A virtual base class defining methods for SAX parsing
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.
A virtual Class: No objects may be created from it.
signature(content = "ANY", .state = "SAXState"): ...
signature(name = "ANY", .state = "SAXState"): ...
signature(name = "ANY", base = "ANY", sysId = "ANY", publicId = "ANY", notationName = "ANY", .state = "SAXState"): ...
signature(target = "ANY", content = "ANY", .state = "SAXState"): ...
signature(name = "ANY", atts = "ANY", .state = "SAXState"): ...
signature(content = "ANY", .state = "SAXState"): ...
Duncan Temple Lang
# 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\n")
.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@commentsPlease choose more modern alternatives, such as Google Chrome or Mozilla Firefox.