SAXState-class: A virtual base class defining methods for SAX parsing

Description

This is a degenerate virtual class which others are expected to sub-class when they want to use S4 methods as handler functions for SAX-based XML parsing. The idea is that one can pass both i) a collection of handlers to xmlEventParse which are simply the generic functions for the different SAX actions, and ii) a suitable object to maintain state across the different SAX calls. This is used to perform the method dispatching to get the appropriate behavior for the action. Each of these methods is expected to return the updated state object and the SAX parser will pass this in the next callback.

We define this class here so that we can provide default methods for each of the different handler actions. This allows other programmers to define new classes to maintain state that are sub-class of SAXState and then they do not have to implement methods for each of the different handlers.

Arguments

Objects from the Class

A virtual Class: No objects may be created from it.

References

http://www.w3.org/XML, http://www.xmlsoft.org

Examples

Run this code

# For each element in the document, grab the node name
# and increment the count in an vector for this name.

# We define an S4 class named ElementNameCounter which
# holds the vector of frequency counts for the node names.

 setClass("ElementNameCounter",
             representation(elements = "integer"), contains = "SAXState")

# Define a method for handling the opening/start of any XML node
# in the SAX streams.

 setMethod("startElement.SAX",  c(.state = "ElementNameCounter"),
           function(name, atts, .state = NULL) {

             if(name %in% names(.state@elements))
                 .state@elements[name] = as.integer(.state@elements[name] + 1)
             else
                 .state@elements[name] = as.integer(1)
             .state
           })

 filename = system.file("exampleData", "eurofxref-hist.xml.gz", package = "XML")

# Parse the file, arranging to have our startElement.SAX method invoked.
 z = xmlEventParse(filename, genericSAXHandlers(), state = new("ElementNameCounter"), addContext = FALSE)

 z@elements

  # Get the contents of all the comments in a character vector.

 setClass("MySAXState",
             representation(comments = "character"), contains = "SAXState")

 setMethod("comment.SAX",  c(.state = "MySAXState"),
           function(content, .state = NULL) {
             cat("comment.SAX called for MySAXState
")
             .state@comments <- c(.state@comments, content)
             .state
           })

 filename = system.file("exampleData", "charts.svg", package = "XML")
 st = new("MySAXState")
 z = xmlEventParse(filename, genericSAXHandlers(useDotNames = TRUE), state = st)
 z@comments

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