Decorator design pattern
- 1. STRUCTURAL PATTERNS- DECORATOR DESIGN PATTERN Dr. S. Neelima M.Tech Ph.D CSE Priyadarshini Institute of Science and Technology for Women, KHAMMAM
- 2. Intent Decorator is a structural design pattern. Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to sub classing for extending functionality. Also Known as Wrapper Decorator Design Pattern- Dr. S. Neelima 2
- 3. Motivation Imagine that you’re working on a notification library which lets other programs notify their users about important events. The initial version of the library was based on the Notifier class that had only a few fields, a constructor and a single send method. Decorator Design Pattern- Dr. S. Neelima 3
- 4. Motivation….. Decorator Design Pattern- Dr. S. Neelima 4
- 5. Decorator Design Pattern- Dr. S. Neelima 5
- 6. The client code would need to wrap a basic notifier object into a set of decorators that match the client’s preferences. The resulting objects will be structured as a stack. Decorator Design Pattern- Dr. S. Neelima 6
- 7. Motivation 2. For example, suppose we have a TextView object that displays text in a window. TextView has no scroll bars by default, because we might not always need them. When we do, we can use a ScrollDecorator to add them. Suppose we also want to add a thick black border around the TextView. We can use a BorderDecorator to add this as well. We simply compose the decorators with the TextView to produce the desired result. Decorator Design Pattern- Dr. S. Neelima 7
- 8. Motivation 2. Solution : Decorator Design Pattern- Dr. S. Neelima 8
- 9. Applicability • Use the pattern to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects. • Use the pattern for responsibilities that can be withdrawn. • Use the pattern when it’s awkward or not possible to extend an object’s behavior using inheritance. Decorator Design Pattern- Dr. S. Neelima 9
- 10. Structure Decorator Design Pattern- Dr. S. Neelima 10
- 11. Participants • Component (VisualComponent) • Concrete Component (TextView) • Decorator • ConcreteDecorator Decorator Design Pattern- Dr. S. Neelima 11 Collaborations Decorator forwards requests to its Component object. It may optionally perform additional operations before and after forwarding the request.
- 12. Consequences Pros • You can extend an object’s behavior without making a new subclass. • You can add or remove responsibilities from an object at runtime. • You can combine several behaviors by wrapping an object into multiple decorators. • Single Responsibility Principle. You can divide a monolithic class that implements many possible variants of behavior into several smaller classes. Cons • It’s hard to remove a specific wrapper from the wrappers stack. • It’s hard to implement a decorator in such a way that its behavior doesn’t depend on the order in the decorators stack. • The initial configuration code of layers might look pretty ugly. Decorator Design Pattern- Dr. S. Neelima 12
- 13. Implement • Make sure your business domain can be represented as a primary component with multiple optional layers over it. • Figure out what methods are common to both the primary component and the optional layers. Create a component interface and declare those methods there. • Create a concrete component class and define the base behavior in it. • Create a base decorator class. It should have a field for storing a reference to a wrapped object. The field should be declared with the component interface type to allow linking to concrete components as well as decorators. The base decorator must delegate all work to the wrapped object. Decorator Design Pattern- Dr. S. Neelima 13
- 14. Implement …. • Make sure all classes implement the component interface. • Create concrete decorators by extending them from the base decorator. A concrete decorator must execute its behavior before or after the call to the parent method (which always delegates to the wrapped object). • The client code must be responsible for creating decorators and composing them in the way the client needs. Decorator Design Pattern- Dr. S. Neelima 14
- 15. Known Uses Decorator Design Pattern- Dr. S. Neelima 15 • InterViews ,ET++ and the ObjectWorksSmalltalk class library. • More exotic applications of Decorator are the DebuggingGlyph from InterViews and the PassivityWrapper from ParcPlace Smalltalk.
- 16. Related Patterns • Adapter changes the interface of an existing object, while Decorator enhances an object without changing its interface. • Composite and Decorator have similar structure diagrams since both rely on recursive composition to organize an open-ended number of objects. • Decorator lets you change the skin of an object, while Strategy lets you change the guts. Decorator Design Pattern- Dr. S. Neelima 16
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