Παράδειγμα προτύπου σχεδίου

Το παρακάτω κείμενο (Σπινέλλης και Ράπτης 2000) περιγράφει με τη μορφή ενός προτύπου σχεδίου τη σύνδεση εξαρτημάτων:

Component Composition - Structural

Intent

The component composition pattern identifies the primary methods of encapsulated component composition and integration.

Motivation

Encapsulated components do not operate in a vacuum. They are composed to create more powerful components and integrated within an object-based system to provide specialised services. Moreover, composition of encapsulated components with component glue can be used to provide efficient access to off-line data, graphical user interfaces, and a multitude of other component-based services. As an example a spelling checker can be easily constructed by composing the translate, sort, unique, and common components, while the gluing of a editbox and listbox components can be used to provide a GUI front end.

Applicability

Many of the problems solved under the Unix programming environment using shell programming constructs and pipelines can be transformed to component composition structures. Of particular relevance are sequences of filter type components, where each one receives a data stream, performs some operations on it, and forwards it to another filter to perform some other operations. Examples include pipelines of tools that process text, images, sound, and object code. Meunier [13] describes a complete pattern language for a ``Pipes and Filters Architecture'' that can be used as a base to structure applications.

Structure

Figure 4: A spell checker with a GUI.

Figure 4 depicts the component interaction diagram of a filter-based spell checker built from Unix-mined and glue components. The text to be spell-checked is retrieved from the GUI edit box using a data source glue component. It is transformed into a list of words using the translate component which is a direct equivalent of the Unix tr command. The word list is then transformed into a sorted list of unique words using the sort and unique components which correspond to the Unix sort and uniq commands. At the same time, the system dictionary and a user dictionary are passed using appropriate file connectors to the merge component which merges two sorted streams; the merge component is a specialisation of sort which provides this functionality. Finally, the two sorted streams of words to be spelled and acceptable words are checked by common - derived from the Unix comm command - which outputs a list of words contained in the first stream and not contained in the second one. This stream of misspelled words is sent using the ListBoxSink glue component to a GUI list box. It is important to note that the integration of GUI elements using the same component object paradigm and the merging of two data streams could not be implemented using the standard Unix linear pipeline system.

Participants

The components composed are object instances of either active process components that are connected to existing data sources and sinks, or connector and glue components (pipes and environment interfacing classes) that provide such sources and sinks.

Consequences

Using the component composition pattern it is possible to implement sophisticated component interaction topologies. In addition, it is possible to package together existing components to provide new standard and reusable components.

Implementations

The implementation of the composition pattern is independent of the component-framework used. Most relevant decisions are taken when implementing the encapsulation and the glue patterns. Designs based on the composition pattern should be portable across different component frameworks.