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Open source XML editors examined

Despite the numerous benefits of XML, users and developers that work with it realize that XML's versatility is a mixed blessing. How does one design an effective editor for a data format that can be used to express virtually any kind of structured content? A visual editing idiom that is appropriate for developing a user interface with XUL content may not be appropriate for writing an essay with DocBook content. In many cases it becomes necessary to construct a specialized editing environment that provides a unique interface and produces valid XML code, such as Glade or OpenOffice.org.

If one were to build a generic XML editor that could support a multitude of vastly disparate content types, how would it work? The structure of a given XML content type is usually specified in a Document Type Definition (DTD) or a Schema. An application can use this data to introduce specializations and interface refinements. Many XML editors employ document meta-data to provide context-sensitive assistance and automation mechanisms.

KXml Editor is a lightweight XML editor and viewer that interfaces exceptionally well with other KDE applications. You can configure Konqueror to use the KXml Editor KPart as an XML view interface component. With its extensive assortment of excellent keyboard shortcuts, KXml Editor facilitates rapid development of data-oriented XML content. Input of element character data requires the user to employ a textbox dialog window that decreases the fluidity and efficiency of the editing experience. KXml Editor allows the user to utilize XPath nomenclature to select a single specific node, but it does not completely support the XPath query syntax, and queries that target multiple nodes are presently ignored. KXml Editor provides no support for content processing or validation, and it cannot open XML documents that contain syntax errors. KXml Editor is usable and efficient, but its limitations decrease its value as an editing tool. It is my XML viewer of choice, and it is useful for rapid production of simple XML documents, but it is less efficient than a conventional editor for DocBook authoring.

Bluefish is a powerful Web development environment that supports a number of markup and programming languages. Despite its extensive feature set, it doesn't provide much support for XML yet. Bluefish has a built-in project manager and an extensible build system that works well for Java and C projects. The intrepid user will easily be able to configure Bluefish to support external XML validation and processing utilities. Bluefish has an impressive function reference system that greatly expedites development of Python and PHP. Unfortunately, it does not provide support for XSL. Bluefish is a great tool for diverse development projects that encompass multiple technologies, but its not the best solution for typical XML authoring tasks.

Quanta is an extremely powerful and extensible Web development environment that has excellent support for XML editing, and provides users with several ways to extend its abilities. Quanta has a nice project manager and a great interface. Quanta's editor supports folding, syntax highlighting, context-sensitive tag completion, and automatic tag closure. Quanta has a built-in XML validation utility, and a really cool XSL debugger that supports breakpoints and stepping. It has a decent template system that automates creation of a wide variety of document types (including DocBook) and it can create new custom templates that you can save for global use or associate with a particular project.

You can use Kommander to write powerful graphical script utilities that extend the functionality of Quanta. Quanta's versatile action system lets you associate Kommander scripts and XML tags with keyboard shortcuts, menu items, and toolbar buttons. You can create more sophisticated extensions as KParts and integrate them with Quanta through its plug-in interface. Quanta comes with plug-ins for CVS management, multi-file search and replace, image map editing, link analysis, and XSL debugging. It is even possible to integrate KXml Editor into Quanta as an additional document view. Quanta has a tag attribute editor, and a tag tree interface that lets users efficiently navigate large documents. Quanta's elegant split WYSIWYG mode makes Web development a pleasure, and its highly configurable editor component and action system allow users to effectively utilize both the keyboard and the mouse.

Screem is an effective Web development environment that is simple and powerful. Its interface lacks the refinement of Quanta or Bluefish, but it is moderately innovative, and provides some unique advantages. Screem supports context-sensitive tag completion, brace matching, real-time validation, and a unique, intelligent tag closure system. I was particularly impressed with the real-time validation mechanism, which highlights any content that invalidates the document.

Screem is not as customizable as Quanta or Bluefish, but it has a text insertion macro feature, and a system that enables users to add external commands to the menu. It has an integrated symbol viewer and a function reference interface, neither of which impressed me. The symbol viewer doesn't seem to work with C or Python, and the reference system implementation is poor. In addition to the traditional text view, Screem has a tree view that displays document structure, and an attribute input interface that provides a complete list of valid attributes associated with the currently selected tag.

As a Vim user, I tend to dislike the limited potential for customization that plagues modern development environments. I shudder at the prospect of using an editor that doesn't allow me to dynamically add new features at run time. Vim customization is really more of an art than a science. Experienced Vim users like to do things their own way, with whatever scripting language they feel most comfortable with.

Tobias Reif has produced an excellent guide to XML editing with Vim. It is a must-read for any Vim user interested in XML development. In his guide, Reif shows how users can leverage Ruby scripts and third-party command-line utilities like Jing and XMLStarlet to construct a complete, versatile XML authoring environment.

Emacs users may be interested in nXML mode, which provides Emacs with an assortment of nifty XML authoring features, like context-sensitive tag completion, intelligent tag closure, and real-time validation. It also provides several useful keyboard shortcuts that simplify moving around XML documents. nXML mode employs Relax-NG (RNG) schemas rather than DTDs. RNG is a powerful and flexible schema language based on TREX and RELAX. The limited availability of existing RNG schemas diminishes the usefulness of nXML in some contexts, but for DocBook editing, nXML is a good choice. Emacs users may also be interested in XSLT-process mode, which adds XSLT processing and debugging to Emacs. The XSLT-process-mode debugger supports break points and stepping, and displays XSLT stack frames and variables in the speed bar.

For unique insight into the future of XML editing, take a look at Conglomerate, an innovative XML editor that features a creative interface particularly suitable for DocBook authoring. Conglomerate uses nested boxes to represent the document structure, and between-line annotations to indicate the presence of inline tags. This unconventional and intuitive presentation manages to remain unobtrusive, and conveys a lot of information about the content of the document. Conglomerate uses DTDs to provide element insertion context menus. Users can customize Conglomerate's view interface with specialized display specs and external plug-ins. Yet despite the quality of its interface, Conglomerate still needs a lot of work. The rendering engine is far too slow, and the application lacks stability.

Conclusions

All of these programs lack XPath support. Users should be able to utilize XPath for filtering and programmatic manipulation of document content. Many of the programs support a multitude of other technologies at the expense of better XML support. The tools designed exclusively for XML either have mediocre interfaces or lack performance. Despite these deficiencies, some of these applications are superior to expensive proprietary alternatives such as Softquad's XMetal and Wattle's XMLWriter.