document.evaluate的详细用法

使用 Greasemonkey 时会遇到的功能最为强大的一个工具就是 evaluate 函数。通过使用XPath这种查询语言，它可以用来寻找页面中的元素，属性和文本。

举个例子来说，如果您想获得某个页面上的全部链接。您也许会想到使用document.getElementsByTagName ('a')；但是如果您还要继续检查是否每个链接都具有href属性，因为<a>还可以用来作为锚名称使用，这时，您需要使用Firefox 内建的XPath支持去获取全部具有href属性的<a>元素。



例子: 获取页面上的全部链接

var allLinks, thisLink;
allLinks = document.evaluate(
'//a[@href]',
document,
null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
null);
for (var i = 0; i < allLinks.snapshotLength; i++) {
thisLink = allLinks.snapshotItem(i);
// do something with thisLink
}

这里，document.evaluate 是关键的部分。 它把 XPath 查询语句作为一个字符串，其它的参数稍后再做解释。 这条 XPath 查询语句可以找到全部具有href属性的<a>元素，并将它们按照随机的顺序依次返回。(这就是说，第一个被返回的元素并一定也是页面上的第一个这样的元素。) 随后，您就可以用 allLinks.snapshotItem(i) 函数访问每一个元素。

XPath表达式所能做到的甚至会使您惊讶。请看下面这个例子，它获取了全部具有title属性的元素。



例子: 获取全部具有title属性的元素

var allElements, thisElement;
allElements = document.evaluate(
'//*[@title]',
document,
null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
null);
for (var i = 0; i < allElements.snapshotLength; i++) {
thisElement = allElements.snapshotItem(i);
switch (thisElement.nodeName.toUpperCase()) {
case 'A':
// this is a link, do something
break;
case 'IMG':
// this is an image, do something else
break;
default:
// do something with other kinds of HTML elements
}
}

如果您已经引用了某个元素（例如上面的 thisElement），您就可以用 thisElement.nodeName 来替代它所对应的在 HTML 页面中的标签名称。如果被访问的这个页面是以 text/html 的方式被服务器执行, 那么标签名称总是用大写子母返回，不论它在原始页面是如何定义的。 如果页面是 application/xhtml+xml 方式的, 那么标签名称就会以小写子母返回。 不论哪种情况，我总是用 thisElement.nodeName.toUpperCase() 得到大写的标签名称。

这是另外一个 XPath 查询，它返回了 div 中的一个特殊的类。



例子: 获取 div 中的 sponsoredlink 类

var allDivs, thisDiv;
allDivs = document.evaluate(
"//div[@class='sponsoredlink']",
document,
null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
null);
for (var i = 0; i < allDivs.snapshotLength; i++) {
thisDiv = allDivs.snapshotItem(i);
// do something with thisDiv
}

注意我在 XPath 查询语句外使用了双引号，这样在语句内部就可以使用单引号。

在 document.evaluate 函数中有很多参数。第二个参数 (在前两个例子中都是docoment) 可以是一个元素， XPath 查询只返回包含在这个元素内的元素。所以，如果您已经引用了一个元素(比如, 通过 document.getElementById 或者 通过 document.getElementsByTagName 得到的数组中的一个元素), 那么您就可以限制查询只返回这个元素的子元素。

第三个参数是对一个叫做 namespace resolver 函数的引用, 它只有在工作在 application/xhtml+xml 类型的页面上的用户脚本中是有效的。即使您对它不是很了解也没有关系，因为那种类型的页面不是很多，您可能一次也遇不到。如果您很想知道它是如何使用的，请参考 Mozilla XPath documentation (http://www-jcsu.jesus.cam.ac.uk/~jg307/mozilla/xpath-tutorial.html)，那里解释了它的用法。

