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<HTML><HEAD><TITLE>Visual Basic in 12 Easy Lessons vel04.htm </TITLE><LINK REL="ToC" HREF="index.htm"><LINK REL="Index" HREF="htindex.htm"><LINK REL="Next" HREF="velp02.htm"><LINK REL="Previous" HREF="vel03.htm"></HEAD><BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080"><A NAME="I0"></A><H2>Visual Basic in 12 Easy Lessons vel04.htm</H2><P ALIGN=LEFT><A HREF="vel03.htm" TARGET="_self"><IMG SRC="purprev.gif" WIDTH = 32 HEIGHT = 32 BORDER = 0 ALT="Previous Page"></A><A HREF="index.htm" TARGET="_self"><IMG SRC="purtoc.gif" WIDTH = 32 HEIGHT = 32 BORDER = 0 ALT="TOC"></A><A HREF="velp02.htm" TARGET="_self"><IMG SRC="purnext.gif" WIDTH = 32 HEIGHT = 32 BORDER = 0 ALT="Next Page"></A><HR ALIGN=CENTER><P><UL><UL><UL><LI><A HREF="#E68E32" >What You'll Learn</A><LI><A HREF="#E68E33" >Event-Driven Environments</A><LI><A HREF="#E68E34" >Control Properties</A><LI><A HREF="#E68E35" >Naming Conventions</A><LI><A HREF="#E68E36" >More Consistency: AUTOLOAD.MAK and CONSTANT.TXT</A><LI><A HREF="#E68E37" >Quick to the Draw!</A><LI><A HREF="#E68E38" >Homework</A><UL><LI><A HREF="#E69E32" >General Knowledge</A><LI><A HREF="#E69E33" >Find the Bug</A><LI><A HREF="#E69E34" >Extra Credit</A></UL></UL></UL></UL><HR ALIGN=CENTER><A NAME="E66E5"></A><H1 ALIGN=CENTER><CENTER><FONT SIZE=6 COLOR="#FF0000"><B>Lesson 2, Unit 4</B></FONT></CENTER></H1><BR><A NAME="E67E8"></A><H2 ALIGN=CENTER><CENTER><FONT SIZE=6 COLOR="#FF0000"><B>Bare-Bones Programs</B></FONT></CENTER></H2><BR><BR><A NAME="E68E32"></A><H3 ALIGN=CENTER><CENTER><FONT SIZE=5 COLOR="#FF0000"><B>What You'll Learn</B></FONT></CENTER></H3><BR><UL><LI> [lb] Event-driven environments<BR><BR><LI> [lb] Control properties<BR><BR><LI> [lb] Naming conventions<BR><BR><LI> [lb] AUTOLOAD.MAK and CONSTANT.TXT<BR><BR></UL><P>In this unit, you will create your own program with Visual Basic from scratch You will write a fully-working Windows program that displays a resizable window, a command button, and a control button with a control button menu.<BR><P>Before you create your first application, however, you must understand how Visual Basic programs interact with the Windows environment.<BR><BR><A NAME="E68E33"></A><H3 ALIGN=CENTER><CENTER><FONT SIZE=5 COLOR="#FF0000"><B>Event-Driven Environments</B></FONT></CENTER></H3><BR><P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>A Windows program behaves differently from a DOS-based program. Instead of the program controlling the user, the user controls the program. When the user responds to a menu or control, Windows generates an event that describes the particular action.<BR><P>Picture yourself driving down the road. All kinds of controls are at your fingertips—steering wheel, blinkers, headlights, breaks, gas pedal, gear shift, radio knobs, rearview mirror, and air conditioning and heater controls. At any time, you might press the brake, turn left, turn on the radio, adjust the mirror, or speed up. The driving conditions, not the physical order of the controls, determine what you do next.<BR><P>When you press the gas pedal, does your foot alone make the car go faster? The answer is no. You would have to have a very powerful foot to be able to speed up a car already going 50 miles per hour. The foot pedal causes an event to happen. That event is that more gas is fed to the car for fuel to burn and the car goes faster. Your car performs its job and makes adjustments based on the events that you trigger.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><I>Definition: </I>An <I>event</I> can be a mouse move, a mouse click, a keystroke, or a control response.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>Using a Windows program is like driving a car in the following respect: You do not always perform the same actions, and the actions that you perform cause certain events to occur. In Windows terminology, an <I>event</I> is the action that the user takes. Whenever the user clicks the mouse, presses a key, responds to a control, or selects from a menu, an event happens. Windows constantly monitors the running Visual Basic program, looking for events.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Note: </B>Too many things can happen in a GUI-based program for it to follow a straight, sequential pattern. If you were to run a DOS-based accounting program, the program more than likely would present you with a menu of limited choices. Only after you select from the menu, does the program take you through the next step. In a Windows program, you are offered a selection of controls, and you can respond to any control in any order. The program must be able to sense when an event takes place and to handle it accordingly.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>Windows and Visual Basic constantly monitor running programs. When the user clicks a command button or performs any other kind of event, Windows intercepts the event and sends it to Visual Basic.<BR><P>Visual Basic does not respond to some special system events, such as when the user presses Alt+Tab to switch to another application running in memory. That is why Windows must interpret all events and pass the ones handled by Visual Basic to Visual Basic. Figure 4.1 shows the relationship between events and event procedures. Notice that Windows always intercepts the events and passes the appropriate events to the Visual Basic program, where Visual Basic then does something in response to them if event procedures are available.