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VB Classes

VB Classes. BICS546. Adding a Class to a Project. Project/Add Class Class will be saved in a .CLS file Steps: Adding properties Declare Public variables in the General Declaration section Property procedures: Property Let / Property Get functions Adding methods Adding events

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VB Classes

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  1. VB Classes BICS546

  2. Adding a Class to a Project • Project/Add Class • Class will be saved in a .CLS file • Steps: • Adding properties • Declare Public variables in the General Declaration section • Property procedures: Property Let / Property Get functions • Adding methods • Adding events • Default events: Class_Initialize(), Class_Terminate • Private variables and procedures can be created for internal use.

  3. Anatomy of a Class Module Class Module Exposed Part Hidden Part Public Variables & Property Procedures Private Variables Public Procedures & Functions Private Procedures & Functions

  4. Class Code Example Option Explicit Public Eid As String Public Ename As String Public salary As Double Public Function tax() As Double tax = salary * 0.1 End Function

  5. Creating Property with Property Procedures • Implementing a property with a public variable the property value cannot be validated. • We can create read-only or write-once properties with property procedures.

  6. Property Procedure Code Example(Demo:Class Module & Builder) Public SSN as String Public Ename as String Public DateHired as Date Private hiddenJobCode as Long Public Property Let JobCode(ByVal jobcodeArg as Long) If jobcodeArg < 1 Or JobCodeArg > 4 Then hiddenJobCode = 1 Else hiddenJobCode = JobCodeArg End IF End Property Public Property Get JobCode() as Long JobCode = hiddenJobCode End Property

  7. Implementing a Read Only Property • Use only the Property Get procedure. • Ex. Create a YearsEmployed property from the DateHired property: Public Property Get YearsEmployed() As Long YearsEmployed = (Date – DateHired)/365 End Property • Note: It is similar to a calculated field in database. • How to create a write-once property?

  8. Collections • Collections are used to store lists of objects. • More flexible than array: • No need to declare the number of objects in a collection, no need to ReDim. • Objects can be added, deleted at any position. • Object can be retrieved from a collection by a key. • A collection’s name usually end with a “s”.

  9. Using Collections • Define a collection: • Ex. Private Customers as New Collection • Methods: • ADD: Add object to a collection • Dim Customer as New clsCustomer • Customers.Add(Customer) • Add an object with a key: • Customers.Add(Customer, Customer.CID) • Item: Retrieve an object from a collection with a position index (base 1) or with a key. • Set Customer = Customers.Item(1) • Set Customer = Customer.Item(“C101”) • Count: Return the number of objects in a collection. • Remove: Delete an object with a position index or key.

  10. Iterating Through a Collection Dim Customer as clsCustomer Dim Indx as Long For Indx = 1 to Customers.Count set Customer = Customers.Item(Indx) … class operations … Next Indx For Each Customer In Customers class operations Next Customer

  11. Implementing a 1:M Relationship With Class Hierarchy Class Department Public did As String Public dname As String Public emps As New Collection Class Employee Public eid As String Public ename As String Public salary As Currency

  12. To Add a New Emp Private Dep As New dept Private Sub Form_Load() Dep.did = "mis" Dep.dname = "Management IS" End Sub Private Sub Command1_Click() Dim newemp As New emp With newemp .eid = Text1 .ename = Text2 .salary = Val(Text3) End With Text1 = vbNullString Text2 = vbNullString Text3 = vbNullString Dep.emps.Add newemp End Sub

  13. To Find Total Salary Private Sub Command3_Click() Dim temp As emp Dim totalsal As Double totalsal = 0 For Each temp In Dep.emps totalsal = totalsal + temp.salary Next temp Label5.Visible = True Label5.Caption = "TotalSalary=" & CStr(totalsal) End Sub

  14. Deploying Classes as ActiveX Servers • New Project/ActiveX DLL • Create the classes you need, or open the classes previously created. • Set the class’s Instancing property to 5 – multiuse. • To compile, File/Make .DLL • VB automatically registers the DLL.

  15. Using the DLL Option Explicit Dim test As ActiveXEmp.emp Dim test2 As Object Private Sub Form_Load() Set test = New ActiveXEmp.emp Set test2 = CreateObject("ActiveXEmp.emp") test.eid = "hi" test2.eid = "there" Text1.Text = test.eid Text2.Text = test2.eid End Sub

  16. Database Access with Classes VB Chapter 7

  17. Data-Aware Classes • Classes that serve as data source. • Single-Record-Handling Classes • Retrieves a single record from the database and makes it available to your application in the form of an object. • The fields in the record are exposed as the object’s properties. • Any actions performed by the data (updates, calculations, etc.) are exposed as the object’s methods.

  18. Data-Aware Classes (VB Chap 7) • A data-aware class has a DataMembers collection. • Steps to Setting up the data-aware class: • 1. Open a new Standard EXE project. • 2. Project/Add Class • 3. Use the Properties window to change the class’s DataSourceBehavior to 1 – vbDataSource • 4. Project/Reference/Microsoft Activex Data Object • 5. In the declaration section of the class, create a private Recordset object.

  19. 6. In the class initialize event, write code to create the Recordset object. • 7. Write code to register Data Member names to DataMembers collection. • Example: DataMembers.Add “DataMemberName” • 8. Use the GetDataMember event to return a recordset object based on DataMember name. • 9. Write methods to control recordset. Note: Some steps can be automated by:Project/Add Class Module /Data Source

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