Visual Basic编程指南：为Pick程序员量身打造

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"Visual Basic for Pick程序员" 本书《Visual Basic for Pick程序员》由Mark Brown撰写，是一本专为Pick编程背景的开发者设计的Visual Basic教程。尽管出版时间可能较早（1996-1998年），但作为国外的经典教材，它依然具有一定的参考价值。Pick Systems Inc.是该书的版权所有者，并对书中可能存在的错误或遗漏不承担任何责任。 Visual Basic (VB) 是微软开发的一种可视化编程环境，主要用于创建Windows应用程序。对于熟悉Pick编程语言的开发者来说，这本书可能提供了一个过渡到VB的桥梁。Pick语言，如D3、Pick BASIC等，是Pick Systems Inc.注册商标，它们在数据库管理和业务应用方面有广泛应用。而VB则是一种面向对象的编程语言，以其易学易用和强大的Windows集成特性而闻名。书中可能涵盖了VB的基础知识，如语法、控件、事件处理、窗体设计以及与数据库的交互等内容。对于Pick程序员来说，理解VB如何处理数据、如何构建用户界面以及如何利用其强大的库和组件可能是重点学习部分。此外，书中可能还涉及了如何将Pick系统的概念和技能应用到VB编程中，以便于在新的开发环境中实现平滑过渡。作者Mark Brown可能在书中提到了一些转换的关键点，比如 Pick系统的开放架构(Pick Open Architecture)与VB的集成，以及如何利用VB的高级特性（如Advanced Pick中的FlashBASIC）来实现类似的功能。同时，书中可能会讨论到 Pick/BASIC的一些特性，如AQL查询语言，以及如何在VB中实现类似的数据操作。尽管书中的文档版本（D3 Version: 7.1）和出版日期（1998年）表明这是一份较早的资源，但VB的基本概念和编程原则至今仍然适用。对于希望拓宽技能范围，从Pick系统转向更广泛IT领域的开发者来说，这本书可能是一个宝贵的参考资料。然而，要确保结合最新的技术发展和更新的VB版本进行学习，以确保知识的时效性。《Visual Basic for Pick程序员》旨在帮助那些有Pick编程背景的开发者理解和掌握Visual Basic，从而更好地适应Windows应用程序的开发环境。书中包含的详细信息和实用指导，对于想要跨越技术鸿沟的程序员来说，是一份不可多得的学习资料。

Page 6 Visual Basic for Pick Programmers

FieldA needs to have a value before FieldB can be entered, you will want to have

the GotFocus event of FieldB check for FieldA contents and set the focus back to

FieldA if it is empty. So make sure there is no code in the FieldA GotFocus event

which might make it skip. If you are setting focus due to an error condition,

always notify the user. Since there is no easy way to disable the tab key or the

mouse, you can’t depend on serial input anymore. Stuff happens, but it doesn’t

always happen in the right sequence.

And, as long as we are talking about the KeyPress event, there is another

significant feature of Visual Basic you should be aware of. The form itself has a

property called KeyPreview. If this property is set to True, the form looks at

every keystroke and filters it, making a form-level KeyPress event active. Use

this event to look for such things as function keys and user-defined control-key

sequences.

For example, your users may be used to Pick System’s Update Processor and are

in the habit of using Ctrl+D and Ctrl+F to cruise through your database. You can

attach code to the text box that looks for these and acts accordingly. But you

DON’T want to have to attach code to perform function key processing into

every text box on your form. So make the form’s KeyPreview property true and

attach code to the form’s KeyDown event like this:

Private Sub Form_KeyDown (KeyCode As Integer, Shift As Integer)

Select Case KeyCode

Case vbKeyF1: MsgBox "F1 is your friend."

Case vbKeyF2: MsgBox "F2 could copy text."

Case vbKeyF3: MsgBox "F3 could paste text."

Case vbKeyF4: MsgBox "F4 could format text."

End Select

End Sub

Okay. So maybe you want to perform some tasks that are a bit more meaningful.

This is just an example. But you get my point. You can detect keystrokes at two

different levels, the form and the individual field. Another example would be

numeric-only fields.

If you have a data field that you know beyond doubt cannot contain anything but

numeric characters, you might make a KeyPress event like this:

Private Sub NumField_KeyPress(KeyAscii as Integer)

If Not Chr(KeyAscii) Like “#” Then KeyAscii = 0

End Sub

This code will check the character represented by the KeyAscii decimal value. If

that character is numeric (Like “#”) then it’s okay. If not, then change it to 0. The

0 value will be ignored and the string won’t change.

Other examples of events would be the form’s Load and Unload events that take

place when the form module is loaded to and unloaded from memory. If the

mouse is clicked on a control, there is a Click or DblClick event. There are

Likes and Unlikes Page 7

GotFocus and LostFocus events to trap the fact that an object has taken or given

up the focus.

There is a Timer object that has a Timer event. This event can be used to initiate

timed processing such as time-date displays, process monitoring and other

periodic routines.

Learn these events and when they occur to streamline your code. Let the system

do as much for you as it can before you start intensive coding projects.

