HOWTO: Handle RDO 2.0 Asynchronous Processing WithEvents

• Microsoft Visual Basic Enterprise Edition for Windows, version 5.0
• Microsoft Visual Studio, Enterprise Edition 97

SUMMARY

RDO 2.0 gives you the ability to handle asynchronous processing more elegantly than RDO 1.0. Because Visual Basic 5.0 allows objects to be dimensioned WithEvents, you can use the events of the object as if it were a visual control, such as a textbox, rather than constantly referring to properties such as StillExecuting.

MORE INFORMATION

This project is an example of dimensioning object's WithEvents using the events of RDO 2.0's rdoConnection object. Be sure to change the connect string and query string to correctly connect to and query your database.

The sample establishes an asynchronous connection to the database and runs an asynchronous query. When the asynchronous calls are made, the CommandButton on the form is disabled. The cbton is then re-enabled in the rdoConnection's QueryComplete and Connect event. This code is more elegant than using a timer to monitor the StillExecuting property repeatedly.

In the code below, note that the rdoConnection object variable MyConn is treated almost like a visual control. From the code window, the MyConn object variable is available from the dropdown list of available objects.

Step-by-Step Example

1. Begin by opening a new project and adding a reference to the Microsoft Remote Data Object 2.0. To add a reference, select References from the Project menu.

2. Place a CommandButton on the form and set its Name property to cmdOpenResultset.

3. Add the following code to the form. Be sure to modify the gstrConnect constant accordingly to ensure that RDO will connect to your database:

         Const gstrConnect As String = "DRIVER={SQL Server};" & _
                                       "SERVER=YourServer;" & _
                                       "DATABASE=pubs;" & _
                                       "UID=sa;" & _
                                       "PWD=;"
         Const gstrSQL As String = "SELECT * FROM Authors"
   
         Dim WithEvents MyConn As rdoConnection
         Dim MyRS As rdoResultset
   
         Private Sub Form_Load()
             Set MyConn = New rdoConnection
             MyConn.Connect = gstrConnect
             MyConn.CursorDriver = rdUseOdbc
             MyConn.EstablishConnection rdDriverNoPrompt, _
                                        False, _
                                        rdAsyncEnable
             cmdOpenResultset.Enabled = False
             Caption = "Opening Connection..."
         End Sub
   
         Private Sub cmdOpenResultset_Click()
             Set MyRS = MyConn.OpenResultset(gstrSQL, _
                                             rdOpenStatic, _
                                             rdConcurReadOnly, _
                                             rdExecDirect + rdAsyncEnable)
             cmdOpenResultset.Enabled = False
             Caption = "Running Query..."
         End Sub
   
         Private Sub MyConn_Connect(ByVal ErrorOccurred As Boolean)
             If ErrorOccurred Then
                 Caption = "Error Occurred!"
             Else
                 Caption = "Connection Established!"
                 cmdOpenResultset.Enabled = True
             End If
         End Sub
   
         Private Sub MyConn_QueryComplete(ByVal Query As RDO.rdoQuery, _
                                          ByVal ErrorOccurred As Boolean)
             IF ErrorOccured Then
                 Caption = "Error Occurred!"
             Else
                 Caption = "Query Complete!"
                 cmdOpenResultset.Enabled = True
             End IF
         End Sub
   
   

4. Run the project.

REFERENCES

For more information on RDO 2.0's new functionality or on dimensioning objects WithEvents, please refer to the Visual Basic Help files or to Books Online.