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Einführung in den Gebrauch von XPath in JavaScript

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Dieses Dokument beschreibt die Schnittstelle zu XPath in JavaScript intern, in Erweiterungen und in Webseiten. Mozilla implementiert einen großen Teil von DOM 3 XPath, sodass XPath auf HTML- und XML-Dokumente angewendet werden kann.

Die Hauptschnittstelle für die Anwendung von XPath ist die evaluate-Methode des document-Objekts.

document.evaluate

Diese Methode wertet  XPath Ausdrücke in Bezug auf ein XML basierendes Dokument (einschließlich HTML-Documente) aus und gibt ein XPathResult-Objekt zurück, das ein Einzelknoten oder eine Zusammenstellung mehrerer Knoten sein kann. Die vorhandene Dokumentation dieser Methode ist unter document.evaluate zu finden, sie ist jedoch für unseren jetzigen Bedarf ziemlich knapp gehalten. Eine umfangreichere Betrachtung wird nachfolgend beschrieben.

var xpathResult = document.evaluate( xpathExpression, contextNode, namespaceResolver, resultType, result );

Parameter

Die evaluate Funktion nimmt insgesamt fünf Parameter entgegen:

  • xpathExpression: Eine Zeichenkette, die den auszuwertenden XPath-Ausdrucks beinhaltet.
  • contextNode: Ein Bezugsknoten im Dokument, auf den sich die Auswertung von xpathExpression beziehen soll, einschließlich sämtlicher Kindknoten. Am häufigsten wird hierfür der document-Knoten verwendet.
  • namespaceResolver: A function that will be passed any namespace prefixes contained within xpathExpression which returns a string representing the namespace URI associated with that prefix. This enables conversion between the prefixes used in the XPath expressions and the possibly different prefixes used in the document. The function can be either:
    • Created by using the createNSResolver method of a XPathEvaluator object. You should use this virtually all of the time.
    • null, which can be used for HTML documents or when no namespace prefixes are used. Note that, if the xpathExpression contains a namespace prefix, this will result in a DOMException being thrown with the code NAMESPACE_ERR.
    • A custom user-defined function. See the Using a User Defined Namespace Resolver section in the appendix for details.
  • resultType: Eine Konstante, die den gewünschten Rückgabetyp als Ergebnis der Auswertung angibt. Die am häufigsten übergebene Konstante ist XPathResult.ANY_TYPE, die eine Rückgabe der Ergebnisse in in einer Weise liefert, die den sebstverständlichen Erwartungen am ehesten entspricht. Im Anhang ist ein Abschnitt zu finden, der eine vollständige Liste der der verfügbaren Konstanten enthält. Sie werden weiter unten im Abschnitt  "Festlegung des Rückgabetyps" erläutert.
  • result: Wird ein vorhandenes XPathResult-Objekt angegeben, so wird es für die Ergebnisrückgabe wiederverwendet. Bei der Angabe von null wird ein neues XPathResult-Objekt erzeugt.

Rückgabewert

Returns xpathResult, which is an XPathResult object of the type specified in the resultType parameter. The XPathResult Interface is defined here.

Einen Standard-Namensraumauflöser (Default Namespace Resolver) implementieren

Wir erstellen einen Namensraumauflöser mit Hilfe der createNSResolver-Methode des document-Objekts.

var nsResolver = document.createNSResolver( contextNode.ownerDocument == null ? contextNode.documentElement : contextNode.ownerDocument.documentElement );

Or alternatively by using the <code>createNSResolver</code> method of a <code>XPathEvaluator</code> object. <pre> var xpEvaluator = new XPathEvaluator(); var nsResolver = xpEvaluator.createNSResolver( contextNode.ownerDocument == null ? contextNode.documentElement : contextNode.ownerDocument.documentElement ); </pre> And then pass document.evaluate, the nsResolver variable as the namespaceResolver parameter.

Note: XPath defines QNames without a prefix to match only elements in the null namespace. There is no way in XPath to pick up the default namespace as applied to a regular element reference (e.g., p[@id='_myid'] for xmlns='http://www.w3.org/1999/xhtml'). To match default elements in a non-null namespace, you either have to refer to a particular element using a form such as ['namespace-uri()='http://www.w3.org/1999/xhtml' and name()='p' and @id='_myid'] (this approach works well for dynamic XPath's where the namespaces might not be known) or use prefixed name tests, and create a namespace resolver mapping the prefix to the namespace. Read more on how to create a user defined namespace resolver, if you wish to take the latter approach.

