XML (version 0.99-4)

xmlEventParse: XML Event/Callback element-wise Parser

Description

Reads and processes the contents of an XML file or string by invoking user-level functions associated with different components of the XML tree. These include beginning and end of XML elements, comments, CDATA (escaped character data), entities, processing instructions, etc. This allows the caller to create the appropriate data structure from the XML document contents rather than the default tree (see xmlTreeParse). Functions for specific tags/elements can be used in addition to the standard callback names.

Usage

xmlEventParse(file, handlers=xmlEventHandler(), ignoreBlanks = FALSE, addContext=TRUE,
               useTagName=TRUE, asText = FALSE, trim=TRUE, useExpat=FALSE, isURL = FALSE,
                state = NULL, replaceEntities = TRUE, validate = FALSE,
                 saxVersion = 1)

Arguments

file

the source of the XML content. This can be a string givinging the name of a file or remote URL, the XML itself, a connection object, or a function. If this is a string, and asText is TRUE, the value is the XML con

handlers

a closure object that contains functions which will be invoked as the XML components in the document are encountered by the parser. The standard functions are startElement(), endElement() comment(), externa

ignoreBlanks

a logical value indicating whether text elements made up entirely of white space should be included in the resulting `tree'.

addContext

logical value indicating whether the callback functions in `handlers' should be invoked with contextual information about the parser and the position in the tree, such as node depth, path indices for the node relative the root, etc. If this is True, ea

useTagName

logical value indicating whether the callback mechanism should look for a function matching the tag name in the startElement and endElement events, before calling the default handler functions. This allows the caller to handle different element typ

asText

logical value indicating that the first argument, `file', should be treated as the XML text to parse, not the name of a file. This allows the contents of documents to be retrieved from different sources (e.g. HTTP servers, XML-RPC, e

trim

whether to strip white space from the beginning and end of text strings.

useExpat

a logical value indicating whether to use the expat SAX parser, or to default to the libxml. If this is TRUE, the library must have been compiled with support for expat. See supportsExpat.

isURL

indicates whether the file argument refers to a URL (accessible via ftp or http) or a regular file on the system. If asText is TRUE, this should not be specified.

state

an optional S object that is passed to the callbacks and can be modified to communicate state between the callbacks. If this is given, the callbacks should accept an argument named .state and it should return an object that will be u

replaceEntities

logical value indicating whether to substitute entity references with their text directly. This should be left as False. The text still appears as the value of the node, but there is more information about its source, allowing the parse to be

saxVersion

an integer value which should be either 1 or 2. This specifies which SAX interface to use in the C code. The essential difference is the number of arguments passed to the startElement handler function(s). In addition to the name

validate

Currently, this has no effect as the libxml2 parser uses a document structure to do validation. a logical indicating whether to use a validating parser or not, or in other words check the contents against the DTD specification. If this is true, warn

Value

• The return value is the `handlers' argument. It is assumed that this is a closure and that the callback functions have manipulated variables local to it and that the caller knows how to extract this.

Notes

The libxml parser can read URLs via http or ftp. It does not require the support of wget as used in other parts of R, but uses its own facilities to connect to remote servers.

Details

This is now implemented using the libxml parser. Originally, this was implemented via the Expat XML parser by Jim Clark (http://www.jclark.com).

References

http://www.w3.org/XML, http://www.jclark.com/xml

See Also

xmlTreeParse

Examples

fileName <- system.file("exampleData", "mtcars.xml", package="XML")

   # Print the name of each XML tag encountered at the beginning of each
   # tag.
   # Uses the libxml SAX parser.
 xmlEventParse(fileName,
                list(startElement=function(name, attrs){
                                    cat(name,"")
                                  }),
                useTagName=FALSE, addContext = FALSE)


# Parse the text rather than a file or URL by reading the URL's contents
  # and making it a single string. Then call xmlEventParse
xmlURL <- "http://www.omegahat.org/Scripts/Data/mtcars.xml"
xmlText <- paste(scan(xmlURL, what="",sep="\n"),"\n",collapse="\n")
xmlEventParse(xmlText, asText=TRUE)

    # Using a state object to share mutable data across callbacks
f <- system.file("exampleData", "gnumeric.xml", package = "XML")
zz <- xmlEventParse(f,
                    handlers = list(startElement=function(name, atts, .state) {
                                                     .state = .state + 1
                                                     print(.state)
                                                     .state
                                                 }), state = 0)
print(zz)




    # Illustrate the startDocument and endDocument handlers.
xmlEventParse(fileName,
               handlers = list(startDocument = function() {
                                                 cat("Starting document
")
                                               },
                               endDocument = function() {
                                                 cat("ending document
")
                                             }),
               saxVersion = 2)




if(libxmlVersion()$major >= 2) {


 startElement = function(x, ...) cat(x, "")


 xmlEventParse(file(f), handlers = list(startElement = startElement))


 # Parse with a function providing the input as needed.
 xmlConnection = 
  function(con) {

   if(is.character(con))
     con = file(con, "r")
  
   if(isOpen(con, "r"))
     open(con, "r")

   function(len) {

     if(len < 0) {
        close(con)
        return(character(0))
     }

      x = character(0)
      tmp = ""
    while(length(tmp) > 0 && nchar(tmp) == 0) {
      tmp = readLines(con, 1)
      if(length(tmp) == 0)
        break
      if(nchar(tmp) == 0)
        x = append(x, "")
      else
        x = tmp
    }
    if(length(tmp) == 0)
      return(tmp)
  
    x = paste(x, collapse="")

    x
  }
 }

 ff = xmlConnection(f)
 xmlEventParse(ff, handlers = list(startElement = startElement))

  # Parse from a connection. Each time the parser needs more input, it
  # calls readLines(<con>, 1)
 xmlEventParse(file(f),  handlers = list(startElement = startElement))


  # using SAX 2
 h = list(startElement = function(name, attrs, namespace, allNamespaces){ 
                                 cat("Starting", name,"")
                                 if(length(attrs))
                                     print(attrs)
                                 print(namespace)
                                 print(allNamespaces)
                         },
          endElement = function(name, uri) {
                          cat("Finishing", name, "")
            }) 
 xmlEventParse(system.file("exampleData", "namespaces.xml", package="XML"), handlers = h, saxVersion = 2)

}