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XMLモジュール

• QtのXMLアーキテクチャ概要
• Qt SAX2 クラス
• SAX2序論
• 特徴
• 特徴を通じての名前空間サポート
• サマリ
• プロパティ
• Qt DOM クラス
• DOM序論
• Namespaces序論
• Qt XML ドキュメントで使用されている規約

QtのXMLアーキテクチャ概要

XMLモジュールは、 SAX2 (XMLのための簡単なAPI)インタフェースとDOM Level2(ドキュメントObject Model)の実装を使用することでよくまとめられたXMLパーサを提供します。

SAXはXMLパーサのためのイベントベースの標準インターフェースです。 Qtインタフェースは SAX2 Java実装のデザインに従います。 命名体系は、Qt命名規則に合うように適合されています。 SAX2 の詳細は http://www.saxproject.orgで見つけることができます。

SAX2 フィルタとリーダファクトリのサポートは開発中です。QtインプリメンテーションはJavaインタフェースの現在のSAX1互換性のクラスを含んでいません。Qtの SAX2 のクラスへの序論に関して、 Qt SAX2のクラス を見てください。

DOM Level2はXMLインタフェースへのXMLドキュメントの構成要素を木構造に写像するW3C Recommendationです。 DOM Level2の仕様については http://www.w3.org/DOM/ で参照可能です。QtのDOMのクラスに関する詳しい情報は Qt DOMクラス で提供されています。

SAXとDOMサポートに加えて、Qtの Q3Picture のクラスはSVGを読み書きすることができます。 追加のXMLサポートは、例えばQt XMLクラスでのSOAPやMMLをサポートするクラスなどを提供している Qtソリューション グループによって提供されます。

QtXml モジュール は以下のSAXとDOMのクラスを提供します:

Qt SAX2クラス

SAX2序論

SAX2 インタフェースは文書情報をユーザに提供するイベント駆動のメカニズムです。 このコンテキストでの"イベント"とは、パーサからのレポートを指します。例えば、開始TAGや終了TAGが現れます。

それをより抽象的でなくするには、以下の例を考えてください:

    <quote>A quotation.</quote>

上のドキュメントを読んでいる間(通常、 SAX2 パーサは"reader"として参照されます)、3回のイベントが発生するでしょう:

1. スタートTAGが発生します (<quote>).
2. 文字データ (言い換えれば テキスト) "A quotation."が発見されます.
3. 終了TAGが構文解析されます (</quote>).

そのようなイベントが現れるたびにパーサはそれを報告します; これらのイベントに応じるためにイベントハンドラをセットアップすることができます。

これはドキュメントをXMLに読み込む速くて簡単なアプローチですが、データが順次格納されて、単純に扱われて、連続的に廃棄されるので、操作は難しいです。 DOMインタフェース は、木構造で全体のドキュメントを読んで、保存します; これで、より多くのメモリを取りますが、ドキュメントの構造を操作するのは、より簡単になります。

Qt XMLモジュールは抽象的なクラス、 QXmlReaderを提供します。これは潜在的 SAX2 リーダのためにインタフェースを定義しています。Qtはリーダインプリメンテーションである、サブクラス化を通して適合させやすい QXmlSimpleReaderを含んでいます。

リーダは特別なハンドラクラスを通して構文解析されたイベントを報告します：

これらのクラスはインタフェースについて記述された抽象的なクラスです。 QXmlDefaultHandler クラスはデフォルトのインプリメンテーションでは"何もしない"クラスです。そのため、ユーザは QXmlDefaultHandler 機能の関心のある部分のみオーバロードすればよいのです。

入力XMLデータを読み込むため、特別なクラスの QXmlInputSource は使用されます。

既に言及されたものは別として、以下の SAX2 サポートのクラスは追加の役に立つ機能性を提供します:

特徴

The behaviour of an XML reader depends on its support for certain optional features. For example, a reader may have the feature "report attributes used for namespace declarations and prefixes along with the local name of a tag". Like every other feature this has a unique name represented by a URI: it is called http://xml.org/sax/features/namespace-prefixes.

The Qt SAX2 implementation can report whether the reader has particular functionality using the QXmlReader::hasFeature() function. Available features can be tested with QXmlReader::feature(), and switched on or off using QXmlReader::setFeature().

Consider the example

    <document xmlns:book = 'http://trolltech.com/fnord/book/'
              xmlns      = 'http://trolltech.com/fnord/' >

A reader that does not support the http://xml.org/sax/features/namespace-prefixes feature would report the element name document but not its attributes xmlns:book and xmlns with their values. A reader with the feature http://xml.org/sax/features/namespace-prefixes reports the namespace attributes if the feature is switched on.

