Figure 1 SAX Interface Descriptions
Interface
Description
ContentHandler
Primary SAX interface that models the Infoset's core information items
ErrorHandler
Models fatal errors, errors, and warnings (as per XML 1.0)
DTDHandler
Models unparsed entities and notations
EntityResolver
Allows an application to perform custom resolution of external entity identifiers
LexicalHandler
Models noncore lexical information (comments, CDATA sections, entity references, and so on)
DeclHandler
Models element and attribute declarations
XMLReader
Makes it possible to tie the previously listed interfaces together in order to process a complete document information item
Attributes
Models a collection of attributes
Locator
Provides contextual information about the caller
Figure 2 SAX Interface Names
Java Language
MSXML COM and C++
MSXML COM and Visual Basic
ContentHandler
ISAXContentHandler
IVBSAXContentHandler
ErrorHandler
ISAXErrorHandler
IVBSAXErrorHandler
DTDHandler
ISAXDTDHandler
IVBSAXDTDHandler
EntityResolver
ISAXEntityResolver
IVBSAXEntityResolver
LexicalHandler
ISAXLexicalHandler
IVBSAXLexicalHandler
DeclHandler
ISAXDeclHandler
IVBSAXDeclHandler
XMLReader
ISAXXMLReader
IVBSAXXMLReader
Attributes
ISAXAttributes
IVBSAXAttributes
Locator
ISAXLocator
IVBSAXLocator
Figure 3 ContentHandler Members
Method
Description
startDocument
Signals the beginning of a document.
endDocument
Signals the end of a document.
startElement
Signals the beginning of an element.
endElement
Signals the end of an element.
startPrefixMapping
Signals the beginning of a prefix-URI Namespace mapping scope.
endPrefixMapping
Signals the end of a prefix URI mapping scope.
Characters
Signals character data.
ignorableWhitespace
Signals ignorable whitespace in element content. This method is not called in the current implementation because the parser is nonvalidating.
processingInstruction
Signals a processing instruction.
skippedEntity
Signals a skipped entity.
Property
Description
DocumentLocator
Receives a Locator interface reference, which provides methods for returning the column number, line number, PublicID, or SystemID from the caller for a current item.
Figure 4 Using a Locator Object
Public Sub GenerateXML(ch as IVBSAXContentHandler)
  Dim atts as New CVBSAXAttributesImpl
  Dim loc as New CVBSAXLocatorImpl
  set ch.documentLocator = loc
  loc.setLineNumber 0
  loc.setColumnNumber 0
  loc.setSystemId 'http://www.develop.com/foo.xml'
  loc.setPublicId ''
  ch.startDocument 
          ' the ch implementation can look at the Locator
          ' information during any method call
  ch.endDocument
End Sub
Figure 5 CSerializer Excerpt
' CSerializer is a generic XML 1.0 serializer for IVBSAXContentHandler
Implements IVBSAXContentHandler

' some helper methods omitted...
•••

' IVBSAXContentHandler method implementations

Private Sub IVBSAXContentHandler_characters(ByVal strChars As String)
    appendXML strChars
End Sub

Private Property Set IVBSAXContentHandler_documentLocator(ByVal loc _
        As MSXML2.IVBSAXLocator)
    Set m_locator = loc
End Property

Private Sub IVBSAXContentHandler_endDocument()
End Sub

Private Sub IVBSAXContentHandler_endElement(ByVal strNamespaceURI As _ 
        String, ByVal strLocalName As String, ByVal strQName As String)
    appendXML "</" & strQName & ">"
End Sub

Private Sub IVBSAXContentHandler_endPrefixMapping(ByVal strPrefix As _
        String)
End Sub

Private Sub IVBSAXContentHandler_ignorableWhitespace(ByVal strChars _
        As String)
    appendXML strChars
End Sub

Private Sub IVBSAXContentHandler_processingInstruction(ByVal strTarget _
        As String, ByVal strData As String)
    appendXML "<?" & strTarget & " " & strData & "?>"
End Sub

Private Sub IVBSAXContentHandler_skippedEntity(ByVal strName As String)
    MsgBox "###warning: skipped entity: " & strName
    printLocation
End Sub
' intialize state
Private Sub IVBSAXContentHandler_startDocument()
    resetNSDecls
End Sub

