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<head>
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<title>XParts - Extending KParts</title>
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</head>
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<body>
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<center>
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<h1>XParts - Extending KParts</h1>
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<small>Matthias Ettrich, Simon Hausmann, Lars Knoll</small>
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</center>
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<p>This article briefly describe the concepts, architecture and
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reasoning behind the XParts technology. The purpose of XParts is
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to extend KParts over language, toolkit, process and machine
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bounderies. XParts makes it possible to write KDE components with
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almost any toolkit or language an author prefers or to turn
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existing applications into KDE components quite easily.
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<p>In addition, XParts is also an important glueing technology to
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make KParts available in other component based systems or to
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utilize non-KPart components transparently as KParts.
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<h4>Classic KParts</h4>
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<p>In order to understand, what is extending about XParts, first a
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brief overview on how KParts work.
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<img src="kparts.png">
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<p>Imagine an application - for example the integrated file manager
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"Konqueror" - wants to utilize a component that handles the
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"text/html" mimetype. It therefore asks the trader of the KIO
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subsystem whether such a service is available and where. The
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trader uses the system configuration cache to localize an
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appropriate service that fits with the user's preferences. The
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system configuration cache is a service type database
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constructed from the desktop files of a KDE setup. In the case
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of "text/html", the trader will very like return KDE's builtin
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HTML viewer dubbed KHtml. This viewer is is most certainly
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available as a KPart component. The application will then - via
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KLibLoader and KLibFactory - load the shared library object that
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implements the component and create a KPart instance. The
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LibLoader keeps track of any objects created in the loaded
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library and will automatically unload it after all objects have
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been deleted.
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<p>If the application does not only want to display HTML, but
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act as a full featured browser, the plain KPart interface is not
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sufficient. If the user clicks on a link, for example, the HTML
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component has to request a new URL. This kind of interaction is
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defined in the BrowserExtension interface. An application can
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query the KParts for additonal interfaces and get handles to
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them in case those are available. In the example case of KHtml,
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the BrowserExtension interface is exported. In the case of a
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text editor component, it's very likely that the TextEditor
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interface is available.
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<h4>In-process components</h4>
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<p>The beauty of KParts is its simplicity. It's a clean and
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flexible in-process approach with all its advantages:
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<ul>
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<li> lightweight - components share the same application
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context and all its allocated resources.
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<li> synchronious - calls are predictable, there are no
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timeouts to wait for and no events to process in an uncertain
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amount of time.
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<li> stable - neither race conditions nor rare exceptions
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can occur
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<li> extremely powerful - there are virtually no
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limitations to how a component API can look like (including
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passing pointers) or what a plugin can do with an application.
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</ul>
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<p>Those advantages are unvaluable for a lightweight and tightly
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integrated office suite like KOffice. However, there are no silver
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bullets and most certainly there are drawbacks when the system is
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used in settings with different requirements.
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<p>Take the fourth item, it's comprehensive power while
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maintaining simplicity. This was one of the main requirements of
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the KOffice team, and it alone almost determines an in-process
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approach with dynamically loadable shared objects. In a generic
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browser like Konqueror, the requirements for integrated components
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are not as high as with an office suite. In an office suite,
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different components operate on one single document, whereas in a
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browser, the components basically provide different views for
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given Urls. To illustrate this issue, imagine how far the web
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came with such primitive and inflexible component technology like
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Netscape plugins. They did most of what people wanted to do with
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browser plugins, though, and so became a huge success.
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<h4>Out-of-process components</h4>
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<p>To sum this up: for multi-view applications like a generic
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browser, there's no technical argument why out-of-process
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components could not be sufficient. So let's look closer at the
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specific advantages of such a solution.
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<ul>
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<li>With out-of-process components, it's much easier to provide
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applications as components that do not support being loaded
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dynamically as shared library objects. Typical examples are
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programs written in interpreted languages. With a pure in-process
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model, one would have to be able to load the interpreter as
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embedded language.
