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| Source Code |
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| 5 | Internet |   |
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XML PropertyBag |
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Persist and Restore your data in a manageable, reusable and extensible way with XML |
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NOTE:
this code has been superceded by the version at the new site.
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| Overview |
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This ActiveX DLL is derived from an excellent original XML PropertyBag sample
by Aaron Anderson
(Aaron.Anderson@mail.farmcreditbank.com).
It allows you to persist and recreate your own objects directly to XML based-files
using a simple interface closely related to VB's PropertyBag object. By
implementing one interface in your class you will gain the ablility to save state
and restore again for any number of objects to and from a single XML file.
Full support for binary data means you can store blobs in your XML file too.
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| About XML |
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The advantages of using XML as a file format for storage in your application
cannot be understated. XML is plain text. You can read and write XML files by hand
(although you might find that a little tedious!) XML is also self-describing. Just
looking at an XML file gives you a good idea what it does and its generally easy
to tell how to read the file. On top of that, XML parsers offer three unique
features not often seen before the success of the HTTP protocol: they're almost always
free, they have a standardised API and they're available for almost any useful computer
you can think of (except Windows CE. Hmmm. Maybe that's not actually a useful
computer anyway).
Anyway, I'm sure you don't need me to tell you that XML is a good idea. Visit Microsoft's XML site for some more ideas and examples! |
| The XMLPropertyBag |
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The XMLPropertyBag object provided with this article comes in two parts: a implementable
interface IXMLPropertyBag and a creatable class XMLPropertyBag. To
take advantage of the XML Property Bag features, your class should implement the IXMLPropertyBag
interface. Doing this effectively adds ReadProperties and WriteProperties
events to your object, passing an XMLPropertyBag in as a parameter. The only
difference between these events and ones in VB's PropertyBag implementations is that
they are prefixed with IXMLPropertyBag_ rather than the base object type name
(e.g. UserControl_ ).
Once you have implemented this interface, to provide XML saving and restoring of values can be done in two steps: firstly you write the code in the ReadProperties and WriteProperties methods, and then you write code in the object's owner to initialise reading and writing of values. Reading and Writing properties is done in almost exactly the same way as with VB's PropertyBag object. The ReadProperty method of the XMLPropertyBag allows you to read the value of a named variant in the property bag and the WriteProperty method does the opposite. The following types of variant can be read and written:
Having implemented these methods, you can then save and restore objects into XML. In the object's owner, create a new instance of the XMLPropertyBag object. If you are saving the object's state, then call the SaveState method on the object, passing in the object to save as the first parameter. You can call SaveState on as many objects as you wish - they will be added to the same XML Document. The optional sName parameter allows you to distinguish between multiple objects of the same type in the same XML file. When complete, you can then get the XML rendering of the objects from the Contents property, which you can then save to whatever storage you are persisting the content to. To restore objects from the XML, set the Contents property to the saved XML. Then you can call RestoreState, passing in each object you wish to restore (and optionally the name if you used that). |
| Quick Start |
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This discussion demonstrates how you can add persistance to a simple class called
in your application. First, assume the class is called cName and has the following
definition:
Option Explicit Private m_sFirstName As String Private m_sLastName As String Public Property Get FirstName() As String FirstName = m_sFirstName End Property Public Property Let FirstName(ByVal sName As String) m_sFirstName = sName End Property Public Property Get LastName() As String FirstName = m_sFirstName End Property Public Property Let LastName(ByVal sName As String) m_sLastName = sName End Property The first step is to implement the IXMLPropertyBag interface. Add this code to the class: Implements IXMLPropertyBag ... Private Sub IXMLPropertyBag_ReadProperties(ByVal PropertyBag As vbalXMLPBag.XMLPropertyBag) ' End Sub Private Sub IXMLPropertyBag_WriteProperties(ByVal PropertyBag As vbalXMLPBag.XMLPropertyBag) ' End Sub Now you can fill in your ReadProperties and WriteProperties implementations: Private Sub IXMLPropertyBag_ReadProperties(ByVal PropertyBag As vbalXMLPBag.XMLPropertyBag) ' FirstName = PropertyBag.ReadProperty("FirstName", "") LastName = PropertyBag.ReadProperty("LastName", "") End Sub Private Sub IXMLPropertyBag_WriteProperties(ByVal PropertyBag As vbalXMLPBag.XMLPropertyBag) ' PropertyBag.WriteProperty "FirstName", FirstName, "" PropertyBag.WriteProperty "LastName", LastName, "" End Sub Once that is done, you can now save and restore cName objects at will. For example, this code: Private m_cN As New cName Public Function SaveName() As String ' Create an instance of the XML Property bag object: Dim oP As New XMLPropertyBag ' Save cName into the property bag: oP.SaveState m_cN ' Now the content is saved in the return value: SaveName = oP.Contents End Sub Public Sub RestoreName(ByVal sXML As String) ' Create an instance of the XML Property bag object: Dim oP As New XMLPropertyBag ' Load the XML into it: oP.Contents = sXML ' Restore the object: oP.RestoreState m_cN End Sub The download code includes a sample demonstrating how to store a Person object which in turn holds a collection of Address objects. Another example is available in the Search and Replace utility. |
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