This is the event-driven or SAX (Simple API for XML)
style parser which process XML without building the tree
but rather identifies tokens in the stream of characters
and passes them to handlers which can make sense of them
in context.
This 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 components include
the beginning and end of XML elements, e.g
<myTag x="1">
and </myTag> respectively,
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)
and so avoids having the entire document in memory.
This is important for large documents and where we would end up with
essentially 2 copies of the data in memory at once, i.e
the tree and the R data structure containing the information taken
from the tree.
When dealing with classes of XML documents whose instances could be large,
this approach is desirable but a little more cumbersome to program
than the standard DOM (Document Object Model) approach provided
by XMLTreeParse.
Note that xmlTreeParse does allow a hybrid style of
processing that allows us to apply handlers to nodes in the tree
as they are being converted to R objects. This is a style of
event-driven or asynchronous calling
In addition to the generic token event handlers such as
"begin an XML element" (the startElement handler), one can
also provide handler functions for specific tags/elements such
as <myTag> with handler elements with the same name as the
XML element of interest, i.e. "myTag" = function(x, attrs).
When the event parser is reading text nodes,
it may call the text handler function with different
sub-strings of the text within the node.
Essentially, the parser collects up n characters into a buffer and
passes this as a single string the text handler and then continues
collecting more text until the buffer is full or there is no more text.
It passes each sub-string to the text handler.
If trim is TRUE, it removes leading and trailing white
space from the substring before calling the text handler. If the
resulting text is empty and ignoreBlanks is TRUE,
then we don't bother calling the text handler function.
So the key thing to remember about dealing with text is that the entire text of a node may come in multiple separate calls to the text handler. A common idiom is to have the text handler concatenate the values it is passed in separate calls and to have the end element handler process the entire text and reset the text variable to be empty.
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, branches = NULL,
useDotNames = length(grep("^\.", names(handlers))) > 0,
error = xmlErrorCumulator(), addFinalizer = NA,
encoding = character())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.
the source of the XML content.
This can be a string giving 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 content.
This allows one to read the content separately from parsing
without having to write it to a file.
If asText is FALSE and a string is passed
for file, this is taken as the name of a
file or remote URI. If one is using the libxml parser (i.e. not expat),
this can be a URI accessed via HTTP or FTP or a compressed local file.
If it is the name of a local file,
it can include ~, environment variables, etc. which will be expanded by R.
(Note this is not the case in S-Plus, as far as I know.)
If a connection is given, the parser incrementally reads one line at
a time by calling the function readLines with
the connection as the first argument (and 1 as the number of
lines to read). The parser calls this function each time it needs
more input.
If invoking the readLines function to get each line is
excessively slow or is inappropriate, one can provide a function as the value
of fileName. Again, when the XML parser needs more content
to process, it invokes this function to get a string.
This function is called with a single argument, the maximum size
of the string that can be returned.
The function is responsible for accessing the correct connection(s),
etc. which is typically done via lexical scoping/environments.
This mechanism allows the user to control how the XML content
is retrieved in very general ways. For example, one might
read from a set of files, starting one when the contents
of the previous file have been consumed. This allows for the
use of hybrid connection objects.
Support for connections and functions in this form is only provided if one is using libxml2 and not libxml version 1.
a closure object that contains functions which will be invoked
as the XML components in the document are encountered by the parser.
The standard function or handler names are
startElement(), endElement()
comment(), getEntity,
entityDeclaration(), processingInstruction(),
text(), cdata(),
startDocument(), and endDocument(),
or alternatively and preferrably,
these names prefixed with a '.',
i.e. .startElement, .comment, ...
The call signature for the entityDeclaration function was changed in
version 1.7-0. Note that in earlier versions, the C routine did not
invoke any R function and so no code will actually break.
Also, we have renamed externalEntity to getEntity.
These were based on the expat parser.
The new signature is
c(name = "character",
type = "integer",
content = "",
system = "character",
public = "character"
)
name gives the name of the entity being
defined.
The type identifies
the type of the entity using the value
of a C-level enumerated constant used in libxml2,
but also gives the human-readable form
as the name of the single element in the integer vector.
