Reader

[Raku LibXML Project] / [LibXML Module] / Reader

class LibXML::Reader

Interface to libxml2 Pull Parser

Synopsis

use LibXML::Reader;

sub dump-node($reader) {
    printf "%d %d %s %d\n", $reader.depth,
                            $reader.nodeType,
                            $reader.name,
                            $reader.isEmptyElement;
}

my LibXML::Reader $reader .= new(file => "file.xml")
       or die "cannot read file.xml\n";
while $reader.read {
    dump-node($reader);
}

use LibXML::Reader;

my LibXML::Reader $reader .= new(file => "file.xml")
       or die "cannot read file.xml\n";
$reader.preservePattern('//table/tr');
$reader.finish;
print $reader.document.Str(:deep);

Description

This is a Raku interface to libxml2's pull-parser implementation xmlTextReader http://xmlsoft.org/html/libxml-xmlreader.html. Pull-parsers (such as StAX in Java, or XmlReader in C#) use an iterator approach to parse XML documents. They are easier to program than event-based parser (SAX) and much more lightweight than tree-based parser (DOM), which load the complete tree into memory.

The Reader acts as a cursor going forward on the document stream and stopping at each node on the way. At every point, the DOM-like methods of the Reader object allow one to examine the current node (name, namespace, attributes, etc.)

The user's code keeps control of the progress and simply calls the read() function repeatedly to progress to the next node in the document order. Other functions provide means for skipping complete sub-trees, or nodes until a specific element, etc.

At every time, only a very limited portion of the document is kept in the memory, which makes the API more memory-efficient than using DOM. However, it is also possible to mix Reader with DOM. At every point the user may copy the current node (optionally expanded into a complete sub-tree) from the processed document to another DOM tree, or to instruct the Reader to collect sub-document in form of a DOM tree consisting of selected nodes.

Reader API also supports namespaces, xml:base, entity handling, DTD, Schema and RelaxNG validation support

The naming of methods compared to libxml2 and C# XmlTextReader has been changed slightly to match the conventions of LibXML. Some functions have been changed or added with respect to the C interface.

Constructor

Depending on the XML source, the Reader object can be created with either of:

my LibXML::Reader $reader .= new( file => "file.xml", ... );
my LibXML::Reader $reader .= new( string => $xml_string, ... );
my LibXML::Reader $reader .= new( io => $file_handle, ... );
my LibXML::Reader $reader .= new( fd => $file_handle.native_descriptor, ... );
my LibXML::Reader $reader .= new( DOM => $dom, ... );

where ... are reader options described below in [Reader options](Reader options) or various parser options described in LibXML::Parser. The constructor recognizes the following XML sources:

Source specification

Str :$file
Read XML from a local file or URL.
Str :$string
Read XML from a string.
IO::Handle :$io
Read XML as a Raku IO::Handle object.
UInt :$fd
Read XML from a file descriptor number. Possibly faster than IO.
LibXML::Document :$DOM
Use reader API to walk through a pre-parsed LibXML::Document.

Reader options

:$encoding
override document encoding.
RelaxNG => $rng-schema
can be used to pass either a LibXML::RelaxNG object or a filename or URL of a RelaxNG schema to the constructor. The schema is then used to validate the document as it is processed.
Schema => $xsd-schema
can be used to pass either a LibXML::Schema object or a filename or URL of a W3C XSD schema to the constructor. The schema is then used to validate the document as it is processed.
...
the reader further supports various parser options described in LibXML::Parser (specifically those labeled by /reader/).

Methods Controlling Parsing Progress

method read

method read() returns Bool

Moves the position to the next node in the stream, exposing its properties.

Returns True if the node was read successfully, False if there is no more nodes to read, or Failure in case of error

method readAttributeValue

method readAttributeValue() returns Bool

Parses an attribute value into one or more Text and EntityReference nodes.

Returns True in case of success, False if the reader was not positioned on an attribute node or all the attribute values have been read, or Failure in case of error.

method readState

method readState() returns UInt

Gets the read state of the reader.

Returns the state value, or Failure in case of error. The module exports constants for the Reader states, see STATES below.

method depth

method depth() returns UInt

The depth of the node in the tree, starts at 0 for the root node.

method next

method next() returns Bool

Skip to the node following the current one in the document order while avoiding the sub-tree if any.

Returns True if the node was read successfully, False if there is no more nodes to read, or Failure in case of error.

method nextElement

method nextElement(
    Str $local-name? where { ... },
    Str $URI?
) returns Bool

Skip nodes following the current one in the document order until a specific element is reached.

The element's name must be equal to a given localname if defined, and its namespace must equal to a given nsURI if defined. Either of the arguments can be undefined (or omitted, in case of the latter or both).

Returns True if the element was found, False if there is no more nodes to read, or Failure in case of error.

method nextPatternMatch

method nextPatternMatch(
    LibXML::Pattern:D $pattern
) returns Bool

Skip nodes following the current one in the document order until an element matching a given compiled pattern is reached.

See LibXML::Pattern for information on compiled patterns. See also the matchesPattern method.

Returns True if the element was found, False if there is no more nodes to read, or Failure in case of error.

method skipSiblings

method skipSiblings() returns Bool

Skip all nodes on the same or lower level until the first node on a higher level is reached.

