1 Chapter 8 Object-Based Programming in VBA

8 Chapter Objectives Declare and use object variables Create procedures that use built-in form methods Find and manipulate data with recordset objects Execute parameter queries in VBA Communicate with other applications with Automation objects

8 Chapter Objectives Find methods through the Object Browser Create class modules Open multiple form instances with user- defined objects Describe the features of a well-designed, object-based application

8 The World According to Methods Access uses objects and properties because people understand and use them in their daily lives and thus can easily apply the concepts to programming Method  Procedure or action that belongs to an object

8 Overview of Access Methods Access methods perform actions Figure 8-1 NewDegree Type procedure

8 Overview of Access Methods Figure 8-1 NewDegree Type procedure (continued)

8 Overview of Access Methods Facts about methods:  RunCommand method  Similar to RunCommand macro action  DoCmd.RunCommand acCmdUndo erases the user’s most recent entry from the form  Open method  When applied to a recordset object, the recordset can access data in the table or query that is specified in the Source argument

8 Overview of Access Methods Facts about methods (continued):  AddNew method  Attached to a recordset object  Causes a recordset to create space for a new record  Update method  Causes the recordset to write the new record to the database  Close method  Removes the reference to the recordset

8 Object Variables To invoke a method, the associated object must be referenced Dot and bang notations  Allow the identification of specific forms, reports, controls, tables, or fields  Can be used when invoking a method

8 Object Variables Dot and bang notation disadvantages:  Can be long  Not very efficient when used repeatedly Object variables  Variables used in VBA procedures that contain a pointer to the item of interest  Efficient reference to Access application objects  Must be used to reference DAO and ADO objects

8 Declaring and Setting Object Variables Declaration statements declare variables as String, Long, Integer, and so on VBA statements are similar to those found in many other procedural languages In VBA, data types used in declaration statements can also include objects

8 Another Technique for Setting Object Variables Declare a control object variable and then set the object variable to the control that currently has the focus  Control is identified by the ActiveControl property of the Screen object  Technique is used when a procedure can work with whatever control has the focus

8 Built-in Methods Related to Microsoft Access Objects Microsoft Access objects, including forms, reports, and applications, support many built-in methods Some objects, including forms, reports, and independent class modules, support the creation of custom methods

8 DoCmd Methods DoCmd objects  Never manipulated by its methods, but those methods frequently manipulate another object DoCmd methods  Exist for almost every action that can be selected from a macro’s action column  Name matches a macro action  Arguments usually correspond to the macro’s action arguments

8 DoCmd Methods Table 8-1 Commonly used DoCmd methods

8 DoCmd Methods Table 8-1 Commonly used DoCmd methods (continued)

8 DoCmd Methods Table 8-1 Commonly used DoCmd methods (continued)

8 The RunCommand Method and Its Intrinsic Constants Action performed by the RunCommand method depends on the value of its command argument  Intrinsic constants is used to specify that value  Exist for almost all of the controls listed in the Access built-in menu bars

8 The RunCommand Method and Its Intrinsic Constants Figure 8-2 Deleting a record from frmStudentQDrops

8 Form and Control Methods Data shown on a form might not be current because:  The recordset generated for the form may not reflect recent changes to the database  Refreshing a recordset refers to the process of updating the fields of all records currently in the recordset  Requerying a form completely regenerates the recordset

8 Form and Control Methods Data on a form may not be current because (continued):  A calculated control may not recalculate its expression after the value of a control used in expression changes  Calculated control should be recalculated  The form may know the correct values to display, but because Access is executing other procedures, it does not have time to display correct values  Form needs repainting

8 Form and Control Methods Figure 8-3 frmCurrentStudents and txtQDrops

8 Methods that Update Displayed Data Access automatically refreshes, recalculates, and repaints forms at Access- defined intervals  Refresh interval  Specified in the database’s Option window

8 Methods that Update Displayed Data Refresh method  To refresh the recordset Recalc method  To cause a recalculation of the form’s calculated controls Repaint method  To force the system to repaint the screen Requery method  To force Access to regenerate a recordset

8 Methods that Update Displayed Data Figure 8-4 Refreshing and requerying a form

8 Methods that Update Displayed Data Table 8-2 Form and control methods

8 Methods that Update Displayed Data Table 8-2 Form and control methods (continued)

8 Report, Module, and Application Methods Application object  Frequently used in Access applications Methods of the Application object:  Echo Method  Used in conjunction with DoCmd’s HourGlass method  Turns screen updating off while several changes occur to the form

8 Report, Module, and Application Methods Table 8-3 Commonly used Application object methods

8 ADO and DAO Methods Methods related to ActiveX Data Objects (ADO) and Data Access Objects (DAO) perform many operations including:  Connecting to the database  Executing queries  Updating data not currently displayed in form ADO is replacing DAO as the standard method for manipulating data in Microsoft products