Anmerkungen

Adapts any DOM node to resolve namespaces so that an XPath expression can be easily evaluated relative to the context of the node where it appeared within the document. This adapter works like the DOM Level 3 method lookupNamespaceURI on nodes in resolving the namespaceURI from a given prefix using the current information available in the node's hierarchy at the time lookupNamespaceURI is called. Also correctly resolves the implicit xml prefix.

Festlegung des Rückgabetyps

The returned variable xpathResult from document.evaluate can either be composed of individual nodes (simple types), or a collection of nodes (node-set types).

Simple Types

When the desired result type in resultType is specified as either:

  • NUMBER_TYPE - a double
  • STRING_TYPE - a string
  • BOOLEAN_TYPE - a boolean

We obtain the returned value of the expression by accessing the following properties respectively of the XPathResult object.

  • numberValue
  • stringValue
  • booleanValue
Beispiel

The following uses the XPath expression count(//p) to obtain the number of <p> elements in a HTML document:

var paragraphCount = document.evaluate( 'count(//p)', document, null, XPathResult.ANY_TYPE, null );

alert( 'This document contains ' + paragraphCount.numberValue + ' paragraph elements' );

Although JavaScript allows us to convert the number to a string for display, the XPath interface will not automatically convert the numerical result if the stringValue property is requested, so the following code will not work:

var paragraphCount = document.evaluate('count(//p)', document, null, XPathResult.ANY_TYPE, null );

alert( 'This document contains ' + paragraphCount.stringValue + ' paragraph elements' );

Instead it will return an exception with the code NS_DOM_TYPE_ERROR.

Node-Set-Typen

The XPathResult object allows node-sets to be returned in 3 principal different types:

Iteratoren

When the specified result type in the resultType parameter is either:

  • UNORDERED_NODE_ITERATOR_TYPE
  • ORDERED_NODE_ITERATOR_TYPE

The XPathResult object returned is a node-set of matched nodes which will behave as an iterator, allowing us to access the individual nodes contained by using the iterateNext() method of the XPathResult.

Once we have iterated over all of the individual matched nodes, iterateNext() will return null.

Note however, that if the document is mutated (the document tree is modified) between iterations that will invalidate the iteration and the invalidIteratorState property of XPathResult is set to true, and a NS_ERROR_DOM_INVALID_STATE_ERR exception is thrown.

Iterator Example
var iterator = document.evaluate('//phoneNumber', documentNode, null, XPathResult.UNORDERED_NODE_ITERATOR_TYPE, null );

try {
  var thisNode = iterator.iterateNext();
  
  while (thisNode) {
    alert( thisNode.textContent );
    thisNode = iterator.iterateNext();
  }	
}
catch (e) {
  dump( 'Error: Document tree modified during iteration ' + e );
}
Momentabbilder (Snapshots)

When the specified result type in the resultType parameter is either:

  • UNORDERED_NODE_SNAPSHOT_TYPE
  • ORDERED_NODE_SNAPSHOT_TYPE

The XPathResult object returned is a static node-set of matched nodes, which allows us to access each node through the snapshotItem(itemNumber) method of the XPathResult object, where itemNumber is the index of the node to be retrieved. The total number of nodes contained can be accessed through the snapshotLength property.

Snapshots do not change with document mutations, so unlike the iterators the snapshot does not become invalid, but it may not correspond to the current document, for example the nodes may have been moved, it might contain nodes that no longer exist, or new nodes could have been added.

Snapshot Example
var nodesSnapshot = document.evaluate('//phoneNumber', documentNode, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null );

for ( var i=0 ; i < nodesSnapshot.snapshotLength; i++ )
{
  dump( nodesSnapshot.snapshotItem(i).textContent );
}
Erster Knoten

When the specified result type in the resultType parameter is either:

  • ANY_UNORDERED_NODE_TYPE
  • FIRST_ORDERED_NODE_TYPE

The XPathResult object returned is only the first found node that matched the XPath expression. This can be accessed through the singleNodeValue property of the XPathResult object. This will be null if the node set is empty.

Note that, for the unordered subtype the single node returned might not be the first in document order, but for the ordered subtype you are guaranteed to get the first matched node in the document order.

First Node Example
var firstPhoneNumber = document.evaluate('//phoneNumber', documentNode, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null );

dump( 'The first phone number found is ' + firstPhoneNumber.singleNodeValue.textContent );

Die Konstante ANY_TYPE

When the result type in the resultType parameter is specified as ANY_TYPE, the XPathResult object returned, will be whatever type that naturally results from the evaluation of the expression.

