CO1403 Visual Basic Programming

1
CO1403 Visual Basic Programming

• Week 4

2
Introduction

• This week we will look at,
• Programming models
• Sequential programming
• Procedural programming
• Event-driven programming
• Events
• Event procedures
• Look at some common events
• Pages 32-35

3
Programming Models

• It is fair to say that we have not even started
  looking at how we write Visual Basic code
• Before you can confidently write your own code,
  you should be able to predict how that code will
  execute
• One of the important things about predicting how
  your code will run is to understand in what
  sequence your lines of code will execute

4
Programming Models

• Our pseudo-code examples have all been of a
  similar ilk
• The algorithm starts at the first line of
  pseudo-code, and ends at the last line
• There is no particular sequence of events as
  specified within the pseudo-code
• However, when writing a Windows application, a
  user can interact with an application in many
  different ways, and are not always prescribed to
  take a particular series of steps

5
Programming Models

• Different programming models have matured over
  the years
• We are about to look at three different models

6
Sequential Programming

• Sequential programming is when one line of code
  is executed after the previous line of code
• Sequential programs have only ONE starting point
• Sequential programs have only ONE ending point

7
Sequential Programming

• To give examples of sequential programs will be
  quite difficult
• People have now gotten used to Windows
  applications, and few people will have used a DOS
  application
• A DOS application ran slightly differently from a
  Windows application

8
Sequential Programming

• A DOS application would operate in the following
  manner,
• You would start the application
• You would follow a pre-determined series of steps
• You would finish the program

9
Sequential Programming

• An example of sequential programming,
• intResult 10 3
• intResult intResult 7
• print (intResult)
• Program starts at the first line of code, and
  finishes after executing the last line of code
• The sequence of execution is set as you are
  writing the code

10
Procedural Programming

• Procedural programming is an extension of
  sequential programming
• Works by grouping together a series of lines of
  code in to a procedure
• Once lines are grouped together, we can give the
  group an identifier
• We can then use this identifier in the rest of
  our program in order to execute the grouped
  together lines of code

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Procedural Programming

• Procedural programming is the same as sequential
  programming in that
• we have a start and a stop
• and that the lines of code are executed in a
  predetermined order
• Grouping together lines of code gives us several
  advantages
• It reuses code that we have typed, rather than
  having to type it all out again
• It prevents mistakes from creeping in to our
  programs

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Procedural Programming

• A 10
• B 20
• C A B
• EdtResult.Text cstr(C)
• A 20
• B 30
• C A B
• EdtResult.Text cstr(C)
• A 30
• B 40
• C A B

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Procedural Programming

• Can be rewritten as,
• Sub DoAdd(A, B)
• C A B
• EdtResult.Text cstr(C)
• End sub
• DoAdd(10, 20)
• DoAdd(20, 30)
• DoAdd(30, 40)

14
Event-Driven

• Event-driven programming is used in most (if not
  all) Windows programming languages
• It is the model followed in Visual Basic
• It is slightly different than the previous
  examples
• Users interact with Windows applications in a
  non-predetermined sequence
• Our programs REACT to user interaction (rather
  than predetermine the order)

15
Event-Driven (Drawn on Board)

• An event-driven program does the following,
• When a program starts it goes in to an infinite
  loop
• The program sits and does nothing except waits
  for a user to interact with the application in
  some way
• When the program detects the user has interacted,
  it exits the infinite loop and executes some
  lines of code
• Once these lines of code are executed (in their
  predetermined order), the program returns to the
  infinite loop
• The infinite loop is only ended when the user
  closes the program

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Event-Driven

• It is important to understand how the
  event-driven model works
• Fortunately, you dont have to understand how to
  write the infinite loop, or how to break out of
  the infinite loop
• Microsoft have written this for you already
• Whenever you create a VB application, this
  event-driven programming model is built in to
  your application

17
Event-Driven

• Looking at an example that should be familiar to
  you all,
• Applying Microsoft Word to this model
• Double click Word icon and program starts
• Once application is loaded in to memory, it just
  sits there and does nothing (Word now in its
  infinite loop waiting for some kind of
  interaction)
• If you type a key, Word recognises this and draws
  the letter on the screen. In the background, a
  keypress was detected by the infinite loop, and
  control was passed to some code that drew the
  letter on the screen

18
Event-Driven

• After the letter is drawn on the screen, control
  is passed back to the infinite loop until some
  other interaction is detected
• No matter what kind of interaction occurs, Word
  merely sits there and waits for you to do
  something
• Once you have finished editing your document, you
  save your work (another interaction) and exit the
  application
• This model can be applied to most (if not all)
  Windows application

19
Events

• Events are important in event-driven programming
• An event is essentially the thing that causes the
  program to leave the infinite loop
• An event is when something happens
• An event is normally caused by some kind of user
  interaction (although this is NOT always the case)

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Events

• For example,
• When we press a key, a keyboard event is
  triggered, and our program drew the key on the
  screen
• Likewise, every time we move our mouse, a mouse
  event is triggered, and the mouse cursor is
  redrawn at a different position on the screen

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Event-Procedures

• When an event is triggered, our infinite loop
  checks to see if it can execute some code in
  response to that event
• As a developer, we can write an event-procedure
• We can associate an event-procedure with a
  particular event, and the code that we write
  within that event-procedure is automatically
  executed

22
Event-Procedures

• Fortunately, Visual Basic makes all this process
  very easy
• VB automatically adds the infinite loop to our
  programs (run a VB application to prove it)
• VB automatically associates an event with an
  event procedure
• All we have to do as developers is write the
  event-procedure

23
Common Events

• There are events in Windows that are common
  across many controls
• Some of those are listed here
• Click event happens when you click the mouse
  button on a control
• GotFocus event happens when the control
  receives focus
• KeyDown event happens when you press a key

24
Common Events

• Paint event happens when the screen is
  repainted
• Load event happens when a form is loaded for
  the first time
• UnLoad event happens when a form is unloaded
  from memory

25
Common Events

• DO NOT WORRY if you cant remember what all the
  events are called
• You are not expected to be able to reel these off
  from memory
• However, you WILL, over time, get to know what
  all these events are used for, and you will quite
  easily refer to the different events