|Using the Visual Basic DataGrid Control|
|Programming - Visual Basic|
|Written by MC Press Contributing Author|
|Tuesday, 31 October 2000 18:00|
Version 6.0 of Microsoft Visual Basic (VB) shipped with several enhancements designed to facilitate database programming using OLE DB and ODBC data access methods. GUI controlssuch as DataGrid, DataList, and DataCombowere also added to allow programmers to quickly prototype data-oriented applications. These controls were designed to take advantage of the ActiveX Data Object (ADO) component library. ADO implements a simplified, access pattern-neutral interface to OLE DB, Microsofts universal database connector layer. Many vendors implement OLE DB providers, or drivers, written to the OLE DB spec so that databases, including the AS/400s, may be accessed with ADO. With the new controls and ADO, it is possible to use VB to create simple, functional database apps with little or no code. Understanding how these controls work is the key to writing more sophisticated applications with ADO and OLE DB in all Windows environments.
The new data access controls of VB 6.0 belong to a class of controls known as data aware, or data bound, controls. These controls can be bound to rows and columns of a relational database source, greatly reducing the programming legwork required to manipulate the data. In essence, you predefine the relationships between the data source and the controls that will display and manipulate the data. Thereafter, any changes to the underlying data source are automatically reflected in the controls display. Likewise, any user changes to editable elements of the control can be made to the underlying data. Perhaps the most anticipated and useful of the VB 6.0 controls is the DataGrid.
Introducing the DataGrid
The DataGrid control combines a data display function (such as the VB Label control) and a data navigation function (such as the VB Adodc control) into a single control. DataGrid sports a highly functional spreadsheet-style user interface. It has many special features for formatting, displaying, updating, and manipulating the data displayed in its rows and columns. As an example for this article, I have used DataGrid and an extremely small amount of code to access an AS/400 file named ORDERS (the file, contained in a *SAVF object, and example code can be downloaded from the MC Web site at www.midrangecomputing.com/mc). You can use ADO objects to open the database
connection and file and to connect those objects to the DataGrid, which will navigate and display the contents of the file.
Before diving into the example, note that the DataGrid and all data-aware controls are fully documented in the Help system that comes with VB 6.0. That documentation is also available on the Web at http:// msdn.microsoft.com/library/devprods/vs6/ vbasic/vbcon98/vbcondataawarecontrols.htm. The example code was developed with Visual Studio on Windows NT 4.0 Workstation with Windows NT Service Pack 3.
Also Visual Studio Service Pack 3 (available at http://msdn.microsoft.com/vbasic/ downloads/ updates.asp) and Client Access service pack SF62608 (available here at www.as400.ibm.com/clientaccess/casp.htm) were installed.
For the example, start with a new Standard EXE project with VBs Project Wizard (select New Project from the File menu). If it is not already there, use the Components Wizard (also found in the Project menu) to add the Microsoft DataGrid Control to the control palette, as shown in Figure 1. If you already have VB 5.0 on your development machine, be careful not to confuse the DataGrid control with the Data Bound Grid control.
Drag and drop a DataGrid component onto the prototype form. At this point, the DataGrid looks like an empty spreadsheet with two cells. You could simply hook it up to an instance of the Adodc control and set Adodcs properties for the connection and file (and not have to write a single line of code!). Instead, as an exercise, Ill have you add the code to open the connection to the data and hook it up to the DataGrid. First, however, you need to add a reference to the ADO component library. From the VB Project menu, select the References option. Select the Microsoft ActiveX Data Objects Library from the list of available references. On the View menu, select Code. Add the following code:
The Form_Load subroutine is called from the VB runtime engine just before the form is displayed. The code uses two ADO objects: Connection (to open the database connection) and Recordset (to open the file), respectively. The Connection.Open method call specifies the IBM Client Access OLE DB provider and your AS/400. The Records.Open method call specifies the file to open (via the ORDERS and adCmdTable parameters), the open connection to use (the Connection parameter), and the locking/access (the adOpenStatic and adLockOptimistic parameters). Finally, you set the DataSource property of DataGrid1 (the default name of the DataGrid dropped into the prototype form) to the Records object.
