1. OOP-Creating Object-Oriented Programs

2. Objects Object - like a noun, a thing
Cmd buttons, text boxes, Recordsets
Properties - like an adjective, characteristics of object
Caption, text, listindex
Methods - like a verb, an action or behavior of object
Clear, movenext, additem
Events - object response to user action or other events
Click, got focus, activate

3. Class Modules
VB allows programmers to create new object types in a class module
Project, Add Class Module
Use "C" as prefix for class module
In the class module you define
Properties
Methods
Event

4. Defining a New Class
Defining your new class is like creating a new tool for the toolbox – the process does not create the object, only a definition of what the object looks like and how it will behave.
You may then create as many instances of the class as you like
e.g. your class may be employee, student, product etc

5. Classes & Instances Items in the toolbox represent Classes
When you add a toolbox item to your project - you add an Instance of the item's Class
Ex. Each textbox on the form is an instance of the textbox class
Use the Object Browser (F2) to examine Classes & their associated Properties, Methods, and Events

6. ComboBox is selected object
Method symbol
Property symbol
Event symbol
Description

7. Cookie Cutter Analogy Class = Cookie cutter
Instantiate = Make a new cookie with the cutter
Instantiated cookies are not all "exactly" the same but they share common characteristics, actions which can be done to them

8. this represents the class Textbox
these represent objects—instances of the class

9. VB & Object Oriented Programming
Big advantage of OOP is the ability to reuse objects.
Reusing objects saves time and effort
You can create three objects from the same class, yet you can set their properties differently.
this represents the class Textbox
these represent objects—instances of the class

10. OOP Characteristics
Object Oriented Programming (OOP) says that a true object oriented language has the following three characteristics;
1) encapsulation,
2) polymorphism,
3) inheritance.

11. Object Oriented Terminology
Encapsulation
Combination of characteristics of an object along with its behavior in "one package"
Cannot make object do anything it doesn't already "know" how to do
Sometimes referred to as data hiding

12. Object Oriented Terminology
Polymorphism
Different classes of objects may have behaviors that are named the same but are implemented differently
Programmers can request an action without knowing exactly what kind of object they have or exactly how it will carry out the action
Ex. Debug.Print, Printer.Print

13. Object Oriented Terminology
Inheritance
Ability to create a new class from an existing class
VB6 cannot do this so we do not consider it true OOP (Object Oriented Programming)
The next release of VB scheduled for release in 2001, VB.Net, is true OOP since it can handle inheritance and polymorphism

14. Object Oriented Terminology
Reusability
The purpose behind Inheritance
VB doesn't exactly allow this
With VB reusability is implemented through
Delegation AND
Superclasses-a base class whose shared code you can call

15. Class Design - Analyze:
Characteristics of your new objects
Characteristics will be properties
Define the properties as variables in the class module
Behaviors of your new objects
Behaviors will be methods
Define the methods as sub procedures and functions in the class module

16. Create a New Class Project, Add Class Module Name with "C" prefix
Define the class properties
Create the class events

17. Define a new class module: Open a new project
Select Add Class Module from the Project menu
Click the New tab and choose Class Module; click Open
Change the class name to one of your choosing (CProduct in this case)
new name
class module selection
New tab

18. Properties of a Class
Declare inside the Class Module in General Declarations
Do not make Public-that would violate encapsulation (each object should be in charge of its own data)
Private mintPatientNum as Integer
Private mdtmDate as Date
Private mstrLastName as String
Private mcurBalance as Currency

19. Assign Values to Properties
Write special property procedures (Tools, Add Procedure,Property) to
Pass the values to the class module
Return values from the class module
Property Let procedure
Sets properties
Property Get procedure
Retrieves properties from a class
Like a function must return a value

20. Property Get
Property Get ProcedureName([Optional argument list] [As DataType] )
Statements inside procedure
ProcedureName=PropertyName
End Property
Example: (remember, Get must return a value)
Property Get LastName () as String
' Retrieve the current value
LastName=mstrLastName
Delared in Gen Dec of Class Module

