Xml-xxpath is an (incomplete) XPath interpreter that is at the moment
bundled with xml-mapping. It is built on top of REXML. xml-mapping
uses xml-xxpath extensively for implementing its node types -- see the
README file and the reference documentation (and the source code) for
details. xml-xxpath, however, does not depend on xml-mapping at all,
and is useful in its own right -- maybe I'll later distribute it as a
seperate library instead of bundling it. For the time being, if you
want to use this XPath implementation stand-alone, you can just rip
the files lib/xml/xxpath.rb, lib/xml/xxpath/steps.rb, and
lib/xml/xxpath_methods.rb out of the xml-mapping distribution and
use them on their own (they do not depend on anything else).
xml-xxpath's XPath support is vastly incomplete (see below), but, in
addition to the normal reading/matching functionality found in other
XPath implementations (i.e. "find all elements in a given XML document
matching a given XPath expression"), xml-xxpath supports write
access. For example, when writing the XPath expression
/foo/bar[3]/baz[@key='hiho'] to the XML document
<foo>
<bar>
<baz key='ab'>hello</baz>
<baz key='xy'>goodbye</baz>
</bar>
</foo>
, you'll get:
<foo>
<bar>
<baz key='ab'>hello</baz>
<baz key='xy'>goodbye</baz>
</bar>
<bar/>
<bar><baz key='hiho'/></bar>
</foo>
This feature is used by xml-mapping when writing (marshalling) Ruby objects to XML, and is actually the reason why I couldn't just use any of the existing XPath implementations, e.g. the one that comes with REXML. Also, the whole xml-xxpath implementation is just 300 lines of Ruby code, it is quite fast (paths are precompiled), and xml-xxpath returns matched elements in the order they appeared in the source document -- I've heard REXML::XPath doesn't do that :)
Some basic knowledge of XPath is helpful for reading this document.
At the moment, xml-xxpath understands XPath expressions of the form
[/]pathelement/[/]pathelement/[/]...,
where each pathelement must be one of these:
-
a simple element name name, e.g.
signature -
an attribute name, @attrname, e.g.
@key -
a combination of an element name and an attribute name and -value, in the form
elt_name[@attr_name='attr_value'] -
an element name and an index,
elt_name[index] -
the "match-all" path element,
* -
.
-
name1
|name2|... -
.[@key='xy'] / self::*[@key='xy'] -
child::*[@key='xy'] -
text()
Xml-xxpath only supports relative paths at this time, i.e. XPath expressions beginning with "/" or "//" will still only find nodes below the node the expression is applied to (as if you had written "./" or ".//", respectively).
Xml-xxpath defines the class XML::XXPath. An instance of that class wraps an XPath expression, the string representation of which must be supplied when constructing the instance. You then call instance methods like first, all or create_new on the instance, supplying the REXML Element the XPath expression should be applied to, and get the results, or, in the case of write access, the element is updated in-place.
:include: xpath_usage.intout
The objects supplied to the all(), first(), and
each() calls must be REXML element nodes, i.e. they must
support messages like elements, attributes etc
(instances of REXML::Element and its subclasses do this). The calls
return the found elements as instances of REXML::Element or
XML::XXPath::Accessors::Attribute. The latter is a wrapper around
attribute nodes that is largely call-compatible to
REXML::Element. This is so you can write things like
path.each{|node|puts node.text} without having to
special-case anything even if the path matches attributes, not just
elements.
As you can see, you can re-use path objects, applying them to different XML elements at will. You should do this because the XPath pattern is stored inside the XPath object in a pre-compiled form, which makes it more efficient.
The path elements of the XPath pattern are applied to the
.elements collection of the passed XML element and its
sub-elements, starting with the first one. This is shown by the
following code:
:include: xpath_docvsroot.intout
A REXML +Document+ object is a REXML +Element+ object whose +elements+ collection consists only of a single member -- the document's root node. The first path element of the XPath -- "foo" in the example -- is matched against that. That is why the path "/bar" in the example doesn't match anything when matched against the document +d+ itself.
An ordinary REXML +Element+ object that represents a node somewhere inside an XML tree has an +elements+ collection that consists of all the element's direct sub-elements. That is why XPath patterns matched against the +firstelt+ element in the example must not start with "/first" (unless there is a child node that is also named "first").
You may pass an :ensure_created=>true option argument to
path.first(elt) / path.all(elt) calls to make sure that path
exists inside the passed XML element elt. If it existed before,
nothing changes, and the call behaves just as it would without the
option argument. If the path didn't exist before, the XML element is
modified such that
-
the path exists afterwards
-
all paths that existed before still exist afterwards
-
the modification is as small as possible (i.e. as few elements as possible are added, additional attributes are added to existing elements if possible etc.)
The created resp. previously existing, matching elements are returned.
Examples:
:include: xpath_ensure_created.intout
Alternatively, you may pass a :create_new=>true option
argument or call create_new (path.create_new(elt) is
equivalent to path.first(elt,:create_new=>true)). In that
case, a new node is created in elt for each path element of path
(or an exception raised if that wasn't possible for any path element).
Examples:
:include: xpath_create_new.intout
This feature is used in xml-mapping by node types like XML::Mapping::ArrayNode, which must create a new instance of the "per-array element path" for each element of the array to be stored in an XML tree.
What is created when the Path "*" is to be created inside an empty XML element? The name of the element to be created isn't known, but still some element must be created. The answer is that xml-xxpath creates a special "unspecified" element whose name must be set by the caller afterwards:
:include: xpath_pathological.intout
The "newelt" object in the last example is an ordinary REXML::Element. xml-xxpath mixes the "unspecified" attribute into that class, as well as into the XML::XXPath::Accessors::Attribute class mentioned above.
doc/xpath_impl_notes.txt contains some documentation on the
implementation of xml-xxpath.
Ruby's.