第四个参数是结果的返回方式。在前面的两个例子中都使用了 XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, 它将结果以随机的方式返回。我使用的几乎全部都是这种方式，但是，出于某种原因，您想让结果以它们在页面上出现的顺序返回，您可以使用 XPathResult.ORDERED_NODE_SNAPSHOT_TYPE 这种方式。Mozilla XPath documentation (http://www-jcsu.jesus.cam.ac.uk/~jg307/mozilla/xpath-tutorial.html)还给出了另外的一些用例。

第五个参数用来合并两次 XPath 查询的结果。 在获得第一次调用 document.evaluate 得到的结果之后，它将两次查询的结果一起返回。在前面的两个例子中，这个参数都用了null，这意味着我们只想获得本次查询的结果。

现在您明白了吗？XPath 既可以很简单，也可以很难，这取决于您要如何使用它。在此我强烈推荐您尽快去阅读 this excellent XPath tutorial (http://www.zvon.org/xxl/XPathTutorial/General/examples.html)，从而了解更多的 XPath 语法。至于 document.evaluate 函数的其它参数, 我几乎从来不使用它们。事实上，您可以自己定义一个函数来封装它们。



例子: 自定义的 xpath 函数

function xpath(query) {
return document.evaluate(query, document, null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
}

在定义了这个函数之后，您就可以调用 xpath('//a[@href]') 来获得某个页面上的全部链接, 或者调用 xpath('//*[@title]') 来获得具有 title 属性的元素。您仍然需要通过 snapshotItem 函数来访问结果中的每一项，这个函数的类型并不是一个规则的Javascript数组。

 

转自: Dive Into Greasemonkey → 公用模式 → 操作所有具有特定属性的元素

http://wiki.mozcn.org/index.php/Firefox:Dive_Into_Greasemonkey

 

附:

 

Using the Mozilla Javascript Interface to XPath

Introduction

This document describes the interface to access XPath functions from javascript

Mozilla implements much of the DOM 3 XPath. This allows XPath expressions to be run against both HTML and XML documents.

The simplest interface to the XPath is the

evaluate

function of the document object,
b5e2
which returns an object of type

XPathResult

:

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

The evaluate function takes a total of five arguments:

xpathExpression

: A string containing an xpath expression to be evaluated

contextNode

: A node in the

document

against which the Xpath expression should be evaluated

namespaceResolver

: A function that takes a string containing a namespace prefix from the

xpathExpression

and returns a string containing the URI to which that prefix corresponds. This enables conversion between the prefixes used in the XPath expressions and the (possibly different) prefixes used in the document

resultType

: A numeric constant indicating the type of result that is returned. These constants are avaliable in the global

XPathResult

object and are defined in the relevaant section of the XPath Spec. For most purposes it's OK to pass in XPathResult.ANY_TYPE which will cause the results of the Xpath expression to be returned as the most natural type

result

:An existing

XPathResult

to use for the results. Passing

null

causes a new

XPathResult

to be created.

A simple example

A simple use of XPath is to extract the level 2 headings of a HTML document. The XPath Expression in this case is simply

//h2

. The code for this is then:

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

Notice that, since HTML does not have namespaces, we have passed in

null

as the

namespaceResolver

. Since we wish to search over the entire document for 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 evaluated the

resultType

property of the returned object. In this case that will evaluate to 4, which, as per the ECMAScript language binding for XPath represents a

UNORDERED_NODE_ITERATOR_TYPE

. This is the default return type when the result of the XPath expression is a node set. It allows us access to a single node at a time and does not make any promises about the order in which the nodes will be returned. To access the returned nodes, we may 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();
}
alert(alertText);

You can ty running this example on the current document:

Level 2 Headings

Once we iterate to a node, we have access to all the standard Mozilla-supported DOM interfaces on that node. After iterating through all the

h1

elements returned from our expression, any further calls to

iterateNext()

will return

null

.

Returning results other than node sets

In many situations, the return value of an XPath expression is not a node set but a simpler type: a number, a string or a boolean value. For expressions of this type, we still obtain an

XPathResult

object as a result of our call to

document.evaluate

but we must access the

numberValue

,

stringValue

or

booleanValue

properties of the

XPathResult

to retrive our results.