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<li>If a component handles the event loop differently from the
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embedding application, an complete event loop merger is
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required. This glueing code can be tricky and might not work well
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in all cases. It's much easier for out-of-process components to
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provide full toolkit independence.
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<li> components of the same type could share one process
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context. Not sure where this is actually useful, but it has most
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certainly some technical beauty attached to it.
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</ul>
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<p>Let's pick a concrete example. Imagine that you - for whatever
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reason - want to offer the Mozilla rendering engine (gecko) as
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KPart, so that users have an an alternative to KDE's builtin
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rendering engine KHtml.
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<p>The first step of such a project is to find out, whether
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Mozilla already is available as a reusable component that could
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form the basis of a KDE integration. And in fact, it is. A small
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library called GtkMozEmbed makes it possible to load the entire
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Mozilla as a single Gtk widget, i.e. the rendering engine gecko,
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the networking protocol implementations, the javascript
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interpreter and whatever else Mozilla.org comes up with. The
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MozEmbed library works pretty similar to KParts. Once
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instantiated, it dynamically loads all libraries required by
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Mozilla. As an interesting side note, all Unix filemanager
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projects that utilize Mozilla (for example the Nautilus
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filemanager) use this library to embed mozilla. This means you are
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in good company using a stock MozEmbed library, as you don't have
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to maintain this code but somebody else will do it for you.
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<p>Now that we have a dynamically loaded Gtk widget, how do we
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turn that into a KPart? Quite straight forward. There is a
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TQGtkWidget extension available for Qt, that lets you use Gtk
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widgets in your Qt applications. You simply create a TQGtkWidget
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with a pointer to the Gtk widget you get from MozEmbed and insert
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that into your KPart. Then you do a few trivial reimplementations
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of the virtual functions of the BrowserExtension interface that
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map to the corresponding functions of Mozilla and you are
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done. The result is a fully functional Konqueror that uses Mozilla
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as backend - or rather a fully functional Mozilla that uses
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Konqueror as graphical user interface, however you want to look at
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it.
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<h4>Trouble ahead</h4>
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<p> While the skedged solution works, there are some unmentioned
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and ugly details. First of all, Mozilla uses the event loop of
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glib, while Konqueror uses Qt. Unfortunatly, mixing both event
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loops is not possible with the current release of glib, unless one
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want to end up with an application that constantly requires some
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CPU to run, even when being idle. While this seems to be ok for
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today's Java virtual machines, it's not acceptable by KDE's
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quality standards. Until glib 2.0 is released, you need to patch
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glib in order to make the TQGtkWidget work properly. No big deal
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for most Linux users, still a hassle. And keep in mind that glib
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is a fairly open system. If the component was written in some
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other toolkit, it might be possible that glueing code is
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impossible to get right, without wasting at least a bit of CPU.
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<p>The second problem is Mozilla's size. It's by no means an
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ordinary component. In fact, it's a magnitude larger than the
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Konqueror framework. And since Mozilla and Konqueror do not share
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the same graphics toolkit, the toolkit's size has to be added to
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that. It seems odd to load and unload such a huge amount of code -
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and it can to lead to all kind of problems when trying to unload
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it again.
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<p>To make things worse, Mozilla wasn't even released as final
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version yet. While it is already quite usable, it's stability is
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still far from being production quality. This doesn't matter too
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much for a standalone browser, but can really hurt with a
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component. A standalone browser usually is supposed to display one
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web page. If it crashes, this page is gone, so the user simply
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tries again. With a generic browser like Konqueror, there is not
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just one component active at a time, but several. There might be
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some directory views, an embedded console, another toplevel window
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window, an imaged preview and much more. A crashing Mozilla would
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take all those component with it - and leave the user with only
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half of its prior desktop.
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<p>Imagine that some users define Mozilla to be the primary
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component to handle text/html in Konqueror. After some testing, all
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works well and they continue using it. A couple of days later, they
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might have forgotten the configuration change they did. Whenever
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they now hit a web page where Mozilla crashes, they will blame
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Konqueror. This we don't want. No code is perfect, but if a crash
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occurs in our code, at least it's our crash. That means, we can fix
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it and we can provide newer versions.
