Class Modules

Standalone class modules differ from form and report class modules in several ways.

First, standalone class modules do not have a built-in user interface, as form and report class modules do. This makes standalone class modules more suited to tasks that do not require an interface, such as performing calculations, looking up data, or modifying a database. When form or report modules require computationally intensive tasks, they can call a standalone class module.

Second, standalone class modules offer Initialize and Terminate events that enable operations that need to take place at the opening and closing of a class instance. Report and form modules do not have these events, but you can perform similar functions with the Load and Close events.

Third, you must use the New keyword to create instances of standalone class modules. Report and form class modules also let you create instances with the DoCmd OpenForm and OpenReport methods as well as by referencing the report or form class module's properties or methods. For example, Form_MyForm.SetFocus opens the MyForm form.

You can create a standalone class module from the Insert menu in VBE. (The same menu offers commands for building a standard module or a procedure.) After creating a class module shell, you can populate it with procedures and declarations, which equip it with custom properties and methods.

Custom Property Functions and Custom Methods

Special property functions make it easier to develop any combination of read-only, write-only, and read/write properties for your classes. If your application permits, you can define properties by simply declaring a public variable. When a class module defines a property with a public variable, it is always a read/write property. The ability to declare custom properties lets you extend the basic Access functionality for forms and reports. In addition, these property functions allow you to create powerful standalone classes.

Your applications can also build custom methods into classes. You can use subprocedures or function procedures to accomplish this. By selectively exposing variables and procedures with the Public keyword, you can narrowly define what methods and properties they expose. This lets your applications define interfaces to your class objects that perform in very specific ways.

Instantiating Classes

The public methods and procedures support programmatic access by procedures outside the class. You must first instantiate the class in a host procedure within another module, using the New keyword. (You use the same keyword to instantiate objects from other classes, such as ADO Connection and Recordset objects. In fact, your applications can instantiate multiple copies of a custom class at the same time—just like the ADO classes.) After instantiating a class, the code in your host procedure manipulates the instance of the class, not the class itself. You can change a property in one instance of a form, but when you instantiate a second instance of the form it appears with the default property setting.

Custom Classes and Events

Although VBA in Access lets you build custom classes with their own properties and methods, you cannot build custom events within those classes. You can, however, design a class that hooks onto a built-in class or type library that you attach. For example, you can build a class module that launches VBA code in response to the ItemAdded and ItemRemoved events of the References collection. This collection tracks links to external type libraries and ActiveX controls. After referencing a library, such as the Microsoft ActiveX Data Objects 2.1 Library, you can build custom events around the ADO events for the Connection and Recordset objects. These events can enable asynchronous data access that lets your application respond to users even while it remains ready to respond to a completed connection or the availability of a fetched set of records.

You use the WithEvents keyword within a Public declaration to point to an object reference that monitors and reports events within an ActiveX control. This keyword is valid only in class modules. You can define multiple variables within a module with the WithEvents keyword, but you cannot create arrays with it. Also, a declaration cannot contain both the WithEvents and New keywords.