Other features include http://xml.org/sax/features/namespace (namespace processing, implies http://xml.org/sax/features/namespace-prefixes) and http://xml.org/sax/features/validation (the ability to report validation errors).

Whilst SAX2 leaves it to the user to define and implement whatever features are required, support for http://xml.org/sax/features/namespace (and thus http://xml.org/sax/features/namespace-prefixes) is mandantory. The QXmlSimpleReader implementation of QXmlReader, supports them, and can do namespace processing.

QXmlSimpleReader is not validating, so it does not support http://xml.org/sax/features/validation.

特徴を通じての名前空間サポート

As we have seen in the previous section we can configure the behavior of the reader when it comes to namespace processing. This is done by setting and unsetting the http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes features.

They influence the reporting behavior in the following way:

1. Namespace prefixes and local parts of elements and attributes can be reported.
2. The qualified names of elements and attributes are reported.
3. QXmlContentHandler::startPrefixMapping() and QXmlContentHandler::endPrefixMapping() are called by the reader.
4. Attributes that declare namespaces (i.e. the attribute xmlns and attributes starting with xmlns:) are reported.

Consider the following element:

    <author xmlns:fnord = 'http://trolltech.com/fnord/'
                 title="Ms"
                 fnord:title="Goddess"
                 name="Eris Kallisti"/>

With http://xml.org/sax/features/namespace-prefixes set to true the reader will report four attributes; but with the namespace-prefixes feature set to false only three, with the xmlns:fnord attribute defining a namespace being "invisible" to the reader.

The http://xml.org/sax/features/namespaces feature is responsible for reporting local names, namespace prefixes and URIs. With http://xml.org/sax/features/namespaces set to true the parser will report title as the local name of the fnord:title attribute, fnord being the namespace prefix and http://trolltech.com/fnord/ as the namespace URI. When http://xml.org/sax/features/namespaces is false none of them are reported.

In the current implementation the Qt XML classes follow the definition that the prefix xmlns itself isn't associated with any namespace at all (see http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using). Therefore even with http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes both set to true the reader won't return either a local name, a namespace prefix or a namespace URI for xmlns:fnord.

This might be changed in the future following the W3C suggestion http://www.w3.org/2000/xmlns/ to associate xmlns with the namespace http://www.w3.org/2000/xmlns.

As the SAX2 standard suggests, QXmlSimpleReader defaults to having http://xml.org/sax/features/namespaces set to true and http://xml.org/sax/features/namespace-prefixes set to false. When changing this behavior using QXmlSimpleReader::setFeature() note that the combination of both features set to false is illegal.

サマリ

QXmlSimpleReader implements the following behavior:

The behavior of the entries marked with an asterisk (*) is not specified by SAX.

プロパティ

Properties are a more general concept. They have a unique name, represented as an URI, but their value is void*. Thus nearly anything can be used as a property value. This concept involves some danger, though: there is no means of ensuring type-safety; the user must take care that they pass the right type. Properties are useful if a reader supports special handler classes.

The URIs used for features and properties often look like URLs, e.g. http://xml.org/sax/features/namespace. This does not mean that the data required is at this address. It is simply a way of defining unique names.

Anyone can define and use new SAX2 properties for their readers. Property support is not mandatory.

To set or query properties the following functions are provided: QXmlReader::setProperty(), QXmlReader::property() and QXmlReader::hasProperty().

Qt DOM クラス

DOM序論

DOM provides an interface to access and change the content and structure of an XML file. It makes a hierarchical view of the document (a tree view). Thus -- in contrast to the SAX2 interface -- an object model of the document is resident in memory after parsing which makes manipulation easy.

All DOM nodes in the document tree are subclasses of QDomNode. The document itself is represented as a QDomDocument object.

Here are the available node classes and their potential child classes:

• QDomDocument: Possible children are
  • QDomElement (at most one)
  • QDomProcessingInstruction
  • QDomComment
  • QDomDocumentType
• QDomDocumentFragment: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomDocumentType: No children
• QDomEntityReference: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomElement: Possible children are
  • QDomElement
  • QDomText
  • QDomComment
  • QDomProcessingInstruction
  • QDomCDATASection
  • QDomEntityReference
• QDomAttr: Possible children are
  • QDomText
  • QDomEntityReference
• QDomProcessingInstruction: No children
• QDomComment: No children
• QDomText: No children
• QDomCDATASection: No children
• QDomEntity: Possible children are
  • QDomElement
  • QDomProcessingInstruction
  • QDomComment
  • QDomText
  • QDomCDATASection
  • QDomEntityReference
• QDomNotation: No children

With QDomNodeList and QDomNamedNodeMap two collection classes are provided: QDomNodeList is a list of nodes, and QDomNamedNodeMap is used to handle unordered sets of nodes (often used for attributes).

The QDomImplementation class allows the user to query features of the DOM implementation.

