Lightweight library for parsing, traversing, and transforming XML in Dart.
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Dart XML

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Dart XML is a lightweight library for parsing, traversing, querying, transforming and building XML documents.

This library is open source, stable and well tested. Development happens on GitHub. Feel free to report issues or create a pull-request there. General questions are best asked on StackOverflow.

The package is hosted on dart packages. Up-to-date class documentation is created with every release.

Tutorial

Installation

Follow the installation instructions on dart packages.

Import the package into your Dart code using:

import 'package:xml/xml.dart' as xml;

Reading and Writing

To read XML input use the top-level function parse(String input):

var bookshelfXml = '''<?xml version="1.0"?>
    <bookshelf>
      <book>
        <title lang="english">Growing a Language</title>
        <price>29.99</price>
      </book>
      <book>
        <title lang="english">Learning XML</title>
        <price>39.95</price>
      </book>
      <price>132.00</price>
    </bookshelf>''';
var document = xml.parse(bookshelfXml);

The resulting object is an instance of XmlDocument. In case the document cannot be parsed, a ParserError is thrown.

To write back the parsed XML document simply call toString(), if you need more control toXmlString(petty: true, indent: '\t'):

print(document.toString());
print(document.toXmlString(pretty: true, indent: '\t'));

Another way to efficiently read an XML document is to use the SAX API (Simple API for XML). Create a reader and provide the event handlers you are interested in. The following snippet prints all the element names in the above XML document: bookshelf, book, title, price, book, title, price and price.

var reader = XmlReader(onStartElement: (name, attributes) => print(name));
reader.parse(bookshelfXml);

Traversing and Querying

Accessors allow to access nodes in the XML tree:

  • attributes returns a list over the attributes of the current node.
  • children returns a list over the children of the current node.

Both lists are mutable and support all common List methods, such as add(XmlNode), addAll(Iterable<XmlNode>), insert(int, XmlNode), and insertAll(int, Iterable<XmlNode>). Trying to add a null value or an unsupported node type throws an XmlNodeTypeError error. Nodes that are already part of a tree are not automatically moved, you need to first create a copy as otherwise an XmlParentError is thrown. XmlDocumentFragment nodes are automatically expanded and copies of their children are added.

There are various methods to traverse the XML tree along its axes:

  • preceding returns an iterable over nodes preceding the opening tag of the current node in document order.
  • descendants returns an iterable over the descendants of the current node in document order. This includes the attributes of the current node, its children, the grandchildren, and so on.
  • following the nodes following the closing tag of the current node in document order.
  • ancestors returns an iterable over the ancestor nodes of the current node, that is the parent, the grandparent, and so on. Note that this is the only iterable that traverses nodes in reverse document order.

For example, the descendants iterator could be used to extract all textual contents from an XML tree:

var textual = document.descendants
    .where((node) => node is xml.XmlText && !node.text.trim().isEmpty)
    .join('\n');
print(textual);

Additionally, there are helpers to find elements with a specific tag:

  • findElements(String name) finds direct children of the current node with the provided tag name.
  • findAllElements(String name) finds direct and indirect children of the current node with the provided tag name.

For example, to find all the nodes with the <title> tag you could write:

var titles = document.findAllElements('title');

The above code returns a lazy iterator that recursively walks the XML document and yields all the element nodes with the requested tag name. To extract the textual contents call text:

titles
    .map((node) => node.text)
    .forEach(print);

This prints Growing a Language and Learning XML.

Similarly, to compute the total price of all the books one could write the following expression:

var total = document.findAllElements('book')
    .map((node) => double.parse(node.findElements('price').single.text))
    .reduce((a, b) => a + b);
print(total);

Note that this first finds all the books, and then extracts the price to avoid counting the price tag that is included in the bookshelf.

Building

To build a new XML document use an XmlBuilder. The builder implements a small set of methods to build complete XML trees. To create the above bookshelf example one would write:

var builder = new xml.XmlBuilder();
builder.processing('xml', 'version="1.0"');
builder.element('bookshelf', nest: () {
  builder.element('book', nest: () {
    builder.element('title', nest: () {
      builder.attribute('lang', 'english');
      builder.text('Growing a Language');
    });
    builder.element('price', nest: 29.99);
  });
  builder.element('book', nest: () {
    builder.element('title', nest: () {
      builder.attribute('lang', 'english');
      builder.text('Learning XML');
    });
    builder.element('price', nest: 39.95);
  });
  builder.element('price', nest: 132.00);
});
var bookshelfXml = builder.build();

Note the element method. It is quite sophisticated and supports many different optional named arguments:

  • The most common is the nest: argument which is used to insert contents into the element. In most cases this will be a function that calls more methods on the builder to define attributes, declare namespaces and add child elements. However, the argument can also be a string or an arbitrary Dart object that is converted to a string and added as a text node.
  • While attributes can be defined from within the element, for simplicity there is also an argument attributes: that takes a map to define simple name-value pairs.
  • Furthermore we can provide an URI as the namespace of the element using namespace: and declare new namespace prefixes using namespaces:. For details see the documentation of the method.

The builder pattern allows you to easily extract repeated parts into specific methods. In the example above, one could put the part that writes a book into a separate method as follows:

buildBook(xml.XmlBuilder builder, String title, String language, num price) {
  builder.element('book', nest: () {
    builder.element('title', nest: () {
      builder.attribute('lang', 'english');
      builder.text(title);
    });
    builder.element('price', nest: price);
  });
}

Misc

Examples

There are numerous packages depending on this package:

Supports

  • Standard well-formed XML (and HTML).
  • Reading documents using an event based API (SAX).
  • Decodes and encodes commonly used character entities.
  • Querying, traversing, and mutating API using Dart principles.
  • Building XML trees using a builder API.

Limitations

  • Doesn't validate namespace declarations.
  • Doesn't validate schema declarations.
  • Doesn't parse and enforce DTD.

History

This library started as an example of the PetitParser library. To my own surprise various people started to use it to read XML files. In April 2014 I was asked to replace the original dart-xml library from John Evans.

License

The MIT License, see LICENSE.