In particular, if the current node occurs in an element, the reader stops at the end tag of the parent element, otherwise it stops at a node immediately following the parent node.

Returns True if successful, False if end of the document is reached, or Failure in case of error.

method nextSibling

method nextSibling() returns Bool

Skips to the node following the current one in the document order while avoiding the sub-tree if any.

Returns True if the element was found, False if there is no more nodes to read, or Failure in case of error.

method nextSiblingElement

method nextSiblingElement(
    Str $name? where { ... },
    Str $URI?
) returns Mu

Like nextElement but only processes sibling elements of the current node (moving forward using nextSibling() rather than read(), internally).

Returns True if the element was found, False if there is no more nodes to read, or Failure in case of error.

method finish

method finish() returns Bool

Skip all remaining nodes in the document, reaching end of the document.

Returns True if successful, False in case of error.

method close

method close() returns Bool

This method releases any resources allocated by the current instance and closes underlying input.

It returns False on failure and True on success. This method is automatically called by the destructor when the reader is forgotten, therefore you do not have to call it directly.

Methods Extracting Information

method name

method name() returns Str

Returns the name of the current node.

Returns:

an element or attribute name of the form [prefix:][name], or
a special-name begining with '#', such as #text, #comment, or #cdata-section.

method nodeType

method nodeType() returns UInt

Returns the type of the current node.

See NODE TYPES below.

method localName

method localName() returns Str

Returns the local name of the node.

Either an element or attribute name of the form [prefix:][name], or a special-name begining with '#', such as #text, #comment, or #cdata-section.

method prefix

method prefix() returns LibXML::Types::NCName

Returns the prefix of the namespace associated with the node.

method namespaceURI

method namespaceURI() returns Str

Returns the URI defining the namespace associated with the node.

method isEmptyElement

method isEmptyElement() returns Bool

Check if the current node is empty.

This is a bit bizarre in the sense that will be considered empty while will not.

method hasValue

method hasValue() returns Bool

Returns True if the node can have a text value.

method value

method value() returns Str

Provides the text value of the node if present or Str:U if not available.

method readInnerXml

method readInnerXml() returns Str

Reads the contents of the current node, including child nodes and markup.

Returns a string containing the XML of the node's content, or Str:U if the current node is neither an element nor attribute, or has no child nodes.

method readOuterXml

method readOuterXml() returns Str

Reads the contents of the current node, including child nodes and markup.

Returns a string containing the XML of the node including its content, or undef if the current node is neither an element nor attribute.

method nodePath

method nodePath() returns Mu

Returns a canonical location path to the current element from the root node to

Namespaced elements are matched by '*', because there is no way to declare prefixes within XPath patterns.
Unlike LibXML::Node::nodePath(), this function does not provide sibling counts (i.e. instead of e.g. '/a/b[1]' and '/a/b[2]' you get '/a/b' for both matches).

method matchesPattern

method matchesPattern(
    LibXML::Pattern:D $pattern
) returns Bool

Returns a true value if the current node matches a compiled pattern.

See LibXML::Pattern for information on compiled patterns.
See also the nextPatternMatch method.

Methods Extracting DOM Nodes

method document

method document() returns Mu

Provides access to the document tree built by the reader.

This function can be used to collect the preserved nodes (see preserveNode() and preservePattern).
CAUTION: Never use this function to modify the tree unless reading of the whole document is completed!

method copyCurrentNode

method copyCurrentNode(
    Bool :$deep
) returns LibXML::Node

This function is similar a DOM function copyNode(). It returns a copy of the currently processed node as a corresponding DOM object.

Use :deep to obtain the full sub-tree.

method preserveNode

method preserveNode() returns LibXML::Node

This tells the XML Reader to preserve the current node in the document tree.

A document tree consisting of the preserved nodes and their content can be obtained using the method document() once parsing is finished.

Returns the node or LibXML::Node:U in case of error.

method preservePattern

method preservePattern(
    Str:D $pattern,
    :%ns
) returns UInt

This tells the XML Reader to preserve all nodes matched by the pattern (which is a streaming XPath subset).

A document tree consisting of the preserved nodes and their content can be obtained using the method document() once parsing is finished.

An :%ns may be used to pass a mapping prefixes used by the XPath to namespace URIs.

The XPath subset available with this function is described at http://www.w3.org/TR/xmlschema-1/#Selector and matches the production

Path ::= ('.//')? ( Step '/' )* ( Step | '@' NameTest )

Returns a positive number in case of success or Failure in case of error

Methods Processing Attributes

method attributeCount

method attributeCount() returns UInt

Provides the number of attributes of the current node.

method hasAttributes

method hasAttributes() returns Bool

Whether the node has attributes.

method getAttribute

method getAttribute(
    Str $name where { ... }
) returns Str

Provides the value of the attribute with the specified qualified name.

Returns a string containing the value of the specified attribute, or Str:U in case of error.

method getAttributeNs

method getAttributeNs(
    Str $local-name where { ... },
    Str $namespace-URI
) returns Str

Provides the value of the specified attribute in a given namespace

method getAttributeNo

method getAttributeNo(
    Int $i where { ... }
) returns Str

Provides the value of the attribute with the specified index relative to the containing element.

method isDefault

method isDefault() returns Bool

Returns True if the current attribute node was generated from the default value defined in the DTD.