Working with XmlDataDocument Events

You must be wondering where XmlDataDocument comes from. Actually, this class works with Extensible Markup Language (XML) documents and is defined in the System.Xml namespace. So before testing this example, you must import the System.Xml namespace. (We discuss the XmlDataDocument object in Chapter 6.)

Note

You might want to skip this section if you're not familiar with the XmlDataDocument object. Chapter 6 covers XmlDataDocument.

The XmlDataDocument events are useful when your application needs to be notified when changes are being made to an XmlDataDocument object. XmlDocument defines the XmlDataDocument events (see Table 5-6).

Table 5-6: XmlDataDocument Events

EVENT	DESCRIPTION
NodeChanged	Occurs when the Value of a node has been changed
NodeChanging	Occurs when the Value of a node is changing
NodeInserted	Occurs when a node inserted into another node
NodeInserting	Occurs when a node is inserting to another node
NodeRemoved	Occurs when a node has been removed
NodeRemoving	Occurs when a node is being removed

The XmlNodeChangedEventHandler method handles these (NodeChanged, NodeChanging, NodeInserted, NodeInserting, NodeRemoved, and NodeRemoving) events. The XmlNodeChangedEventHandler is defined as follows:

 Public Delegate Sub XmlNodeChangedEventHandler(_    ByVal sender As Object, _    ByVal e As XmlNodeChangedEventArgs _ ) 

In this example, sender is the source of the event and e is an XmlNodeChangedEventArgs containing the event data. The XmlNodeChangedEventArgs defines properties (see Table 5-7).

Table 5-7: The XmlNodeChangedEventArgs Properties

PROPERTY	DESCRIPTION
Action	Returns a value indicating the type of node-changed event
NewParent	Returns the value of parent node after the operation is finished
Node	Returns the node that is being added, removed, or changed
OldParent	Returns the value of the parent node before operation started

Listing 5-12 handles the XmlDataDocument events. The XmlDocumentBtn_Click method creates event handlers for NodeChanged, NodeInserted, and NodeRemoved events. MyNodeChangedEvent, MyNodeInsertedEvent, and MyNodeRemoved event handlers are executed when these events are fired. You use LoadXml to load an XML fragment and then use the ReplaceChild and RemoveChild methods to replace and remove document nodes.

Listing 5-12: The XmlDataDocument Event Handling Example

 Private Sub XmlDataDocumentBtn_Click(ByVal sender As System.Object, _   ByVal e As System.EventArgs) Handles XmlDataDocumentBtn.Click     Dim xmlDoc As XmlDocument = New XmlDocument()     xmlDoc.LoadXml("<Record> Some Value </Record>")     ' Add event handlers     AddHandler xmlDoc.NodeChanged, AddressOf NodeChangedEvent_Handler     AddHandler xmlDoc.NodeInserted, AddressOf NodeInsertedEvent_Handler     AddHandler xmlDoc.NodeRemoved, AddressOf NodeRemovedEvent_Handler     Dim root As XmlElement = xmlDoc.DocumentElement     Dim str As String = root.ToString()     Dim xmlDocFragment As XmlDocumentFragment = xmlDoc.CreateDocumentFragment()     xmlDocFragment.InnerXml = _     "<Fragment><SomeData>Fragment Data</SomeData></Fragment>"     ' Replace Node     Dim rootNode As XmlElement = xmlDoc.DocumentElement     rootNode.ReplaceChild(xmlDocFragment, rootNode.LastChild)     ' Remove Node     Dim node As XmlNode = xmlDoc.LastChild     xmlDoc.RemoveChild(node)     ' Remove event handlers     RemoveHandler xmlDoc.NodeChanged, AddressOf NodeChangedEvent_Handler     RemoveHandler xmlDoc.NodeInserted, AddressOf NodeInsertedEvent_Handler     RemoveHandler xmlDoc.NodeRemoved, AddressOf NodeRemovedEvent_Handler   End Sub 

Listing 5-13 shows the NodeChangedEvent handler. The Node property of XmlNodeChangedEventArgs returns an XmlNode object. Using the Node property you can get more information about a node such as its parent node, value, name, namespace, and so on. (See the "Using the XmlNode Class" section of Chapter 6 to learn more about XmlNode.)

Listing 5-13: The NodeChanged Event Handler

 ' NodeChanged event handler Public Sub NodeChangedEvent_Handler(ByVal src As Object, _ ByVal args As XmlNodeChangedEventArgs)   MessageBox.Show("Node Changed Event Fired for node " + args.Node.Name)   If args.Node.Value <> Nothing Then     MessageBox.Show(args.Node.Value)   End If End Sub 

Similar to Listing 5-13, Listing 5-14 and Listing 5-15 show event handlers for the NodeInserted and NodeRemoved events.

Listing 5-14: The NodeInserted Event Handler

 ' NodeInserted event handler   Public Sub NodeInsertedEvent_Handler(ByVal src As Object, _   ByVal args As XmlNodeChangedEventArgs)     MessageBox.Show("Node Inserted Event Fired for node " + args.Node.Name)     If args.Node.Value <> Nothing Then       MessageBox.Show(args.Node.Value)     End If   End Sub 

Listing 5-15: The NodeRemoved Event Handler

 ' Node Removed event handler   Public Sub NodeRemovedEvent_Handler(ByVal src As Object, _   ByVal args As XmlNodeChangedEventArgs)     MessageBox.Show("Node Removed Event Fired for node " + args.Node.Name)     If args.Node.Value <> Nothing Then       MessageBox.Show(args.Node.Value)     End If   End Sub