Decorator pattern

LePUS3 chart of the decorator pattern

In object-oriented programming, the decorator pattern is a design pattern that allows new/additional behaviour to be added to an existing object dynamically.

Introduction

The decorator pattern can be used to make it possible to extend (decorate) the functionality of a certain object at runtime, 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. Pass a Component to the Decorator constructor to initialize the Component pointer;
4. In the Decorator class, redirect all "Component" methods to the "Component" pointer; and
5. In the Decorator class, override any Component method(s) whose behaviour 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).

The decorator pattern is an alternative to subclassing. Subclassing adds behaviour at compile time, and the change affects all instances of the original class; decorating can provide new behaviour at runtime for individual 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 what combinations of extensions will be needed at design time. 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. An example of the decorator pattern is the Java I/O Streams implementation.

Motivation

UML Diagram for the Window Example

As an example, consider a window in a windowing system. To allow scrolling of the window's contents, we 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. We could create a subclass ScrollingWindow that provides them, or we could create a ScrollingWindowDecorator that adds this functionality to existing Window objects. At this point, either solution would be fine.

Now let's assume we also wish the option to add borders to our windows. Again, our original Window class has no support. The ScrollingWindow subclass now poses a problem, because it has effectively created a new kind of window. If we wish to add border support to all windows, we must create subclasses WindowWithBorder and ScrollingWindowWithBorder. Obviously, this problem gets worse with every new feature 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.

Another good example of where a decorator can be desired is when there is a need to restrict access to an object's properties or methods according to some set of rules or perhaps several parallel sets of rules (different user credentials, etc.) In this case instead of implementing the access control in the original object it is left unchanged and unaware of any restrictions on its use, and it is wrapped in an access control decorator object, which can then serve only the permitted subset of the original object's interface.

Example

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

// the Window interface
interface Window {
    public void draw(); // draws the Window
    public String getDescription(); // returns a description of the Window
}
 
 
// implementation of a simple Window without any scrollbars
class SimpleWindow implements Window {
    public void draw() {
        // draw window
    }
 
    public String getDescription() {
        return "simple window";
    }
}

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

// abstract decorator class - note that it implements Window
abstract class WindowDecorator implements Window {
    protected Window decoratedWindow; // the Window being decorated
 
    public WindowDecorator (Window decoratedWindow) {
        this.decoratedWindow = decoratedWindow;
    }
}
 
 
// the first concrete decorator which adds vertical scrollbar functionality
class VerticalScrollBarDecorator extends WindowDecorator {
    public VerticalScrollBarDecorator (Window decoratedWindow) {
        super(decoratedWindow);
    }
 
    public void draw() {
        drawVerticalScrollBar();
        decoratedWindow.draw();
    }
 
    private void drawVerticalScrollBar() {
        // draw the vertical scrollbar
    }
 
    public String getDescription() {
        return decoratedWindow.getDescription() + ", including vertical scrollbars";
    }
}
 
 
// the second concrete decorator which adds horizontal scrollbar functionality
class HorizontalScrollBarDecorator extends WindowDecorator {
    public HorizontalScrollBarDecorator (Window decoratedWindow) {
        super(decoratedWindow);
    }
 
    public void draw() {
        drawHorizontalScrollBar();
        decoratedWindow.draw();
    }
 
    private void drawHorizontalScrollBar() {
        // draw the horizontal scrollbar
    }
 
    public String getDescription() {
        return decoratedWindow.getDescription() + ", including horizontal scrollbars";
    }
}

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:

public class DecoratedWindowTest {
    public static void main(String[] args) {
        // create a decorated Window with horizontal and vertical scrollbars
        Window decoratedWindow = new HorizontalScrollBarDecorator (
                new VerticalScrollBarDecorator(new SimpleWindow()));
 
        // print the Window's description
        System.out.println(decoratedWindow.getDescription());
    }
}

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.

Dynamic languages

The decorator pattern can also be implemented in dynamic languages with neither interfaces nor traditional OOP inheritance.

JavaScript

// Class to be decorated
function Coffee() {
    this.cost = function() {
	return 1;
    };
}
 
// Decorator A
function Milk(coffee) {
    this.cost = function() {
	return coffee.cost() + 0.5;
    };	
}
 
// Decorator B
function Whip(coffee) {
    this.cost = function() {
	return coffee.cost() + 0.7;
    };
}
 
// Decorator C
function Sprinkles(coffee) {
    this.cost = function() {
	return coffee.cost() + 0.2;
    };
}
 
// Here's one way of using it
var coffee = new Milk(new Whip(new Sprinkles(new Coffee())));
alert( coffee.cost() );
 
// Here's another
var coffee = new Coffee();
coffee = new Sprinkles(coffee);
coffee = new Whip(coffee);
coffee = new Milk(coffee);
alert(coffee.cost());

Python

def set_cost_decorator(func):
    def wrapper1(instance, cost):
        func(instance, cost)
    return wrapper1
 
def get_cost_decorator(additional_cost):
    def wrapper1(func):
        def wrapper2(instance):
            return func(instance) + additional_cost
        return wrapper2
    return wrapper1
 
class Coffee(object):
    @set_cost_decorator
    def set_cost(self, cost):
        self.cost = cost
 
    @get_cost_decorator(0.5)
    @get_cost_decorator(0.7)
    @get_cost_decorator(0.2)
    def get_cost(self):
        return self.cost
 
coffee = Coffee()
coffee.set_cost(1.0)
print coffee.get_cost() # 2.4

See also

• Composite pattern
• Adapter pattern
• Abstract class
• Abstract factory
• Immutable object

External links

• Decorator Pattern UML and Code Example
• Decorator pattern description from the Portland Pattern Repository
• Article "The Decorator Design Pattern" (Java) by Antonio García and Stephen Wong
• Article "Using the Decorator Pattern" (Java) by Budi Kurniawan
• Decorator in UML and in LePUS3 (a formal modelling language)
• A PHP approach (PHP)
• A Delphi approach (Delphi)
• A JavaScript implementation
• Decorator Pattern with Ruby in 8 Lines (Ruby)
• PerfectJPattern Open Source Project, Provides componentized implementation of the Decorator Pattern in Java