It could be any of the simple types (NUMBER_TYPE, STRING_TYPE, BOOLEAN_TYPE), but, if the returned result type is a node-set then it will only be an UNORDERED_NODE_ITERATOR_TYPE.

To determine that type after evaluation, we use the resultType property of the XPathResult object. The constant values of this property are defined in the appendix. None Yet =====Any_Type Example===== <pre> </pre>

Beispiele

Innerhalb eines HTML-Dokuments

The following code is intended to be placed in any JavaScript fragment within or linked to the HTML document against which the XPath expression is to be evaluated.

To extract all the <h2> heading elements in an HTML document using XPath, the xpathExpression is simply '//h2'. Where, // is the Recursive Descent Operator that matches elements with the nodeName h2 anywhere in the document tree. The full code for this is: link to introductory xpath doc

var headings = document.evaluate('//h2', document, null, XPathResult.ANY_TYPE, null );

Notice that, since HTML does not have namespaces, we have passed null for the namespaceResolver parameter.

Since we wish to search over the entire document for the headings, we have used the document object itself as the contextNode.

The result of this expression is an XPathResult object. If we wish to know the type of result returned, we may evaluate the resultType property of the returned object. In this case, that will evaluate to 4, an UNORDERED_NODE_ITERATOR_TYPE. This is the default return type when the result of the XPath expression is a node set. It provides access to a single node at a time and may not return nodes in a particular order. To access the returned nodes, we use the iterateNext() method of the returned object:

var thisHeading = headings.iterateNext();

var alertText = 'Level 2 headings in this document are:\n'

while (thisHeading) {
  alertText += thisHeading.textContent + '\n';
  thisHeading = headings.iterateNext();
}

Once we iterate to a node, we have access to all the standard DOM interfaces on that node. After iterating through all the h2 elements returned from our expression, any further calls to iterateNext() will return null.

Auswertung an einem XML-Dokument innerhalb einer Erweiterung

The following uses an XML document located at chrome://yourextension/content/peopleDB.xml as an example.

<?xml version="1.0"?>
<people xmlns:xul = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" >
  <person>
	<name first="george" last="bush" />
	<address street="1600 pennsylvania avenue" city="washington" country="usa"/>
	<phoneNumber>202-456-1111</phoneNumber>
  </person>
  <person>
	<name first="tony" last="blair" />
	<address street="10 downing street" city="london" country="uk"/>
	<phoneNumber>020 7925 0918</phoneNumber>
  </person>
</people>

To make the contents of the XML document available within the extension, we create an XMLHttpRequest object to load the document synchronously, the variable xmlDoc will contain the document as an XMLDocument object against which we can use the evaluate method

JavaScript used in the extensions xul/js documents.

var req = new XMLHttpRequest();

req.open("GET", "chrome://yourextension/content/peopleDB.xml", false); 
req.send(null);

var xmlDoc = req.responseXML;		

var nsResolver = xmlDoc.createNSResolver( xmlDoc.ownerDocument == null ? xmlDoc.documentElement : xmlDoc.ownerDocument.documentElement);

var personIterator = xmlDoc.evaluate('//person', xmlDoc, nsResolver, XPathResult.ANY_TYPE, null );

Anmerkung

When the XPathResult object is not defined, the constants can be retreived in privileged code using Components.interfaces.nsIDOMXPathResult.ANY_TYPE (CI.nsIDOMXPathResult). Similarly, an XPathEvaluator can be created using:

Components.classes["@mozilla.org/dom/xpath-evaluator;1"].createInstance(Components.interfaces.nsIDOMXPathEvaluator)

Anhang

Einen benutzerdefinierten Namensauflöser implementieren

This is an example for illustration only. This function will need to take namespace prefixes from the xpathExpression and return the URI that corresponds to that prefix. For example, the expression:

'//xhtml:td/mathml:math'

will select all MathML expressions that are the children of (X)HTML table data cell elements.

In order to associate the 'mathml:' prefix with the namespace URI 'http://www.w3.org/1998/Math/MathML' and 'xhtml:' with the URI 'http://www.w3.org/1999/xhtml' we provide a function:

function nsResolver(prefix) {
  var ns = {
    'xhtml' : 'http://www.w3.org/1999/xhtml',
    'mathml': 'http://www.w3.org/1998/Math/MathML'
  };
  return ns[prefix] || null;
}

Our call to document.evaluate would then looks like:

document.evaluate( '//xhtml:td/mathml:math', document, nsResolver, XPathResult.ANY_TYPE, null );

Einen Standard-Namensrauf für XML-Dokumente implementieren

As noted in the Implementing a Default Namespace Resolver previously, the default resolver does not handle the default namespace for XML documents. For example with this document:

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
    <entry />
    <entry />
    <entry />
</feed>

doc.evaluate('//entry', doc, nsResolver, XPathResult.ANY_TYPE, null) will return an empty set, where nsResolver is the resolver returned by createNSResolver. Passing a null resolver doesn't work any better, either.

