Decorator pattern

Intent

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:

1. Subclass the original Component class into a Decorator class (see UML diagram);
2. In the Decorator class, add a Component pointer as a field;
3. In the Decorator class, pass a Component to the Decorator constructor to initialize the Component pointer;
4. In the Decorator class, forward all Component methods to the Component pointer; and
5. In the ConcreteDecorator class, override any Component method(s) whose behavior needs to be modified.

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.

The decorator pattern is an alternative to subclassing. Subclassing adds behavior at compile time, and the change affects all instances of the original class; decorating can provide new behavior at run-time for selective objects.

This difference becomes most important when there are several independent ways of extending functionality. In some object-oriented programming languages, classes cannot be created at runtime, and it is typically not possible to predict, at design time, what combinations of extensions will be needed. This would mean that a new class would have to be made for every possible combination. By contrast, decorators are objects, created at runtime, and can be combined on a per-use basis. The I/O Streams implementations of both Java and the .NET Framework incorporate the decorator pattern.

Motivation

As an example, consider a window in a windowing system. To allow scrolling of the window's contents, one may wish to add horizontal or vertical scrollbars to it, as appropriate. Assume windows are represented by instances of the Window class, and assume this class has no functionality for adding scrollbars. One could create a subclass ScrollingWindow that provides them, or create a ScrollingWindowDecorator that adds this functionality to existing Window objects. At this point, either solution would be fine.

Now, assume one also desires the ability to add borders to windows. Again, the original Window class has no support. The ScrollingWindow subclass now poses a problem, because it has effectively created a new kind of window. If one wishes to add border support to many but not all windows, one must create subclasses WindowWithBorder and ScrollingWindowWithBorder etc. This problem gets worse with every new feature or window subtype to be added. For the decorator solution, we simply create a new BorderedWindowDecorator—at runtime, we can decorate existing windows with the ScrollingWindowDecorator or the BorderedWindowDecorator or both, as we see fit. Notice that if the functionality needs to be added to all Windows, you could modify the base class and that will do. On the other hand, sometimes (e.g., using external frameworks) it is not possible, legal, or convenient to modify the base class.

Note, in the previous example, that the "SimpleWindow" and "WindowDecorator" classes implement the "Window" interface, which defines the "draw()" method and the "getDescription()" method, that are required in this scenario, in order to decorate a window control.

Usage

A decorator makes it possible to add or alter behavior of an interface at run-time. Alternatively, the adapter can be used when the wrapper must respect a particular interface and must support polymorphic behavior, and the Facade when an easier or simpler interface to an underlying object is desired.

C++

Two options are presented here, first a dynamic, runtime-composable decorator (has issues with calling decorated functions unless proxied explicitly) and a decorator that uses mixin inheritance.

First example (window/scrolling scenario)

The following Java example illustrates the use of decorators using the window/scrolling scenario.

The following classes contain the decorators for all Window classes, including the decorator classes themselves.

Here's a test program that creates a Window instance which is fully decorated (i.e., with vertical and horizontal scrollbars), and prints its description:

Below is the JUnit test class for the Test Driven Development

The output of this program is "simple window, including vertical scrollbars, including horizontal scrollbars". Notice how the getDescription method of the two decorators first retrieve the decorated Window's description and decorates it with a suffix.

Second example (coffee making scenario)

The next Java example illustrates the use of decorators using coffee making scenario. In this example, the scenario only includes cost and ingredients.

The following classes contain the decorators for all Coffee classes, including the decorator classes themselves..

Here's a test program that creates a Coffee instance which is fully decorated (with milk and sprinkles), and calculate cost of coffee and prints its ingredients:

The output of this program is given below:

Cost: 1.0; Ingredients: CoffeeCost: 1.5; Ingredients: Coffee, MilkCost: 1.7; Ingredients: Coffee, Milk, Sprinkles

Python

Python includes a more natural way of decorating a function by using an annotation on the function that is decorated.

Crystal

Output:

Cost: 1.0; Ingredients: CoffeeCost: 1.5; Ingredients: Coffee, MilkCost: 1.7; Ingredients: Coffee, Milk, Sprinkles