<BR><P><B> <A HREF="04vel01.gif">Figure 4.1. Windows intercepts events before it </B><B>passes them to Visual Basic.</A></B><BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><I>Definition: </I>An <I>event procedure</I> responds to an event.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>When Visual Basic gets an event from Windows, it checks whether the programmer wrote an <I>event procedure</I> for the event. If an event procedure exists, Visual Basic executes the event. In the CONTROLS.MAK program that you ran in the previous unit, clicking the Next Control command button caused the next control in the program's repertoire of controls to appear. The only way that the program could respond to the Next Control command button was for an event procedure to be written for that particular event—namely, the command button keypress.<BR><P>A program can contain controls and still not respond to all events. In CONTROLS.MAK, for example, when you select the check boxes, nothing happens except that the boxes are selected or deselected. The check box controls handle the checking and unchecking of the boxes so that no event procedure is needed to do that. Once you select check boxes, though, the program does absolutely nothing with the selected check boxes because there is no event procedure inside CONTROLS.MAK that does any work when the user selects a check box. The check boxes exist in that program only to illustrate how they operate.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Warning: </B>Most, but not all events, are user-triggered. Some internal Windows events can take place that trigger actions. Likewise, you can direct a program to respond to timed intervals, such as ticking a clock forward every second. The interval of time would be an event in that case.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>Is all this talk about events and event procedures getting technical? Actually, you will see that the implementation of event capturing and event procedures is extremely easy. The Visual Basic environment is set up to create event procedures for you when you request them. The bottom line is this: When you want your Windows program to respond to an event, make sure that you write an event procedure for it. If you want to ignore certain events, don't write event procedures for them.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Tip: </B>Most of the events that you want to handle are obvious. For example, if you add a command button control to a form, you want to do something when the user clicks the command button. However, if the user tries to drag the command button with the mouse, you probably want to ignore that event because the user should rarely be allowed to move controls during the execution of the program.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>Almost anything that can happen during the execution of a Windows program can be an event. Once you design a Windows program form and place controls on it, you must write code that responds to events. That code is made up of event procedures. In a way, each event procedure is like a miniature program that you write for each control whose event should cause an action to take place.<BR><BR><A NAME="E68E34"></A><H3 ALIGN=CENTER><CENTER><FONT SIZE=5 COLOR="#FF0000"><B>Control Properties</B></FONT></CENTER></H3><BR><P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>All controls are different and work differently. You use different controls for different things. Even among controls of the same type, however, differences exist. Those differences reside in the control properties.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><I>Definition: </I>A <I>property</I> determines how a control differs from other controls.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>Consider the three controls shown in Figure 4.2. All three controls are command buttons even though they all look different. The user can click any of them, but each has a different set of properties.<BR><P><B> <A HREF="04vel02.gif">Figure 4.2. Three command buttons with different </B><B>properties.</A></B><BR><P>The top command button has the size seen most often in Windows programs. Visual Basic automatically assigns this size when you add command buttons to your applications unless you specify something different. The second command button is much wider than the top one, and no access keystroke is available. Its caption is not displayed in boldfaced letters, and the font is not standard. The third command button is very small, and its caption is italicized. Because small italics on command buttons are not always easy to read, be careful about using them with small controls.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Tip: </B>Less is usually better.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>Adding too much of anything is usually worse than better. The application often requires that command buttons differ from their default size, but try to stay as consistent as possible. It is rarely a good idea to put more than one font on more than one command button that appears on the same form.