A simple example is using the XPath expression

count(//p)

to obtain the number of paragraphs in a HTML document:

var paragraphCount = document.evaluate("count(//p)", document, null, XPathResult.ANY_TYPE, null).numberValue;
alert("This document contains " + paragraphCount + " paragraphs");
}

Count Paragraphs

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).stringValue;
alert("This document contains " + paragraphCount + " paragraphs");
}

Instead it will return

NS_DOM_TYPE_ERROR

. It is possible to request a specific type of return value by altering the

resultType

property. In order to force a string return type, we can pass the constant

XPathResult.STRING_TYPE

, so the following code will work:

var paragraphCount = document.evaluate("count(//p)", document, null, XPathResult.STRING_TYPE, null).stringValue;
alert("This document contains " + paragraphCount + " paragraphs");
}

The constants corresponding to the other simple types follow the same naming pattern:

XPathResult.STRING_TYPE

: Return the result as a string where possible

XPathResult.NUMBER_TYPE

: Return the result as a floating point number where possible

XPathResult.BOOLEAN_TYPE

: Return the result as a boolean where possible

Other types of nodesets

In the earlier example of reading all the level 2 headings in a document, the nodeset returned was of type

UNORDERED_NODE_ITERATOR_TYPE

. The XPath interface allows nodesets to be returned in a variety of different ways. There are three principal caategories of nodesets that can be returned:

Iterators: An iterator which allows access to one node at at time. The next node can be accessed with the

iterateNext()

method of the

XpathResult()

object. If the document is modified between the XPath evaluation, the

invalidIteratorState

property becomes

true

Snapshots: A static list of nodes that match the XPathExpression, accessed through the

snapshotItem(itemNumber)

method of the

XPathResult

, where

itemNumber

is the index of the node to be retrived. The number of nodes returned can be accessed through the

snapshotLength

property. Snapshots do not change with document mutations and so may contain nodes that no longer exist or an incomplete set of results.

First nodes: Only the first found node matching the XPath expression is returned. This may be accessed through the

singleNodeValue

property of the

XPathResult

object.

For each of these nodeset types, there is a subtype that is guaranteed to retain document order and one that is not. The full list of typecodes that can be passed as a

resultType

argument is:

XPathResult.UNORDERED_NODE_ITERATOR_TYPE

: Iterator, Unordered

XPathResult.ORDERED_NODE_ITERATOR_TYPE

: Iterator, Ordered

XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE

: Snapshot, Unordered

XPathResult.ORDERED_NODE_SNAPSHOT_TYPE

: Snapshot, Ordered

XPathResult.ANY_UNORDERED_NODE_TYPE

: Single node, Unordered

XPathResult.FIRST_ORDERED_NODE_TYPE

: Single node, Ordered

For example, //XXX - want an example with add and remove nodes and probably also with contextNode != document

Using XPath with XML

All the examples so far have been designed to work with HTML documents in which there are no namespaces. In order to use XPath on XML documents, one must provide a function for converting namespace prefixes in the xpath expression to those in the document. Such a function can be provided explicitly or can be created from a node in the target document.

Implementing a Namespace Resolver

Namespace resolvers are simply functions that take namespace prefixes from the XPath expression and return the corresponding URI. For example, the expression:

//html:td/mathml:math

Might be designed to select all MathML expressions that are the children of HTML table elements. To assosiate the

mathml:

prefix with the namespace URL

http://www.w3.org/1998/Math/MathML

and

html:

with the URL

http://www.w3.org/1999/xhtml

we can provide a function:

function NSResolver(prefix) {
if(prefix == 'html') {
return 'http://www.w3.org/1999/xhtml';
}
else if(prefix == 'mathml') {
return 'http://www.w3.org/1998/Math/MathML'
}
else  {
//this shouldn't ever happen
return null;
}
}

Our call to

document.evaluate

then looks like:

document.evaluate("//html:td/mathml:math", document, NSResolver, XPathResult.ANY_TYPE, null);