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Decorator pattern

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Decorator pattern

In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, dynamically, without affecting the behavior of other instances of the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern as well as to the Open-Closed Principle, by allowing the functionality of a class to be extended without being modified. Decorator use can be more efficient than subclassing, because an object's behavior can be augmented without defining an entirely new object.

The decorator design pattern is one of the twenty-three Gang-of-Four design patterns; these describe how to solve recurring design problems and design flexible and reusable object-oriented software—that is, objects which are easier to implement, change, test, and reuse.

The decorator pattern provides a flexible alternative to subclassing for extending functionality. When using subclassing, different subclasses extend a class in different ways. However, an extension is bound to the class at compile-time and can't be changed at run-time. The decorator pattern allows responsibilities to be added (and removed from) an object dynamically at run-time. It is achieved by defining Decorator objects that

This allows working with different Decorator objects to extend the functionality of an object dynamically at run-time.

The decorator pattern can be used to extend (decorate) the functionality of a certain object statically, or in some cases at run-time, independently of other instances of the same class, provided some groundwork is done at design time. This is achieved by designing a new Decorator class that wraps the original class. This wrapping could be achieved by the following sequence of steps:

This pattern is designed so that multiple decorators can be stacked on top of each other, each time adding a new functionality to the overridden method(s).

Note that decorators and the original class object share a common set of features. In the previous diagram, the operation() method was available in both the decorated and undecorated versions.

The decoration features (e.g., methods, properties, or other members) are usually defined by an interface, mixin (a.k.a. trait) or class inheritance which is shared by the decorators and the decorated object. In the previous example, the class Component is inherited by both the ConcreteComponent and the subclasses that descend from Decorator.

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