R: XML output streams

xmlOutputBuffer {XML} | R Documentation

## XML output streams

### Description

These two functions provide different ways to construct XML documents incrementally. They provide a single, common interface for adding and closing tags, and inserting nodes. The buffer version stores the XML representation as a string. The DOM version builds the tree of XML node objects entirely within R.

### Usage

    xmlOutputBuffer(dtd=NULL, nameSpace="", buf=NULL,
                                 nsURI=NULL, header="<?xml version=\"1.0\"?>")

    xmlOutputDOM(tag="doc", attrs = NULL, dtd=NULL,
                              nameSpace=NULL, nsURI=character(0),
                              xmlDeclaration = NULL)

### Arguments

`dtd` | a DTD object (see `parseDTD` and `xmlTreeParse`) which contains specifications about what elements are valid within other elements and what attributes are supported by different elements. This can be used to validate the document as it is being constructed incrementally.
`attrs` | attributes for the top-level node, in the form of a named vector or list.
`nameSpace` | the default namespace identifier to be used when an element is created without an explicit namespace. This provides a convenient way to specify the default name space that appers in tags throughout the resulting document.
`buf` | a connection object or a string into which the XML content is written. This is currently a simplistic implementation since we will use the OOP-style classes from the Omegahat projects in the future.
`nsURI` | the URI or value for the name space which is used when declaring the namespace. For `xmlOuputDOM`, this is a named character vector with each element giving the name space identifier and the corresponding URI, \ e.g `c(shelp = "http://www.omegahat.net/XML/SHelp")`
`header` | if non-NULL, this is immediately written to the output stream allowing one to control the initial section of the XML document.
`tag` | the name of the top-level node/element in the DOM being created.
`xmlDeclaration` | a logical value or a string. If this is a logical value and `TRUE`, the default <?xml version='1.0'?> processing instruction is emitted at the top of the document. If it is `FALSE`, no xml declaration is emitted at the top of the document. If this is provided as a string, the contents of this is added as the content of the processing instruction. A version='1.0' is added if there is no 'version=' content within the given string.

### Details

These functions create a closure instance which provides methods or functions that operate on shared data used to represent the contents of the XML document being created and the current state of that creation.

### Value

Both of these functions return a list of functions which operate on the XML data in a shared environment.

`value` | get the contents of the XML document as they are currently defined.
`addTag` | add a new element to the document, specifying its name and attributes. This allows the tag to be left open so that new elements will be added as children of it.
`closeTag` | close the currently open tag, indicating that new elements will be added, by default, as siblings of this one.
`reset` | discard the current contents of the document so that we can start over and free the resources (memory) associated with this document.

The following are specific to `xmlOutputDOM`:

`addNode` | insert an complete `XMLNode` object into the currently active (i.e. open) node.
`current` | obtain the path or collection of indices to to the currently active/open node from the root node.

### Author(s)

Duncan Temple Lang

### References

http://www.omegahat.net/RSXML, http://www.w3.org/xml

### See Also

`xmlTree` for a native/internal (C-level) representation of the tree, `xmlNode`, `xmlTextNode`, `append.xmlNode`

And a different representation of a tree is available via `xmlHashTree`.

### Examples

     con <- xmlOutputDOM()
    con$addTag("author", "Duncan Temple Lang")
    con$addTag("address",  close=FALSE)
     con$addTag("office", "2C-259")
     con$addTag("street", "Mountain Avenue.")
     con$addTag("phone", close = FALSE)
       con$addTag("area", "908", attrs=c(state="NJ"))
       con$addTag("number", "582-3217")
     con$closeTag() # phone
    con$closeTag() # address

    con$addTag("section", close = FALSE)
     con$addNode(xmlTextNode("This is some text "))
     con$addTag("a","and a link", attrs=c(href="http://www.omegahat.net"))
     con$addNode(xmlTextNode("and some follow up text"))

     con$addTag("subsection", close = FALSE)
       con$addNode(xmlTextNode("some addtional text "))
       con$addTag("a", attrs=c(href="http://www.omegahat.net"), close=FALSE)
         con$addNode(xmlTextNode("the content of the link"))
       con$closeTag() # a
     con$closeTag() # "subsection"
    con$closeTag() # section

     d <- xmlOutputDOM()
     d$addPI("S", "plot(1:10)")
     d$addCData('x <- list(1, a="&");\nx[[2]]')
     d$addComment("A comment")
     print(d$value())
     print(d$value(), indent = FALSE, tagSeparator = "")

     d = xmlOutputDOM("bob", xmlDeclaration = TRUE)
     print(d$value())

     d = xmlOutputDOM("bob", xmlDeclaration = "encoding='UTF-8'")
     print(d$value())

     d = xmlOutputBuffer("bob", header = "<?xml version='1.0' encoding='UTF-8'?>",
                          dtd = "foo.dtd")
     d$addTag("bob")
     cat(d$value())

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