The possible values are
"Internal_General",
"External_General_Parsed",
"External_General_Unparsed", "Internal_Parameter",
"External_Parameter", "Internal_Predefined".
If we are dealing with an internal entity,
the content will be the string containing
the value of the entity.
If we are dealing with an external entity,
then content will be a character vector of length
0, i.e. empty.
Instead, either or both of the system and public
arguments will be non-empty and identify the
location of the external content.
system will be a string containing a URI, if non-empty,
and public corresponds to the PUBLIC identifier used
to identify content using an SGML-like approach.
The use of PUBLIC identifiers is less common.
a logical value indicating whether text elements made up entirely of white space should be included in the resulting `tree'.
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, each callback function should support ....
a logical value.
If this is TRUE, when the SAX parser signals an event for the
start of an XML element, it will first look for an element in the
list of handler functions whose name matches (exactly) the name of
the XML element. If such an element is found, that function is
invoked. Otherwise, the generic startElement handler function
is invoked. The benefit of this is that the author of the handler
functions can write node-specific handlers for the different element
names in a document and not have to establish a mechanism to invoke
these functions within the startElement function. This is done
by the XML package directly.
If the value is FALSE, then the startElement handler
function will be called without any effort to find a node-specific
handler. If there are no node-specific handlers, specifying
FALSE for this parameter will make the computations very
slightly faster.
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, etc.) and still use this parser.
whether to strip white space from the beginning and end of text strings.
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.
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.
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 used as the updated value of this state object.
The new value can be any S object and will be passed to the next
callback where again it will be updated by that functions return
value, and so on.
If this not specified in the call to xmlEventParse,
no .state argument is passed to the callbacks. This makes the
interface compatible with previous releases.
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 reversed with full reference information.
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). Under SAX 2, in addition to the name of
the element and the named-attributes vector, two additional arguments
are provided.
The first identifies the namespace of the element.
This is a named character vector of length 1,
with the value being the URI of the namespace and the name
being the prefix that identifies that namespace within the document.
For example, xmlns:r="http://www.r-project.org"
would be passed as c(r = "http://www.r-project.org").
If there is no prefix because the namespace is being used as the
default, the result of calling names on
the string is "".
The second additional argument (the fourth in total) gives the collection of all the namespaces
defined within this element.
Again, this is a named character vector.
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, warning messages will be displayed about errors in the DTD and/or document, but the parsing will proceed except for the presence of terminal errors.
a named list of functions.
Each element identifies an XML element name.
If an XML element of that name is encountered in
the SAX stream, the stream is processed until the
end of that element and an internal node (see
xmlTreeParse and its useInternalNodes parameter)
is created. The function in our branches list corresponding to this
XML element is then invoked with the (internal) node as the only
argument.
This allows one to use the DOM model on a sub-tree of the entire
document and thus use both SAX and DOM together to get the
efficiency of SAX and the simpler programming model of DOM.
Note that the branches mechanism works top-down and does not
work for nested tags. If one specifies an element name in the
branches argument, e.g. myNode, and
there is a nested myNode instance within a branch, the branches
handler will not be called for that nested instance.
If there is an instance where this is problematic, please
contact the maintainer of this package.
One can cause the parser to collect a branch without identifying
the node within the branches list. Specifically, within
a regular start-element handler, one can return a function
whose class is SAXBranchFunction.
The SAX parser recognizes this and collects up the branch
starting at the current node being processed and when it is
complete, invokes this function.
This allows us to dynamically determine which nodes to treat as
branches rather than just matching names. This is necessary when
a node name has different meanings in different parts of the XML
hierarchy, e.g. dict in an iTunes song list.
See the file itunesSax2.R inthe examples for an example of this.
This is a two step process. In the future, we might make it so that the R function handling the start-element event could directly collect the branch and continue its operations without having to call another function asynchronously.
a logical value
indicating whether to use the
newer format for identifying general element function handlers
with the '.' prefix, e.g. .text, .comment, .startElement.
