Alison Balteramp;#039;s Mastering Microsoft Office Access 1002003 [Electronic resources] نسخه متنی

Practical Examples: Class Modules

To best understand how to benefit from the use of Class modules, it is beneficial to see them in action. This chapter covers three examples. The first shows the use of a file information class. Each instance of the class is used to house information about a particular file. The second illustrates how the use of a customer class facilitates the process of dealing with customer data. The third is a system information class. It retrieves and then provides information about the computer system. An additional example of the use of Class modules is found in Chapter 16. It shows how a custom error class facilitates the process of implementing error handling within your application.

The FileInformation Class

It is a common application requirement to be able to extract the drive, path, or short filename from a complete filename. Although you can obtain these pieces of information using a Standard Code module and functions, placing the functions in a Class module makes them easier to work with. The FileInformation class contained in the Chap13Ex sample database contains four properties: FullFileName, Drive, Path, and Name. The user of the class sets the FullFileName property. It contains the complete filename and path of the file whose parts the user wishes to extract. The Drive, Path, and Name properties of the class contain the drive, path, and name of the file specified in the FullFileName property. Listing 13.1 shows the Property Let and Property Get procedures, as well as the Private variables associated with these properties.

Listing 13.1 The Property Declarations for the FileInformation Class

Notice that the Drive, Path, and Name properties have no associated Property Let routines. This is because these properties are read-only properties from outside the class. When the code sets the FullFileName property, it executes the GetDrive, GetPath, and GetName routines. Each of these routines populates the appropriate Private variables so that they can be retrieved in the Property Get routines. Listing 13.2 shows the GetDrive, GetPath, and GetName subroutines.

Listing 13.2 The GetDrive, GetPath, and GetName Routines

The GetDrive routine extracts the characters to the left of the colon, including the colon, thereby extracting the drive. The GetPath routine locates the last backslash in the filename. The string to the left of the last backslash contains the path name. Finally, the GetName routine extracts everything to the right of the last backslash.

Note that the GetDrive, GetPath, and GetName routines are private to the Class module. This means that their code cannot be executed from outside the Class module. The code shown in Listing 13.3 illustrates how the code within the Class module is used.

Listing 13.3 Using the FileInformation Class

This code, found in the frmFileInformation form in CHAP13EX, declares a FileInformation variable. As long as the user has entered a filename, the code instantiates an instance of the FileInformation class. It sets the FullFileName property equal to the value contained in the txtFullFileName text box. This causes the GetDrive, GetPath, and GetName routines to execute, thereby populating the Private variables contained within the class. The code then retrieves the Drive, Path, and Name property values and places them in text boxes on the form.

Using a Collection to Manipulate Multiple Instances of the FileInformation Class

The idea of using a collection to manipulate multiple instances of a class was discussed in the "Working with Custom Collections" section of this chapter. It is illustrated in Listing 13.4.

Listing 13.4 Using a Collection to Manipulate Multiple Instances of the FileInformation Class

The code receives a directory path (including a trailing backslash) as a parameter. It creates and instantiates a Collection object. It then executes the Dir function, which retrieves the name of the first file in the specified directory. As long as it finds at least one file, it executes the code within the Do Until loop. The code creates an instance of the FileInformation class. The FullFileName property of the instance is then set equal to the directory name concatenated with the filename. The most important line of code in the routine is then executed, adding the instance of the FileInformation class to the collection. This enables the instance to persist. The code calls the Dir function to retrieve the name of the next file in the specified directory, and the process is repeated until no additional filenames are located.

After the code adds all the instances of the FileInformation class to the collection, it uses the For...Each loop to iterate through all items in the collection. It retrieves the Drive, Path, and Name properties of each item in the collection and prints them to the Debug window. Notice that the code uses a variant variable to iterate through the elements of the Collection object.

The Data Access Class

Building a data access class greatly facilitates the process of dealing with data, particularly when the data within a table is accessed from numerous forms or numerous databases. By encapsulating the data access activities into a Class module, you can better ensure that all the forms and applications treat the data consistently. Each field within the table becomes a property of the class. This is illustrated by the private declarations and Property Let and Property Get routines shown in Listing 13.5.

Listing 13.5 The Private Variables and Property Let and Property Get Routines Used by the Data Access Class

The Initialize event of the class, shown in Chapter 14, "What Are ActiveX Data Objects and Data Access Objects, and Why Are They Important?" For now, it's only important to understand the basics. The example sets the LockType of the recordset to adLockOptimistic, and the CursorType of the recordset to adOpenDynamic. The combination of these two property settings renders the recordset's data updateable.

Listing 13.6 The Initialize Event of the Client Class

After the code opens the recordset, the contents of the first record in the recordset must be available as properties of the class. This is necessary so that the contents of the first record can be displayed in the frmClients form. The Scatter method, shown in Listing 13.7, accomplishes this task.

Listing 13.7 The Scatter Method of the Client Class

The Scatter method simply takes the contents of the fields in the current record (in this case, the first record) and stores them in Private variables that are accessed by the Property Get and Property Let routines within the class. The variables are then used by the Form_Load event of the frmClients form, shown in Listing 13.8.

Listing 13.8 The Form_Load Routine of the frmClients Form

The Form_Load event instantiates the Client class, causing the Initialize event of the class to execute. The Scatter method of the class executes, and then the code populates the text boxes on the form with the contents of the ClientID, CompanyName, Address, and City properties of the class. The frmClient form, populated with data from the first record in the tblClients table, is shown in Figure 13.3.

After the code displays the first record, the user can opt to move to the next record in the recordset. Listing 13.9 shows the Click event of the cmdNext command button on the frmClients form, which calls the MoveNext method of the class and then displays the contents of the class's properties.

