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ISBN: 978-972-8939-25-0 © 2010 IADIS2. RELATED WORKThe analysis of related approaches shows that little research has been done to facilitate service-basedprovision and dynamic integration of UI components using description languages like UISDL. The Mixupapproach (Yu et al. 2007) introduces a development and runtime environment to compose web applications atthe presentation layer based on existing UI components. The abstract UI-only component model enablesintegration into existing composite applications and defines the presentation state, properties to represent theappearance, events to signal state changes and operations to react to state changes triggered by other UIcomponents. To describe UI components, the approach uses XPIL (Extensible Presentation IntegrationLanguage) as a declarative description language that solely contains a small set of elements describing theabstract component model similar to WSDL. Unlike UISDL, XPIL solely describes the interface of UIcomponents and does not distinguish between an abstract and concrete part to enable the distribution andflexible integration of different component implementations. Furthermore, component metadata is notdescribed, making the registration and discovery in a repository complicated. We will show that especiallythe consideration of these two aspects is vital for the realization of UI Services.Since the Mixup approach only provides a UI-only component model, the authors introduced a platformfor universal composition called mashArt (Daniel et al. 2009). The universal component model extends themodel presented in (Yu et al. 2007) and encapsulates UI, application and data components by defining theirstate, properties, events and operations. The specification of a component has the form of an abstractcomponent descriptor as a main part of the mashArt Description Language (MDL), which has a very similarstructure to XPIL, but aims at a universal component description. However, MDL does not provide metadatato enable its registration and discovery in a repository. Furthermore, it lacks a separation into abstract andconcrete parts, leading to the above-mentioned drawback.The Service-Oriented User Interface Modelling and Composition (SOAUI) approach (Tsai et al. 2008)proposes the discovery and dynamic integration of UI components into a system design process of serviceorientedapplications at runtime. The specification is based on proprietary Microsoft XAML (Microsoft2010) and describes the offered functionality of a UI component. The authors intend to establish a unified UIspecification standard to share components between heterogeneous systems and platforms. They mainlyfocus on the management and retrieval of UI components, while our work explicitly includes their servicebasedprovision and dynamic integration. Widgets (Cáceres 2009) or gadgets (Google 2008) are artifacts thatstatically combine a piece of application logic with a UI component to a small application. They couldbenefit from UI Services described with UISDL, since the choice of the concrete UI componentimplementation to be used for the widget can be left open until runtime, leaving it up to personal preferencesof the user or the technical context in which the widget/gadget is used.In summary, only very few approaches exist that use a declarative description language for UIcomponents. Unlike UISDL, these approaches do not sufficiently address the service-based provision anddynamic integration of UI components through UI Service regarding the separation of concrete and abstractparts, explicit support for metadata, and for dynamic integration at runtime.3. USER INTERFACE SERVICE DESCRIPTION LANGUAGEThis section presents the UISDL in detail. Starting with the introduction of our UI component model and adiscussion of requirements for the language, design decisions and language concepts are presented. We alsomention language concepts that are inherited from WSDL as a classical service description language.3.1 UI Component ModelThe basis for UI Services, and thereby for UISDL, is the specification of a generic UI component model. OurUI component model abstracts from implementation details by defining UI components as a set of abstractproperties, events, and operations.Properties represent the externally visible UI component state. Common properties of an electronic mapare, for example, the geographic coordinate of its center or its type (normal, satellite, hybrid). Events aretriggered by UI components to publish state changes to other components, usually in response to user176