder, the complete expression preceded by an equal sign will be displayed. And lastly, if you used the Code Builder, the event property will display "[Event Procedure]."

It is important to note that "wiring" an event is an important step in making sure the program runs as expected. Even if you have written the event handler to do all the necessary tasks when an event occurs, if it is not wired to that event, then the event handler will never be executed. An event in Access is not wired if the event property is empty. Remember, even if an event handler is present in the code module, it will not be called if the event property is blank.

Main Loop
The final piece of the puzzle is called the "main loop" because it's a function that coninually monitors for events and is responsible for calling the appropriate event handler when an event fires. Fortunately for us, Access already provides this function so we don't have to worry about it.

The main point of this article is to explain that you, as the programmer, can write as many event handlers as you like and has the option to wire any of them to the event you want to handle. And unless you "wire" the event to its event handler, the application may not run as expected. Please note that Access will automatically "wire" the event handlers you've written if you used the "build" button next to each event property. And as was mentioned in the UtterAccess thread, you can also go straight to the VBA Editor window and write a procedure based on an Access event template, and Access will automatically wire that event handler to the appropriate event based on the template you used.

See the UtterAccess discussion that started it all.

Original post date: July 5, 2013

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