<BR><P>One of the most important steps you take when you write Visual Basic programs is setting control properties. When this book teaches you how to add a new control to an application, you will learn about virtually all of its properties. The next unit, for instance, shows you how to place command buttons. Before you learn about placement, though, you will read about the command button's 25 properties.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Note: </B>Rarely will you have to change all the properties of a control when you place the control on the form. Nevertheless, if you see every property that is available when you learn a new control, you will know what you can—and cannot—do it.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE>Even Forms Have Properties: Visual Basic programmers use the generic term <I>object</I> for controls that they place on the form. Actually, even the form is an occurrence of an object. The word <I>object</I> means different things in different computer languages. Although faintly related, a Visual Basic object has little to do with the objects that you find in object-oriented programming languages such as C++.<BR>Every object in a Visual Basic program has properties. Even the forms have properties. As you can see from Figure 4.2, you can add descriptive titles to forms. The title of a form is one of its property settings. The background color is another property that you can set. Although you should stick to forms with white backgrounds for the sake of consistency—it is used for most Windows programs—you, the programmer, can change it.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><B>Tip: </B>Where do you add properties for controls?<BR>In the Properties window. (Good name, huh?)</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P><FONT COLOR="#FF8000"><B><I>Review: </I></B></FONT>Each control has a different set of property values. You often set initial property values when you place controls on the form. During the execution of the program, your code also often changes property values. The CONTROLS.MAK application, for example, changes a command button's caption property from Press Me to Once Again and back to Press Me. I added the original Press Me caption when I added the command button to the form. Then I used code to change the caption during the program's execution. What enables Visual Basic to know when to change the caption? When the user presses the command button, a command button click event occurs. Then the event procedure written for that particular event changes the caption. Read on, true Visual Basic believer, and you will see that event procedures are neither as difficult, nor as complicated, as they might first sound.<BR><BR><A NAME="E68E35"></A><H3 ALIGN=CENTER><CENTER><FONT SIZE=5 COLOR="#FF0000"><B>Naming Conventions</B></FONT></CENTER></H3><BR><P><FONT COLOR="#FF8000"><B><I>Concept: </I></B></FONT>Although you will learn all about control properties throughout this book, one control property is worth learning about early on—the Name property. All controls have a Name property. The Name property labels each particular control. Without a unique name, you could not distinguish one control from another inside the Visual Basic code. Although Visual Basic assigns default names to all controls, get in the habit of changing those names to something more descriptive so that you can remember them more easily as you add to the program.<BR><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><I>Definition: </I>A <I>naming convention</I> is a set of naming rules.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><P>The first property that you should always set is the Name property. This unit does not tell you <I>how</I> to set the Name property; that is covered in the next lesson. You must know in advance, however, that Visual Basic programmers do not arbitrarily assign names to controls. Programmers who want to make their programming lives less stressful follow prescribed naming conventions when they choose names for their controls.<BR><P>A convention is not just a group of people gathered for the weekend. A convention is a standard set of rules that you follow. You are already familiar with naming conventions—for example, the file-naming conventions used in Windows and DOS-based computers (up to eight characters for the name and up to three characters for the extension). In Visual Basic, the naming convention for controls is simple. When you decide on a name for your control, be sure to stay within these boundaries:<BR><UL><LI> [lb] Names can be as short as one character or as long as 40 characters.<BR><BR><LI> [lb] Names must begin with a letter of the alphabet and can be in either uppercase or lowercase letters.<BR><BR><LI> [lb] After the initial letter, names can contain letters, numbers, or underscores in names.<BR><BR></UL><BLOCKQUOTE><BLOCKQUOTE><HR ALIGN=CENTER><BR><NOTE><I>Definition: </I>A <I>reserved word</I> is a Visual Basic command.</NOTE><BR><HR ALIGN=CENTER></BLOCKQUOTE></BLOCKQUOTE><OL><LI> [lb] Names cannot be the same as a reserved word. <A HREF="velxb.htm">Appendix B</A> lists all the reserved words in Visual Basic.<BR><BR></OL><UL><LI> [lb] Names should make sense. For instance, although you could name an exiting command button Rose, cmdExit is better; it is self-documenting and easier to remember.<BR><BR></UL><P>The following are valid names:<BR><UL><UL>⌨️ 快捷键说明
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