Visual Basic Concepts
Perhaps the most important (and often overlooked) part of creating an application in Visual Basic is the design phase. While it's obvious that you need to design a user interface for your application, it may not be as obvious that you need to design the structure of the code. The way you structure your application can make a difference in its performance as well as in the maintainability and usability of your code.
The code in a Visual Basic application is organized in a hierarchical fashion. A typical application consists of one or more modules: a form module for each form in the application, optional standard modules for shared code, and optional class modules. Each module contains one or more procedures that contain the code: event procedures, Sub or Function procedures, and Property procedures.
Determining which procedures belong in which module depends somewhat on the type of application that you are creating. Because Visual Basic is based on objects, it helps to think of your application in terms of the objects that it represents. The design of the sample application for this chapter, Vcr.vbp, is based on the objects that comprise a video cassette recorder and a television. The VCR application consists of two form modules, a standard module, and two class modules. You can use the Object Browser to examine the structure of the project (Figure 5.2).
Figure 5.2 The structure of the VCR project is shown in the Object Browser
The main form for the VCR application (frmVCR) is a visual representation of a combination VCR and television screen (Figure 5.3). It is composed of several objects that model those found in the real world version. A group of Command buttons (cmdPlay, cmdRecord, and so on) mimic the buttons used to operate a VCR. The software VCR also contains a clock (lblTime), a channel indicator (lblChannel), function indicators (shpPlay, shpRecord, and so on), and a "picture tube" (picTV). The event procedures for all of these objects are contained in the Vcr.frm form module.
Figure 5.3 The main form for the VCR application
In many cases there are repetitive procedures that are shared by more than one object. For example, when the Play, Rewind, or Record buttons are "pushed," the Pause and Stop buttons need to be enabled. Rather than repeat this code in each button's Click event procedure, it's better to create a shared Sub procedure that can be called by any button. If these procedures need to be modified in the future, all of the modifications can be done in one place. This and other shared procedures are contained in the standard module, Vcr.bas.
Some parts of a VCR aren't visible, such as the tape transport mechanism or the logic for recording a television program. Likewise, some of the functions of the software VCR have no visual representation. These are implemented as two class modules: Recorder.cls and Tape.cls. Code to initiate the "recording" process is contained in the clsRecorder module; code to control the direction and speed of the "tape" is contained in the clsTape module. The classes defined in these modules have no direct references to any of the objects in the forms. Because they are independent code modules, they could easily be reused to build an audio recorder without any modifications.
In addition to designing the structure of your code, it's important to establish naming conventions. By default, Visual Basic names the first form in a project Form1, the second Form2, and so on. If you have several forms in an application, it's a good idea to give them meaningful names to avoid confusion when writing or editing your code. Some suggested naming conventions are presented in "Visual Basic Coding Conventions."
As you learn more about objects and writing code, you can refer to the VCR sample application for examples of various different coding techniques.