One possible workaround is to create a custom resolver that returns the correct default namespace (the Atom namespace in this case). Note that you still have to use some namespace prefix in your XPath expression, so that the resolver function will be able to change it to your required namespace. E.g.:

function resolver() {
    return 'http://www.w3.org/2005/Atom';
}
doc.evaluate('//myns:entry', doc, resolver, XPathResult.ANY_TYPE, null)

Note that a more complex resolver will be required if the document uses multiple namespaces.

An approach which might work better (and allow namespaces not to be known ahead of time) is described in the next section.

Using XPath functions to reference elements with a default namespace

Another approach to match default elements in a non-null namespace (and one which works well for dynamic XPath expressions where the namespaces might not be known), involves referring to a particular element using a form such as [namespace-uri()='http://www.w3.org/1999/xhtml' and name()='p' and @id='_myid']. This circumvents the problem of an XPath query not being able to detect the default namespace on a regularly labeled element.

Getting specifically namespaced elements and attributes regardless of prefix

If one wishes to provide flexibility in namespaces (as they are intended) by not necessarily requiring a particular prefix to be used when finding a namespaced element or attribute, one must use special techniques.

While one can adapt the approach in the above section to test for namespaced elements regardless of the prefix chosen (using local-name() in combination with namespace-uri() instead of name()), a more challenging situation occurs, however, if one wishes to grab an element with a particular namespaced attribute in a predicate (given the absence of implementation-independent variables in XPath 1.0).

For example, one might try (incorrectly) to grab an element with a namespaced attribute as follows: var xpathlink = someElements[local-name(@*)="href" and namespace-uri(@*)='http://www.w3.org/1999/xlink'];

This could inadvertently grab some elements if one of its attributes existed that had a local name of "href", but it was a different attribute which had the targeted (XLink) namespace (instead of @href).

In order to accurately grab elements with the XLink @href attribute (without also being confined to predefined prefixes in a namespace resolver), one could obtain them as follows:

var xpathEls = 'someElements[@*[local-name() = "href" and namespace-uri() = "http://www.w3.org/1999/xlink"]]'; // Grabs elements with any single attribute that has both the local name 'href' and the XLink namespace
var thislevel = xml.evaluate(xpathEls, xml, null, XPathResult.ANY_TYPE, null);
var thisitemEl = thislevel.iterateNext();

In XPathResult definierte Konstanten

Result Type Defined Constant Value Description
ANY_TYPE 0 A result set containing whatever type naturally results from evaluation of the expression. Note that if the result is a node-set then UNORDERED_NODE_ITERATOR_TYPE is always the resulting type.
NUMBER_TYPE 1 A result containing a single number. This is useful for example, in an XPath expression using the count() function.
STRING_TYPE 2 A result containing a single string.
BOOLEAN_TYPE 3 A result containing a single boolean value. This is useful for example, in an XPath expression using the not() function.
UNORDERED_NODE_ITERATOR_TYPE 4 A result node-set containing all the nodes matching the expression. The nodes may not necessarily be in the same order that they appear in the document.
ORDERED_NODE_ITERATOR_TYPE 5 A result node-set containing all the nodes matching the expression. The nodes in the result set are in the same order that they appear in the document.
UNORDERED_NODE_SNAPSHOT_TYPE 6 A result node-set containing snapshots of all the nodes matching the expression. The nodes may not necessarily be in the same order that they appear in the document.
ORDERED_NODE_SNAPSHOT_TYPE 7 A result node-set containing snapshots of all the nodes matching the expression. The nodes in the result set are in the same order that they appear in the document.
ANY_UNORDERED_NODE_TYPE 8 A result node-set containing any single node that matches the expression. The node is not necessarily the first node in the document that matches the expression.
FIRST_ORDERED_NODE_TYPE 9 A result node-set containing the first node in the document that matches the expression.

Siehe auch

Original Document Information

  • Based Upon Original Document Mozilla XPath Tutorial
  • Original Source Author: James Graham.
  • Other Contributors: James Thompson.
  • Last Updated Date: 2006-3-25.

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