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<p>Thus, from a maintainance and support point of you, it is not
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acceptable for KDE to run code inprocess that is not actually
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maintained or controlled by the team, at least not in the default
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setup.
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<h4>Out-of-process components</h4>
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<p>For the given reasons, it makes a lot of sense to extend KParts
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over process bounderies. In addition, we also win a high degree of
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toolkit and language independency.
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<p>To make this work, we have to identify the streamable parts of
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the KParts interface and offer them via some kind of middleware.
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<p>We chose KDE's native desktop middleware, the desktop
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communication protocol (DCOP) to establish the communication. In
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addition to the fact that DCOP was explicitely designed for these
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kind of tasks, there are some more benefits:
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<ul>
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<li> DCOP runs already on the desktop, i.e. there are no additonal costs
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in terms of resource consumption.
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<li> Does not put any limitations onto the interfaces as long as
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data types are streamable
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<li> Server architecture makes it easy and robust to detect
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crashes on either side.
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</ul>
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There are several DCOP implementations available. The reference
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implementation is the one using C++ and Qt that is used in KDE
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applications. For Mozilla, we would choose a plain ANSI-C
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implementation that uses glib.
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<p>The following picture shows the interface structure:
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<p> <img src="xparts.png">
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<p>The main thing that differs from KParts is the
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<em>XPartHost</em> interface that is responsible for embedding a
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part. The missing link now is a standard KPart component that
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implements the <em>XPartHost</em> interface. Via this
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<b>KXPartHost</b> component, it is possible to use any XPart
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transparently as KPart without changing a single line of code:
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<p>
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<img src="kxparthost.png">
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<p>On the other side of the fence, we need an implementation of
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the <em>XPartManager</em> interface and can serve us with
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<em>XPart</em> interfaces. We provide this through the
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relatively highlevel and generic classes GtkXPartManger and
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GtkXPart, as shown in the next picture:
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<p>
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<img src="gtkxpart.png">
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<p> The GtkXPart is a standard Gtk widget that can have a MozEmbed
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widget as child widget. The only code that is necessary to write
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is the code used to connect the <em>BrowserExtension</em>
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interface to the corresponding functions of Mozilla.
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<h4>External KParts</h4>
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<p>The same technique can now be used to utilize standard KPart
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components in an out-of-process fashion via the XPart system. All
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we need is a KXPartManager that wraps standard KParts in
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KXParts. The KXParts then export the <em>XPart</em> interface. The
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complete structure is shown in the next picture:
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<p><img src="kxpart.png">
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<h4>Conclusion</h4> <p> Although the implementation of the
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external mozilla part is more a proof of concept than a finished
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xpart, we have shown a clean way to realize out of process
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components on top of KParts. It could also be shown that this
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approach is both language and toolkit independent.
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<p>To accomplish this task, not a <em>single</em> line of code
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in konqueror had to be changed. All we did was providing yet
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another independent KPart component.
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<p>By writing a small wrapper it is possible to embed any kind of
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visual component. In addition, we can provide generic wrappers for
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any kind of visual component model, as long as those models are
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powerful enough to describe their interfaces and GUI requirements
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at runtime. This includes KParts (eg. KOffice components), Bonobo
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components (like the Nautilus MP3 viewer) and Uno components
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provided by OpenOffice (formerly known as StarOffice).
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<hr>
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<address><a href="mailto:ettrich@kde.org">Matthias Ettrich</a></address>
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<address><a href="mailto:hausmann@kde.org">Simon Hausmann</a></address>
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<address><a href="mailto:knoll@kde.org">Lars Knoll</a></address>
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<!-- Created: Tue Oct 17 18:08:25 CEST 2000 -->
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<!-- hhmts start -->
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Last modified: Tue Apr 3 20:39:13 CEST 2001
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<!-- hhmts end -->
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</body>
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</html>
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