XPath Context¶

An XML::XPath::Context holds the state for XPath evaluation — registered namespaces, the context node, and an optional object cache. For one-off queries, doc.find and node.find create a context automatically. For repeated queries, creating a context explicitly avoids redundant setup.

Creating a Context¶

context = XML::XPath::Context.new(doc)

Registering Namespaces¶

context.register_namespace('atom', 'http://www.w3.org/2005/Atom')
context.register_namespace('dc', 'http://purl.org/dc/elements/1.1/')

entries = context.find('//atom:entry')

You can also register from a hash, array, or directly from a node's namespace declarations:

# From a hash
context.register_namespaces('atom' => 'http://www.w3.org/2005/Atom')

# From an array of "prefix:uri" strings
context.register_namespaces(['atom:http://www.w3.org/2005/Atom'])

# From a node's namespace declarations
context.register_namespaces_from_node(doc.root)

Setting the Context Node¶

By default the context node is the document root. Change it to evaluate XPath relative to a different node:

context.node = doc.root.find_first('//chapter')
sections = context.find('section')  # relative to chapter

Caching¶

Enable the XPath object cache to reuse internal XPath objects across queries, reducing allocations:

context.enable_cache     # default cache size
context.enable_cache(100) # specific size

# When done
context.disable_cache

Return Types¶

context.find returns different types depending on the XPath expression:

XPath expression	Ruby return type
`//book`	`XML::XPath::Object` (node set)
`count(//book)`	`Float`
`string(//title)`	`String`
`1 = 1`	`true` or `false`

An XML::XPath::Object for a node set is enumerable:

results = context.find('//book')
results.each { |node| puts node.name }
results.length
results.first
results[0]