Handling Application Dependencies
Configurations can vary from one machine to another, making application dependencies one of the more difficult aspects of development and deployment. Let's take a look at some ways you can deal with one dependency issue in particular - references.
The following examples tie into one another so begin by creating a new database for the code that follows.
Installing Files from an Attachment Field
Certain dependencies can also be stored and subsequently installed from an attachment field. This is particularly useful if you are distributing library databases with your application. Using an attachment field allows you to create setup routines that are built in to the database.
To use an attachment field to contain the dependent files, begin by creating a table with the following schema. Save the table as USysTblDependentFiles when you're done.
As the MDB file format does not support the Attachment data type, this will work only in the new ACCDB file format.
|Field Name||Field Type (Size)||Description|
|DependentFiles||Attachment||Contains the dependent files|
|Component||Description of the attachment contents. Used in the WHERE clause of a recordset that retrieves setup files and allows you to define multiple files that make up a given component.|
In our example, we will store a single ACCDB file in an attachment field. This file will be used as a reference in our application.
Create a new class module called Installer and add the following code:
' installs a file from tblAttachments Public Function InstallFromAttachment(stComponent As String, _ stDestPath As String) As Long On Error GoTo err_handler:
Therefore, we'll need two Recordset objects.
Dim rs1 As DAO.Recordset2 Dim rs2 As DAO.Recordset2 Dim fld As DAO.Field2 Dim stSQL As String Dim stDestFile As String
Next, we want to open the dependent files table for the selected component and get the recordset of attachments. This will be stored in the variable named rs2.
' open the attachment table and look for the specified attachment stSQL = "SELECT * FROM USysTblDependentFiles " & _ "WHERE Component = '" & stComponent & "'" Set rs1 = CurrentDb().OpenRecordset(stSQL) Set rs2 = rs1.Fields("DependentFiles").Value
Because the attachment field may store multiple attachments, we should walk through the entire recordset of files. We'll get the FileData field for each attachment and save it to the hard drive using the SaveToFile method. The FileData field in the attachment recordset contains the actual file.
If (Not (rs2.EOF)) Then ' save the attachment data - may be multiple attachments While (Not rs2.EOF) Set fld = rs2("FileData") stDestFile = stDestPath & "\" & rs2("FileName") fld.SaveToFile stDestFile ' next attachment rs2.MoveNext Wend Else Err.Raise vbObjectError + 513, TypeName(Me), _ "Cannot find files for component: " & stComponent End If
We're raising an error here instead of displaying a message box because this is a class module. The last thing we should do is clean up our objects and then exit the routine:
cleanup: DoEvents rs2.Close: Set rs2 = Nothing rs1.Close: Set rs1 = Nothing Set fld = Nothing Exit Function err_handler: If (Err = 3839) Then ' file already exists - delete it VBA.Kill stDestFile Resume ElseIf (Err = 91) Then Err.Clear Exit Function ElseIf (Err <> 0) Then MsgBox "Unexpected error: " & Err.Description, vbCritical Resume cleanup End If End Function
The 3839 error code shown in the error handler is raised when the file being saved already exists. In our case, we're deleting the file and updating it from the one in the attachment field.
To test this routine, insert multiple attachments into the USysTblDependentFiles table. Then, add the following test code.
Public Sub InstallAttachmentsTest() Dim objInstaller As New Installer objInstaller.InstallFromAttachment "<YourComponentName>", CurrentProject.Path Set objInstaller = Nothing End Sub
This code will install files from the dependent files table to the directory where the database resides. Be sure to change the text YourComponentName to match that in your table.
In this tutorial:
- Creating an Automated Build
- Design the Form
- Retrieving Information from the Source Database
- Building the Application
- Creating the Target Database
- Set Build Properties on the Target Database
- Setting Build Properties on the Source Database
- Deleting Data from Tables
- Calculating the Version Number
- Handling Application Dependencies
- Updating References
- Testing Reference Fix-Up
- Late Binding
- Licensing Your Applications
- Number of Records
- Restricting the Number of Times an Application is Launched
- Registering an Application
- Creating a Registration Web Service
- Validate the Registration Using the Web Service
- Miscellaneous Deployment Scenarios
- Create the Client Application
- Testing the Versioning Server
- Re-Linking Tables Automatically
- Programmatically Creating DSNs
- Creating a User DSN
- Ensuring an Application Runs Locally