' serialize the element's start tag including attributes & namespace decls
Private Sub IVBSAXContentHandler_startElement(ByVal strNamespaceURI _
        As String, ByVal strLocalName As String, ByVal strQName As _
        String, ByVal oAttributes As MSXML2.IVBSAXAttributes)
    ' begin tag
    appendXML "<" & strQName
    ' serialize attributes
    Dim i As Integer
    For i = 0 To oAttributes.length - 1
        appendXML " " & oAttributes.getQName(i) & "='" & _
                  oAttributes.getValue(i) & "'"
    Next
    ' serialize namespace decls 
    If Not m_currentNSDecls Is Nothing Then
        Dim nsdecl As CNSMapping
        For Each nsdecl In m_currentNSDecls
            If nsdecl.prefix = "" Then
                appendXML " xmlns='" & nsdecl.uri & "'"
            Else
                appendXML " xmlns:" & nsdecl.prefix & "='" & nsdecl.uri & _
                    "'"
            End If
        Next
    End If
    resetNSDecls    
    ' close the start tag
    appendXML ">"
End Sub

' save any namespace declarations so they can be serialized in the next 
' startElement call
Private Sub IVBSAXContentHandler_startPrefixMapping(ByVal strPrefix As _
        String, ByVal strURI As String)
    Dim nsdecl As CNSMapping
    Set nsdecl = New CNSMapping
    nsdecl.prefix = strPrefix
    nsdecl.uri = strURI
    m_currentNSDecls.Add nsdecl
End Sub
Figure 6 startElement
Private Sub IVBSAXContentHandler_startElement(ByVal _
        strNamespaceURI As String, ByVal strLocalName As String, _
        ByVal strQName As String, ByVal oAttributes As _
        MSXML2.IVBSAXAttributes)
    If strNamespaceURI = "urn:www-develop-com:invoices" Then
        Select Case strLocalName
            Case "Invoice"
                m_ctx.Push stateInvoice
                Set m_inv = New CInvoice
            Case "InvoiceID"
                m_ctx.Push stateInvoiceID
            Case "CustomerName"
                m_ctx.Push stateCustomerName
            •••
        End Select
    End If
End Sub
Figure 7 CDeserializer Implementation
Option Explicit

Implements IVBSAXContentHandler

' reference to deserialized CInvoice instance
Dim m_inv As CInvoice
' current state
Dim m_curSku As String
Dim m_curDesc As String
Dim m_curPrice As String

' stack of state identifiers
Dim m_ctx As CStack
' state identifiers
Private Enum InvoiceStates
    stateInvoice = 1
    stateInvoiceID
    stateCustomerName
    stateLineItems
    stateLineItem
    stateSku
    stateDescription
    statePrice
    stateUnknown
End Enum

' returns the generated CInvoice instance
Public Function getInvoice() As CInvoice
    Set getInvoice = m_inv
End Function

Private Sub Class_Initialize()
    Set m_ctx = New CStack ' stack for managing element states
End Sub

Private Sub IVBSAXContentHandler_characters(ByVal strChars As String)
    ' populate the CInvoice instance with the appropriate data
    If Not m_inv Is Nothing Then
        Select Case m_ctx.Peek()
            Case stateInvoiceID
                m_inv.invoiceID = strChars
            Case stateCustomerName
                m_inv.customerName = strChars
            Case stateSku
                m_curSku = strChars
            Case stateDescription
                m_curDesc = strChars
            Case statePrice
                m_curPrice = strChars
                m_inv.addItem m_curSku, m_curDesc, CDbl(m_curPrice)
            Case Else
                ' do nothing
        End Select
    End If
End Sub

' manage state machine in startElement/endElement
Private Sub IVBSAXContentHandler_startElement(ByVal strNamespaceURI As _
        String, ByVal strLocalName As String, ByVal strQName As String, _
        ByVal oAttributes As MSXML2.IVBSAXAttributes)
    If strNamespaceURI = "urn:www-develop-com:invoices" Then
        ' if we're processing the right document (per namespace) then
        ' push the state identifier onto stack for the current element
        Select Case strLocalName
            Case "Invoice"
                m_ctx.Push stateInvoice
                Set m_inv = New CInvoice
            Case "InvoiceID"
                m_ctx.Push stateInvoiceID
            Case "CustomerName"
                m_ctx.Push stateCustomerName
            Case "LineItems"
                m_ctx.Push stateLineItems
            Case "LineItem"
                m_ctx.Push stateLineItem
            Case "Sku"
                m_ctx.Push stateSku
            Case "Description"
                m_ctx.Push stateDescription
            Case "Price"
                m_ctx.Push statePrice
            Case Else
                m_ctx.Push stateUnknown
        End Select
    End If
End Sub

Private Sub IVBSAXContentHandler_endElement(ByVal strNamespaceURI As _
        String, ByVal strLocalName As String, ByVal strQName As String)
    If strNamespaceURI = "urn:www-develop-com:invoices" Then
        ' if we're processing the right document (per namespace)
        ' pop the current element state off stack
        m_ctx.Pop
        ' if a LineItem element, reset LineItem context variables
        If strLocalName = "LineItem" Then
            m_curSku = ""
            m_curDesc = ""
            m_curPrice = ""
        End If
    End If
End Sub