Listing 13.9 The Click Event of the cmdNext Command Button

The cmdNext click event calls the MoveNext method of the class. The MoveNext method is responsible for moving forward from record to record within the class. It uses the module-level recordset object set in the Initialize event of the class. This appears in Listing 13.10.

Listing 13.10 The MoveNext Method of the Client Class

The MoveNext method first tests to see whether the end of the recordset has been reached. If not, the MoveNext method is used to move to the next record in the recordset. If the end of the recordset is encountered, the code moves back to the last record. The Scatter method is called to update the values of the module-level variables. The cmdNext Click event then retrieves these values via the Property Get routines, to update the data displayed on the form.

The cmdPrevious Click event of the frmClients form is similar to the cmdNext Click event. In Listing 13.11, it calls the MovePrevious method of the class and then displays the contents of the class's properties.

Listing 13.11 The Click Event of the cmdPrevious Command Button

The Click event of the cmdPrevious command button first calls the MovePrevious method of the class. The MovePrevious method of the class is similar to the MoveNext method. In Listing 13.12, it moves to and displays the previous record in the recordset.

Listing 13.12 The MovePrevious Method

The MovePrevious method first tests to see whether the record pointer is before the first record in the recordset. If not, the MovePrevious method moves to the previous record in the recordset. If it encounters the beginning of the recordset, the code moves forward to the first record. The code calls the Scatter method to update the values of the module-level variables. These variables are then retrieved by the Property Get routines initiated by the Click event of the cmdPrevious command button.

The Client class enables the user to edit the data in the underlying recordset. The user simply enters data into the form's unbound text boxes. After users enters the data, they click Save. The Click event of the cmdSave command button saves the form's data to the underlying recordset, as shown in Listing 13.13.

Listing 13.13 The Click Event of the cmdSave Command Button

The code in the Click event of the cmdSave command button first sets all the properties of the class to the corresponding text box values. It then executes the Save method of the class (see Listing 13.15).

Before we look at the Save method, it is important to explore the code under the Click event of the cmdNew command button (see Listing 13.14). It's very simple: It clears the text boxes on the form, readying them for the entry of the new data. It then sets the value of the AddFlag to True.

Listing 13.14 The Click Event of the cmdNew Command Button

In the case of either an edit or an add, the code in the Save method of the class actually writes the data from the new record to disk. This code is shown in Listing 13.15.

Listing 13.15 The Save Method of the Class

The Save method of the class first determines whether the user is adding or editing data. This is determined by evaluating the mboolAddFlag. The code sets the mboolAddFlag to True when the user clicks the Add button. When editing, the value of the variable is False. If the user is adding the record, the code executes the private routine called AddNew, which appears in Listing 13.16.

Listing 13.16 The AddNew Method of the Class

The AddNew method of the class uses the AddNew method of an ADO recordset to populate a new record with the values contained in the Private variables. The Update method of the recordset object writes the new data to disk. When the Update method executes, the value of the AutoNumber field is assigned and stored in the variable called mlngClientID. This variable is retrieved in the Click event of cmdSave so that the txtClientID text box contains the appropriate value.

Whereas the AddNew method of the class adds the record in the recordset, the Edit method of the class updates the data in an existing record. It appears in Listing 13.17.

Listing 13.17 The Edit Method of the Class

The Edit method uses the Update method of the ADO recordset to take the values in the module-level variables and write them to disk.

The last data task associated with the cmdClients form provides the user with the capability to delete a record from the recordset. The code behind the Click event of the cmdDelete command button appears in Listing 13.18.

Listing 13.18 The Click Event of the cmdDelete Command Button

This code executes the Delete method of the class, shown in Listing 13.19. It uses the Delete method of an ADO recordset to delete the current record from the recordset. After the deletion, the record pointer is sitting on the deleted record. The MoveNext method of the class moves the record pointer to the next valid record. The Click event of the cmdDelete command button then populates the text boxes on the form with the values of the record that the MoveNext method moved to.

Listing 13.19 The Delete Method of the Class

The SystemInformation Class

The process of obtaining system information, such as the amount of free drive space, is usually a somewhat tedious and difficult process. This information is generally available only through the Windows API, covered in Chapter 23, "Exploiting the Power of the Windows API." The execution of Windows API functions is best left to more advanced developers. So how can a junior developer access this important information? If the senior developer encapsulates the complex functionality of the Windows API calls in a Class module, the junior developer can obtain the system information as properties of the class.

The class called SystemInformation is responsible for obtaining information about the hardware, operating system, and system resources. To obtain this information, the Declare statements, type structure declarations, and constant declarations are included in the General Declarations section of the Class module (see Listing 13.20).

Listing 13.20 The Private Variables and Type Structures Required by the SystemInformation Class

The SystemInformation

class contains six read-only properties :
TotalMemory, AvailableMemory, OSVersion, OSBuild, OSPlatform, and Processor. These properties are set within the class and cannot be modified from outside the class. The Property Get functions for the six properties are shown in Listing 13.21.

Listing 13.21 The Property Get Routines Required by the SystemInformation Class

All the work is done in the Initialize event of the class. When the class is instantiated, the Initialize event executes all the Windows API functions necessary to obtain the required system information. The Initialize event of the class is shown in Listing 13.22.

Listing 13.22 The Initialize Event of the SystemInformation Class

The GlobalMemoryStatus Windows API function populates the TotalMemory and AvailableMemory properties. The GetVersionEX function is used to set the OSVersion, OSBuild, and OSPlatform properties. Finally, the GetSystemInfo function populates the Processor property.