8 Methods of the Connection Object Almost all procedures that use ADO objects require a Connection object  Identifies and opens the database that contains the tables and queries that are used within the procedure  Establishes a connection to a database that will be manipulated in VBA code

8 Methods of the Connection Object Methods of a Connection Object:  Close method  Shuts down the pathway to the database  Execute method  Can be used to execute queries

8 Executing Queries in VBA Through the Command Object The Execute method associated with the Command object in VBA causes a query to run Figure 8-6 qryDeleteDeniedProspects

8 Query Parameters Frequently, action queries triggered in VBA code use VBA parameters Advantage of parameters is that the query will retrieve or update different data depending on the parameters’ values To execute a parameter query in VBA, the values of the parameters must be supplied

8 Opening Recordsets Programmers can use recordsets to locate and update data  Must declare and set a recordset object variable  Must open a pathway to the table or query that contains the data through the Open method Recordset objects can be created from other recordset objects through the Clone method

8 Finding Records in a Recordset Bookmark property of a recordset  A string expression that uniquely identifies the current record AbsolutePosition property  A long integer that contains the ordinal position of the current record MoveFirst, MoveLast, MovePrevious, and MoveNext methods  Used for simple movements to new records

8 Finding Records in a Recordset Move method  Used in conjunction with a long integer to move the specified number of records forward Seek method  Used to search a recordset through the primary key or indexes Filter property  Similar to a form’s filter property

8 Editing, Deleting, and Updating Data in a Recordset Programmers frequently use recordsets to edit, delete, and update records Recordset methods specify how the data are to be updated Recordsets must be opened with a LockType that permit updating of the database

8 Using DAO Recordsets Methods of DAO recordset  FindFirst, FindNext, FindPrevious, and FindLast  Similar to ADO’s Find method  AddNew, Update, and Delete  Almost identical to their ADO counterparts  Edit  Must be executed before values in an existing record are changed

8 Using DAO Recordsets RecordsetClone property of a form  Returns a pointer to the recordset NoMatch property  Used to indicate whether the previous Find method failed to find a match

8 ADO and DAO Methods – Some Final Thoughts CompactDatabase method  Associated with the JetEngine object  Makes the database smaller Append method  Places newly created objects in a collection Delete method of a collection object  Removes an object from the collection

8 The Object Browser The Object Browser is available in the Visual Basic Editor (VBE) Figure 8-8 Object Browser for VBA Collection Add method

8 Automation Object Variables and Methods GetObject function  Returns a pointer to objects in another Windows application called the source application Automation object variable  Any variable that references an object contained in another application CreateObject function  Used to create new applications in Access

8 Object-Based Approach to Application Development The object-based philosophy of Access provides some guidance as to how an application should be organized Many objects make up an Access application  Most possess the ability to test or manipulate their own features

8 Object-Based Approach to Application Development Other objects such as collections and forms possess abilities that perform certain operations on themselves  Procedures outside the object need to know only which methods are available and what the methods do A benefit of locating methods with objects that they manipulate is that many objects can use the same method

8 Object-Based Approach to Application Development “Rules of thumb” for designing Access applications:  Organize procedures based on the object being manipulated  Try to build objects that can be reused  Build Objects that are not highly coupled with other objects

8 Opening Multiple Instances of a Form User-defined object  Class object that can be used to create other objects Class object  A model of an object Instances of the class object  Objects that are based on a class

8 Creating and Using Independent Class Modules Independent class modules  Contain sub procedures and functions  Can also contain Let and Get property procedures  Treated as an object Class modules can also invoke events Because a class module is an object, instances of the object can be created

8 Chapter Summary In VBA objects and properties are manipulated and created by methods  Access supplies built-in methods  You can create your own methods Objects can be identified through the dot and bang notations  Object variables shortens notation Object variables can be declared as any recognized ADO, DAO, or Microsoft Access object type

8 Chapter Summary Form objects are associated with several built-in methods  Requery tells form to regenerate its recordset  Recalc, Refresh, and Repaint ensure that Access is displaying the correct data on a form DoCmd object offers many methods for manipulating data on a form

8 Chapter Summary Recordsets can be created with several methods and properties  Recordset methods locate or change data in a table Filling parameters and executing an action query in VBA code updates data Automation object variables allow VBA in Access to use and manipulate objects in other languages such as Word and Excel

8 Chapter Summary Access generates user-defined objects whenever forms and reports are created The combination of the New keyword with a user-defined object can be used to open multiple form instances Assigning an object to a persistent collection prolongs the object’s life