Click the Start button, and the DataGrid will display the contents of the ORDERS file. Notice that the scrollbars control the display and, via the Recordset object, access the records of the file.
Figure 2 shows the DataGrid in action. Note that the DataGrid has many properties that affect the display of the data. By clicking on the button at the beginning of each row, you select that row as the current row. Your VB program can thus reference the users selected row at runtime via the DataGrids Bookmark property. By default, the cells of the DataGrid are editable, allowing the user to change the data in the underlying source if the
OLE DB provider permits. There are simply too many interesting DataGrid features to list here; see the references section at the end of this article for more information.
Because of the intuitive binding between the ADO Recordset and the DataGrid control, it is very easy to change the amount of data displayed by simply changing the query. For example, change the Records.Open method call just listed with the following code:
The previous simple file access format is then replaced with an SQL query, as specified by the adCmdText parameter. This query reduces the amount of fields shown for each row in the DataGrid.
Complex-bound Controls and the AS/400
The DataGrid control is known as a complex-bound control, another subclass of data- bound controls. Complex-bound controls can be bound to entire rows of a data source, whereas simple-bound controls, such as Label or TextBox, can be bound only to a single field. Complex-bound controls are highly integrated with the underlying OLE DB data access engine so that they can efficiently and flexibly access the rows of the data sources they are bound to. Controls like the DataGrid, therefore, are best used with databases that have a fully functional OLE DB provider. In particular, OLE DB driver functionality, such as row references (known as bookmarks) and multirecord locking, is critical. Unfortunately, the OLE DB providers for DB2/400 support neither of these features. Note the following code from the DataGrid example:
This code sets the CursorLocation property of the database connection. It tells the OLE DB provider whether to use client-side or server-side cursors, also known as record pointers, for the connection. Remove this line of code and you will see not bookmarkable or similar errors. Native DB2/400 and Client Access do not support OLE DBs server-side cursor semantics, so this line of code is required to indicate the use of client-side cursors. The intent of server-side cursors is to optimize the connection when you might want only a few records and to allow the contents of the Recordset to be efficiently updateable. The effect of this limitation is that the entire contents of the file or query are fetched from the server when the Recordset object is opened, resulting in very poor performance. For large files, this causes a significant delay in the DataGrid example. Of course, one way to prevent this is to use an SQL statement with very narrow selection criteria to restrict the number of records retrieved from the file to a small subset of the records in the file.
It should be mentioned that Client Access also supports a non-SQL, record-oriented access methodology, but it requires secondary, IBM-supplied Component Object Model (COM) objects, making it a less portable solution. Note also that the Recordset Open method in these examples specifies the adOpenStatic parameter, setting a static cursor type for the Recordsets CursorType property. Data returned in such a method call is static; you cant use the ADO methods provided to add, update, or delete records in the Recordset and expect those changes to be reflected on the server. The only way to make changes to a file via SQL with IBMs OLE DB provider is through SQL language statements. All known SQL-enabled OLE DB providers for the AS/400 suffer from the cursor location limitation; however, HiT Softwares AS/400 OLE DB provider does allow a dynamic cursor type. Thus, in the DataGrid example program, you could change the data in the cells, but, unless
you are using the HiT OLE DB provider and dynamic cursors, the changes wont be made to the file. (Visit Hit Softwares Web site at www.hit.com for a free trial download of its OLE DB provider.)
It can be tricky to effectively use VB 6.0s DataGrid control with the current OLE DB providers for the AS/400, but, with an understanding of how the underlying OLE DB provider is being exercised, you can pick the right problems to solve with this elegant control.
REFERENCES AND RELATED MATERIALS
Client Access Service Pack SF62608 download: www.as400.ibm.com/clientaccess/casp.htm
HiT Software: www.hit.com
IBM Client Access OLE DB Support: www.as400.ibm.com/clientaccess/oledb Figure 1: The Visual Basic Components Wizard lets you add DataGrid.
Figure 2: The example program reads the ORDERS file and displays its contents in the DataGrid.
|Last Updated on Tuesday, 31 October 2000 18:00|