21. Property Let
Property Let ProcedureName([Optional argument list,] Incoming Value [As DataType] )
Statements inside procedure
PropertyName=IncomingValue
End Property
Example:
Property Let LastName () as String
' Assign the property value
mstrLastName=strLastName

22. Instantiating - Creating a New Object Based on a Class
Create an instance of the class by using Dim with the New keyword and specify the class with the As keyword in General Declarations
OR, use Dim in General Declarations and Set Statement in Form_Load

23. Examples of Creating Instance
Dim|Public|Private variablename As New classname
Ex.
Dim mEmployee As New CEmployee
Private mInventory As New CInventory OR
Dim mEmployee As CEmployee
Set mEmployee=New CEmployee
Private mInventory As CInventory
Set mInventory=New CInventory

24. Freeing Resources
When you are finished with the new object instance you created you should free the resources assigned to it
Form_Unload is often a good location
Set the new instance of the object equal to Nothing keyword
Set mEmployee = Nothing

25. Initialize & Terminate Events
Each Class Module has two predefined events you can utilize
Class_Initialize
Triggered when an object is created
Class_Terminate
Triggered when an object is Set equal to Nothing or goes out of scope(ex. declared as local in a sub procedure)

26. Generating Events Most objects generate events
You can create objects that generate events
Objects that Generate events are referred to as:
Event Source or Event Provider
Exs. of events we are used to seeing generated: Click, MouseUp, ADO's WillChangeRecord

27. Responding to Events Forms generally respond to events
Objects that respond to events are referred to as:
Event Sink or Event Consumer
Examples of events we are used to seeing as responding
cmdOK_Click
form_MouseUp

28. Event Examples User clicks a command button
Event Source(Provider)=the Command Button
Form module's command button's click event executes
Event Sink(Consumer)=Form

29. How to generate an event
Declare the event in the General Declarations section of the class module, pass arguments with an event if you wish
Public Event TaskComplete( )
Raise the event in code in the same module the Event was delcared in
If mblnJobFinished Then RaiseEvent TaskComplete End If

30. How to respond to an event
Declare the object using WithEvents
Private WithEvents mMyTask as CMyTask
Instantiate the object using Set
Set mMyTask=New CMyTask
Write the code for the event procedure
When finished release the object variable using Nothing keyword

31. Collections
A Collection Class holds references for a series of objects created from the same class or from different classes
Actually a collection holds references to the objects
You reference by Index Number or a Key
Similar to list box and the associated items in the list box

32. Key for Collection Objects
Key must be a string
Can be used to reference individual objects in the collection
Declare the Key as String at the module level of the Class module for the object (not the collection)
Add Property Get and Let procedures

33. Creating a Collection Create a new class module
Name it as plural of the objects in the collection
CProducts is a Collection of CProduct objects
Declare an object variable "As Collection" (in Gen Declarations) and VB automatically provides:
Add, Remove, and Item methods
Count property

34. Creating a collection cont.
Code the Class_Initialize Event
Set equal to New Collection
Code the Class_Terminate Event
Set equal to Nothing
Code the private function that calculates the next Item number and/or assigns the Key

35. Creating a collection cont.
Code the Add Wrapper Event to add items to the collection
Code the Remove Wrapper Event to remove items to the collection
Code Item Wrapper Event to access individual elements in the collection
Write Property Get and Let for the Count property of the collection

36. Object Browser
Use it to view the properties, methods, events and constants of VB objects
It is more complete than Help but less descriptive
Use it to view the new objects you created in your project

37. Multitier Applications
Common use of classes is to create multitier apps
Each of the functions of a multitier app can be coded in a separate component and stored and run of different machines
Goal is to create components that can be combined and replaced

38. Three-tier Applications
3-tier is the most common implementation of multitier
Tiers
User Services
Business Services
Data Services

39. User Services Tier
User Interface
VB Forms
Controls
Menus

40. Business Services Tier
All Business Logic
May be written in multiple classes
Generally includes
Data Validation
Calculations
Enforcement of Business Rules

41. Data Services Retrieval and Storage of Data Databases Sequential Files
Random Access Files