Design Patterns for Object-Oriented Hypermedia Applications

By: G. Rossi, A. Garrido, S. Carvalho
Published in: PLoPD2
Pages: 177-191
Category: Hypermedia

Summary: Design patterns for object-oriented applications with hypermedia functionality.

Pattern: NavigationStrategy

Pages: 180-185

In conventional hypermedia applications, links are hard-coded from the source node to the target. Define a family of algorithms that decouples the activation of hypermedia links from the computation of their endpoints, allowing different means of obtaining the endpoints and their lazy creation.

• This pattern is similar to Strategy [Gamma+95] in that it allows the designer to define a family of algorithms, making them interchangeable and allowing the Link class to be extended independently of those algorithms. It differs from Strategy [Gamma+95] by including a subhierarchy of factories where the strategy algorithm behaves as Factory Method [Gamma+95], which allows lazy creation of endpoints and navigation strategies.
• AbstractStrategyFactory is similar to Acceptor in that both establish connections lazily. AbstractStrategyFactory is similar to Abstract Factory [Gamma+95], while Acceptor is not.
• This pattern uses Template Method [Gamma+95] to define the abstract algorithm for performing navigation.

Category: Hypermedia

Pattern: NavigationObserver

Pages: 185-189

Hypermedia applications should record navigation in a user-perceptible way. Decouple navigation from the perceivable record of the process, and simplify the construction of navigation history viewers by separating the hypermedia components (nodes and links) from the objects that implement both the record of navigation and its appearance.

• The relationship between viewers and history resembles Observer [Gamma+95]
• History may be implemented with Singleton [Gamma+95]
• Mediator [Gamma+95] can decouple hypermedia components from a history when dealing with multiple browsing sessions.

Category: Hypermedia