If this is FALSE, then the older format
text, comment, startElement, ...
are used. This causes problems when there are indeed nodes
named text or comment or startElement as a
node-specific handler are confused with the corresponding
general handler of the same name. Using TRUE
means that your list of handlers should have names that use
the '.' prefix for these general element handlers.
This is the preferred way to write new code.
a function that is called when an XML error is encountered.
This is called with 6 arguments and is described in xmlTreeParse.
a logical value or identifier for a C routine that controls whether we register finalizers on the intenal node.
a character string (scalar) giving the encoding for the document. This is optional as the document should contain its own encoding information. However, if it doesn't, the caller can specify this for the parser.
Duncan Temple Lang
This is now implemented using the libxml parser. Originally, this was implemented via the Expat XML parser by Jim Clark (http://www.jclark.com/).
xmlTreeParse
xmlStopParser
XMLParserContextFunction
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,"\n")
}),
useTagName=FALSE, addContext = FALSE)
if (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 <- "https://www.omegahat.net/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\n")
},
endDocument = function() {
cat("ending document\n")
}),
saxVersion = 2)
if(libxmlVersion()$major >= 2) {
startElement = function(x, ...) cat(x, "\n")
xmlEventParse(ff <- file(f), handlers = list(startElement = startElement))
close(ff)
# Parse with a function providing the input as needed.
xmlConnection =
function(con) {
if(is.character(con))
con = file(con, "r")
if(isOpen(con, "r"))
open(con, "r")
function(len) {
if(len < 0) {
close(con)
return(character(0))
}
x = character(0)
tmp = ""
while(length(tmp) > 0 && nchar(tmp) == 0) {
tmp = readLines(con, 1)
if(length(tmp) == 0)
break
if(nchar(tmp) == 0)
x = append(x, "\n")
else
x = tmp
}
if(length(tmp) == 0)
return(tmp)
x = paste(x, collapse="")
x
}
}
## this leaves a connection open
## xmlConnection would need amending to return the connection.
ff = xmlConnection(f)
xmlEventParse(ff, handlers = list(startElement = startElement))
# Parse from a connection. Each time the parser needs more input, it
# calls readLines(, 1)
xmlEventParse(ff <-file(f), handlers = list(startElement = startElement))
close(ff)
# using SAX 2
h = list(startElement = function(name, attrs, namespace, allNamespaces){
cat("Starting", name,"\n")
if(length(attrs))
print(attrs)
print(namespace)
print(allNamespaces)
},
endElement = function(name, uri) {
cat("Finishing", name, "\n")
})
xmlEventParse(system.file("exampleData", "namespaces.xml", package="XML"),
handlers = h, saxVersion = 2)
# This example is not very realistic but illustrates how to use the
# branches argument. It forces the creation of complete nodes for
# elements named and extracts the id attribute.
# This could be done directly on the startElement, but this just
# illustrates the mechanism.
filename = system.file("exampleData", "branch.xml", package="XML")
b.counter = function() {
nodes <- character()
f = function(node) { nodes <<- c(nodes, xmlGetAttr(node, "id"))}
list(b = f, nodes = function() nodes)
}
b = b.counter()
invisible(xmlEventParse(filename, branches = b["b"]))
b$nodes()
filename = system.file("exampleData", "branch.xml", package="XML")
invisible(xmlEventParse(filename, branches = list(b = function(node) {
print(names(node))})))
invisible(xmlEventParse(filename, branches = list(b = function(node) {
print(xmlName(xmlChildren(node)[[1]]))})))
}
############################################
# Stopping the parser mid-way and an example of using XMLParserContextFunction.
startElement =
function(ctxt, name, attrs, ...) {
print(ctxt)
print(name)
if(name == "rewriteURI") {
cat("Terminating parser\n")
xmlStopParser(ctxt)
}
}
class(startElement) = "XMLParserContextFunction"
endElement =
function(name, ...)
cat("ending", name, "\n")
fileName = system.file("exampleData", "catalog.xml", package = "XML")
xmlEventParse(fileName, handlers = list(startElement = startElement,
endElement = endElement))
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