First Big Difference: Data Typing and Scope

Visual Basic types data, so you’d better get used to it early. In the Pick/BASIC

world, we typed data on the fly and every field was treated like a string until it

had to be a numeric, and then it had better be a numeric. VB stores data internally

differently, depending on its type, and now we need to know things like, is this a

number or a string, and never the twain do meet. VB will choke if you try to feed

it the wrong type of data. But, on the other hand, we now have a whole spectrum

of data types to use to more efficiently process our information. Here’s a brief list

of the major data types you will need to know about right away. Check out your

favorite VB manual for a complete list.

String

Just like the name implies, this is text data. Strings can be up to

64K long. Strings stored in table “text” fields are limited to 256

bytes. Strings stored as “memo” fields may be the full 64K

long. Be aware of this if you are going to be mixing and

matching a D

database with other vendor’s information

systems. In VB 5.0, this string size increases to over 1GB.

Integer

This is a 16-bit signed integer, +/-32767. This field takes two

bytes of memory or file space to store.

Long

This is a 32-bit signed (long) integer, meaning it’s a whole

number, four bytes long, +/-2,147,483,648. Note, this form is

NOT long enough for telephone numbers if you plan to ever

call anyone in an area code greater than 214.

Single

This is the standard numeric for fields that contain anything

with decimals. This is a single precision floating point number

and it takes 4 bytes. Value range from -3.402823E38 to

-1.401298E-45 for negative values and from 1.401298E-45 to

3.402823E38 for positive values. Don’t let the scientific

notation throw you. This just means that Singles can be very

large or very small. This is the format most of the SQL

processors call “Float”.

Double

Use this for really big or really small numbers. This is a double

precision floating point number with enough decimal positions

Page 8 Visual Basic for Pick Programmers

to convert Italian currency. This field takes 8 bytes. Negative

values range from -1.79769313486232E308 to

-4.94065645841247E-324 and positive values range from

4.94065645841247E-324 to 1.79769313486232E308. Use this

type sparingly. It takes a lot of horsepower to muscle these

floating point fields, even with a co-processor.

Currency

This is a specialized single precision floating point numeric

with a standard 4 decimal places.

Variant

This is actually the most Pick/BASIC-like data type. As its

name implies, it is variable, just what this field contains. It

could be integer, string, or date/time. Use these whenever you

don’t really care about data type (like for for/next loop

counters) and places that don’t specifically need a certain data

type. But be aware, they are MUCH slower because VB has to

figure out what data type they are each time it references them,

and they take an incredible amount of overhead in terms of

memory and disk space.

Boolean

This field type describes a one-byte field with the possible

values 0 and –1 (false and true). These fields may be used in if

statements without the = True or = False modifiers.

Data fields don’t have to be predefined, but if they aren’t, VB assumes they are

variant types and will treat them as such. Most of the time, that’s not bad, but

some times it can be, so most programmers use a thing called OPTION

EXPLICIT in their General Declarations section. This tells your project that all

the fields must be defined before they are used. This brings us the next big

difference between Pick/BASIC and VB.

You can get around some of this by “implicitly” typing your data fields. This

means tagging a special character onto the end of the field name to signify its

type. For example:

indicates a String as in sStr$, name$, address$ and I$.

indicates an Integer as in I%, custno%, areacode% or qty%.

means a Long integer as in phone& or custID&.

Programmers

Note:

This is dangerous and should be used with care. & is

the character VB uses to concatenate two strings (see

below). If you don’t place the space between the field

name and the & sign, the system will interpret it as a

new field with a long integer data type. Also, Microsoft

has announced plans to stop supporting this in some

future release of VB.

Indicates a Single and # means a Double precision floating point number

Page 10 Visual Basic for Pick Programmers

Using these commands in conjunction with something called Hungarian

Notation, you can easily identify the type of data by its name and protect yourself

from mis-typing data within different procedures. For example, the following can

make a programmer’s life much easier:

In your Module, General Declarations section, enter:

DefInt I

DefStr s

DefLng l

DefVar v

Then, in your code modules, names like these will be set to the proper type automatically:

sString = D3Record.brExtractStr(1)

For iCnt = 1 to iMaxCount

call D3RuleModule.brCall(vParam)

In this code snippet, sString, iCnt, iMaxCount and vParam as all assumed to be

string, integer and variant types because the first letter of their names

corresponds to one of the letters setup in your module’s general declarations

section. Remember, to use this, you must remove the “Option Explicit” from

your module.

You can define multiple fields on a single Dim line, but you cannot imply that

they are all the same data type.

Consider the following:

Dim abc, def, ghi as Integer

Logically, you would think that this defines three integer fields. You would be

wrong!! Visual Basic will create abc and def as variants and ghi only as an

integer. This can be a real pain in the enpennage, but that’s how VB works.

Data Scope

We will be talking a lot about Scope and Focus in the rest of this document, so it

behooves us now to explain how Visual Basic keeps track of things. The first and

most important question to a data processor is: Where is the data?

In Pick/BASIC, every field within a given program was available throughout that

program, and not available to other “sub-” programs. If we wanted to pass data

between two routines, we had to use common data fields, or pass parameter lists.

VB works the same way, only with more layers. Let’s start at the top and work

our way down.

We place Global variables, things that we want to be available to all code

throughout the project, in the General Declarations section of a Code Module.

There can be more than one Code Module, so be careful not to use the same

name twice for globals. This is not fatal in itself, as you can reference variables

by full path (object.variablename), but it can lead to debugging nightmares. Code

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jedshen

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