Private Sub IVBSAXContentHandler_startDocument()
    Set m_inv = Nothing
End Sub

Private Sub IVBSAXContentHandler_startPrefixMapping(ByVal strPrefix _
        As String, ByVal strURI As String)
End Sub

Private Sub IVBSAXContentHandler_endPrefixMapping(ByVal strPrefix _ 
        As String)
End Sub

Private Sub IVBSAXContentHandler_ignorableWhitespace(ByVal strChars _
        As String)
End Sub

Private Sub IVBSAXContentHandler_processingInstruction(ByVal strTarget _
        As String, ByVal strData As String)
End Sub

Private Sub IVBSAXContentHandler_skippedEntity(ByVal strName As String)
End Sub

Private Sub IVBSAXContentHandler_endDocument()
End Sub

Private Property Set IVBSAXContentHandler_documentLocator(ByVal RHS _
        As MSXML2.IVBSAXLocator)
End Property
Figure 9 IVBSAXERRORHANDLER Implementation
Implements IVBSAXContentHandler
Implements IVBSAXErrorHandler

Private Sub IVBSAXErrorHandler_error(ByVal oLocator As _    
        MSXML2.IVBSAXLocator, ByVal strError As String, _
        ByVal nErrorCode As Long)
   processError oLocator, strError, nErrorCode
End Sub
Private Sub IVBSAXErrorHandler_fatalError(ByVal oLocator As _
       MSXML2.IVBSAXLocator, ByVal strError As String, _
       ByVal nErrorCode As Long)
   processError oLocator, strError, nErrorCode
End Sub
Private Sub IVBSAXErrorHandler_warning(ByVal oLocator As _
        MSXML2.IVBSAXLocator, ByVal strError As String, _
        ByVal nErrorCode As Long)
   logWarning oLocator, strError, nErrorCode
End Sub
' IVBSAXContentHandler methods omitted for clarity
Figure 10 Creating a SAX Pipeline
' builds a SAX pipeline of 3 filters (CVBSAXXMLFilterImpl) and the
' VBSAXXMLReader class 

Private Sub cmdTestXMLFilters_Click()
    ' clear TextBox
    txtResult = ""
    Dim reader As IVBSAXXMLReader
    Dim filter1 As CVBSAXXMLFilterImpl, filter2 As CVBSAXXMLFilterImpl
    Dim filter3 As CVBSAXXMLFilterImpl
    Dim xmlFilter1 As IVBSAXXMLFilter, xmlFilter2 As IVBSAXXMLFilter
    Dim xmlFilter3 As IVBSAXXMLFilter
    Dim txtStream As Scripting.TextStream
    Set txtStream = createTextStream("c:\temp\out.txt")

    ' create the VBSAXXMLReader30 implementation of IVBSAXXMLReader
    Set reader = New VBSAXXMLReader30

    ' create the first filter & set its message
    Set filter1 = New CVBSAXXMLFilterImpl
    Set xmlFilter1 = filter1
    filter1.setFilterID "FILTER 1"
    filter1.setOutputStream txtStream
    ' hook first filter up to VBSAXXMLReader30 instance
    Set xmlFilter1.Parent = reader

    ' create the second filter & set its message
    Set filter2 = New CVBSAXXMLFilterImpl
    Set xmlFilter2 = filter2
    filter2.setFilterID "FILTER 2"
    filter2.setOutputStream txtStream
    ' hook second filter up to first filter
    Set xmlFilter2.Parent = filter1

    ' create the second filter & set its message
    Set filter3 = New CVBSAXXMLFilterImpl
    Set xmlFilter3 = filter3
    filter3.setFilterID "FILTER 3"
    filter3.setOutputStream txtStream
    ' hook third filter up to second filter
    Set xmlFilter3.Parent = filter2

    ' the pipeline now looks like this:
    ' filter3 <-> filter2 <-> filter1 <-> reader

    ' create an IVBSAXContentHandler implementation (CSerializer)
    ' and register with the last filter in the pipeline
    Set reader = filter3
    Dim ser As CSerializer
    Set ser = New CSerializer
    ser.cacheXML = True
    Set reader.contentHandler = ser
    reader.putProperty "http://xml.org/sax/properties" & _
    "/lexical-handler", ser

    ' parse and display output
    reader.parseURL txtFileName.Text
    txtResult = "Output from filter pipeline written to..." & _
    " & vbNewLine & "c:\temp\out.txt" & vbNewLine & vbNewLine
    txtResult = txtResult & ser.getSerializedXML()
    txtStream.Close

End Sub
Figure 11 Flow of Method Calls

Figure 11 Flow of Method Calls

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