To get started please refer to the QDomDocument documentation.

Namespaces序論

Parts of the Qt XML module documentation assume that you are familiar with XML namespaces. Here we present a brief introduction; skip to Qt XML documentation conventions if you already know this material.

Namespaces are a concept introduced into XML to allow a more modular design. With their help data processing software can easily resolve naming conflicts in XML documents.

Consider the following example:

    <document>
    <book>
      <title>Practical XML</title>
      <author title="Ms" name="Eris Kallisti"/>
      <chapter>
        <title>A Namespace Called fnord</title>
      </chapter>
    </book>
    </document>

Here we find three different uses of the name title. If you wish to process this document you will encounter problems because each of the titles should be displayed in a different manner -- even though they have the same name.

The solution would be to have some means of identifying the first occurrence of title as the title of a book, i.e. to use the title element of a book namespace to distinguish it from, for example, the chapter title, e.g.:

    <book:title>Practical XML</book:title>

book in this case is a prefix denoting the namespace.

Before we can apply a namespace to element or attribute names we must declare it.

Namespaces are URIs like http://trolltech.com/fnord/book/. This does not mean that data must be available at this address; the URI is simply used to provide a unique name.

We declare namespaces in the same way as attributes; strictly speaking they are attributes. To make for example http://trolltech.com/fnord/ the document's default XML namespace xmlns we write

    xmlns="http://trolltech.com/fnord/"

To distinguish the http://trolltech.com/fnord/book/ namespace from the default, we must supply it with a prefix:

    xmlns:book="http://trolltech.com/fnord/book/"

A namespace that is declared like this can be applied to element and attribute names by prepending the appropriate prefix and a ":" delimiter. We have already seen this with the book:title element.

Element names without a prefix belong to the default namespace. This rule does not apply to attributes: an attribute without a prefix does not belong to any of the declared XML namespaces at all. Attributes always belong to the "traditional" namespace of the element in which they appear. A "traditional" namespace is not an XML namespace, it simply means that all attribute names belonging to one element must be different. Later we will see how to assign an XML namespace to an attribute.

Due to the fact that attributes without prefixes are not in any XML namespace there is no collision between the attribute title (that belongs to the author element) and for example the title element within a chapter.

Let's clarify this with an example:

    <document xmlns:book = 'http://trolltech.com/fnord/book/'
              xmlns      = 'http://trolltech.com/fnord/' >
    <book>
      <book:title>Practical XML</book:title>
      <book:author xmlns:fnord = 'http://trolltech.com/fnord/'
                   title="Ms"
                   fnord:title="Goddess"
                   name="Eris Kallisti"/>
      <chapter>
        <title>A Namespace Called fnord</title>
      </chapter>
    </book>
    </document>

Within the document element we have two namespaces declared. The default namespace http://trolltech.com/fnord/ applies to the book element, the chapter element, the appropriate title element and of course to document itself.

The book:author and book:title elements belong to the namespace with the URI http://trolltech.com/fnord/book/.

The two book:author attributes title and name have no XML namespace assigned. They are only members of the "traditional" namespace of the element book:author, meaning that for example two title attributes in book:author are forbidden.

In the above example we circumvent the last rule by adding a title attribute from the http://trolltech.com/fnord/ namespace to book:author: the fnord:title comes from the namespace with the prefix fnord that is declared in the book:author element.

Clearly the fnord namespace has the same namespace URI as the default namespace. So why didn't we simply use the default namespace we'd already declared? The answer is quite complex:

• attributes without a prefix don't belong to any XML namespace at all, not even to the default namespace;
• additionally omitting the prefix would lead to a title-title clash;
• writing it as xmlns:title would declare a new namespace with the prefix title instead of applying the default xmlns namespace.

With the Qt XML classes elements and attributes can be accessed in two ways: either by refering to their qualified names consisting of the namespace prefix and the "real" name (or local name) or by the combination of local name and namespace URI.

More information on XML namespaces can be found at http://www.w3.org/TR/REC-xml-names/.

Qt XML ドキュメントで使用されている規約

The following terms are used to distinguish the parts of names within the context of namespaces:

• The qualified name is the name as it appears in the document. (In the above example book:title is a qualified name.)
• A namespace prefix in a qualified name is the part to the left of the ":". (book is the namespace prefix in book:title.)
• The local part of a name (also refered to as the local name) appears to the right of the ":". (Thus title is the local part of book:title.)
• The namespace URI ("Uniform Resource Identifier") is a unique identifier for a namespace. It looks like a URL (e.g. http://trolltech.com/fnord/ ) but does not require data to be accessible by the given protocol at the named address.

Elements without a ":" (like chapter in the example) do not have a namespace prefix. In this case the local part and the qualified name are identical (i.e. chapter).