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 objects from 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. The decorator pattern is structurally nearly identical to the chain of responsibility pattern, the difference being that in a chain of responsibility, exactly one of the classes handles the request, while for the decorator, all classes handle the request.

Overview

The decorator design pattern is one of the twenty-three well-known GoF 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.

What problems can it solve?

When using subclassing, different subclasses extend a class in different ways. But an extension is bound to the class at compile-time and can't be changed at run-time.

What solution does it describe?

Define Decorator objects that
This allows working with different Decorator objects to extend the functionality of an object dynamically at run-time.
See also the UML class and sequence diagram below.

Intent

The decorator pattern can be used to extend 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 ;
  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 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.
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 are usually defined by an interface, mixin 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 selected 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 interface, 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, a new BorderedWindowDecorator is created. Any combination of ScrollingWindowDecorator or BorderedWindowDecorator can decorate existing windows. If the functionality needs to be added to all Windows, the base class can be modified. On the other hand, sometimes it is not possible, legal, or convenient to modify the base class.
In the previous example, 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.
PatternIntent
AdapterConverts one interface to another so that it matches what the client is expecting
DecoratorDynamically adds responsibility to the interface by wrapping the original code
FacadeProvides a simplified interface

Structure

UML class and sequence diagram

In the above UML class diagram,
the abstract Decorator class maintains a reference
to the decorated object and forwards all requests to it
.
This makes Decorator transparent to clients of Component.
Subclasses implement additional behavior
that should be added to the Component.
The sequence diagram
shows the run-time interactions: The Client object
works through Decorator1 and Decorator2 objects to
extend the functionality of a Component1 object.
The Client calls operation
on Decorator1, which forwards the request to Decorator2.
Decorator2 performs addBehavior after forwarding
the request to Component1 and returns to
Decorator1, which performs addBehavior
and returns to the Client.

Examples

Go


package decolog
import
//Decorate the operation
func Decorate
// package main
package main
import
func DoActionA
func DoActionB

C++

Two options are presented here, first a dynamic, runtime-composable decorator and a decorator that uses mixin inheritance.

Dynamic Decorator


  1. include
  2. include
struct Shape ;
struct Circle : Shape ;
struct ColoredShape : Shape ;
int main


  1. include
  2. include
  3. include
struct WebPage
struct BasicWebPage : WebPage
struct WebPageDecorator : WebPage
struct AuthenticatedWebPage : WebPageDecorator
struct AuthorizedWebPage : WebPageDecorator
int main

Static Decorator (Mixin Inheritance)

This example demonstrates a static Decorator implementation, which is possible due to C++ ability to inherit from the template argument.

  1. include
  2. include
struct Circle ;
template
struct ColoredShape : public T ;
int main

Java

First example (window/scrolling scenario)

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

// The Window interface class
public interface Window
// Implementation of a simple Window without any scrollbars
class SimpleWindow implements 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
// The first concrete decorator which adds vertical scrollbar functionality
class VerticalScrollBarDecorator extends WindowDecorator
// The second concrete decorator which adds horizontal scrollbar functionality
class HorizontalScrollBarDecorator extends WindowDecorator

Here's a test program that creates a Window instance which is fully decorated, and prints its description:

public class DecoratedWindowTest

Below is the JUnit test class for the Test Driven Development

import static org.junit.Assert.assertEquals;
import org.junit.Test;
public class WindowDecoratorTest

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 interface Coffee defines the functionality of Coffee implemented by decorator
public interface Coffee
// Extension of a simple coffee without any extra ingredients
public class SimpleCoffee implements Coffee

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

// Abstract decorator class - note that it implements Coffee interface
public abstract class CoffeeDecorator implements Coffee
// Decorator WithMilk mixes milk into coffee.
// Note it extends CoffeeDecorator.
class WithMilk extends CoffeeDecorator
// Decorator WithSprinkles mixes sprinkles onto coffee.
// Note it extends CoffeeDecorator.
class WithSprinkles extends CoffeeDecorator

Here's a test program that creates a instance which is fully decorated, and calculate cost of coffee and prints its ingredients:

public class Main

The output of this program is given below:

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

PHP


abstract class Component
class ConcreteComponent extends Component
abstract class Decorator extends Component
class ConcreteDecorator1 extends Decorator
class ConcreteDecorator2 extends Decorator
class Client
$client = new Client;
// Result: #quanton81
//Concrete Component: 1000
//Concrete Decorator 1: 500
//Concrete Decorator 2: 500
//Client: 2000

Python

The following Python example, taken from , shows us how to pipeline decorators to dynamically add many behaviors in an object:

"""
Demonstrated decorators in a world of a 10x10 grid of values 0-255.
"""
import random
def s32_to_u16:
if x < 0:
sign = 0xf000
else:
sign = 0
bottom = x & 0x00007fff
return bottom | sign
def seed_from_xy:
return s32_to_u16 |
class RandomSquare:
def __init__:
s.seed_modifier = seed_modifier
def get:
seed = seed_from_xy ^ s.seed_modifier
random.seed
return random.randint
class DataSquare:
def __init__:
s.data = * 10 * 10
def get:
return s.data # yes: these are all 10x10
def set:
s.data = u
class CacheDecorator:
def __init__:
s.decorated = decorated
s.cache = DataSquare
def get:
if s.cache.get None:
s.cache.set
return s.cache.get
class MaxDecorator:
def __init__:
s.decorated = decorated
s.max = max
def get:
if s.decorated.get > s.max:
return s.max
return s.decorated.get
class MinDecorator:
def __init__:
s.decorated = decorated
s.min = min
def get:
if s.decorated.get < s.min:
return s.min
return s.decorated.get
class VisibilityDecorator:
def __init__:
s.decorated = decorated
def get:
return s.decorated.get
def draw:
for y in range:
for x in range:
print "%3d" % s.get,
print
  1. Now, build up a pipeline of decorators:
random_square = RandomSquare
random_cache = CacheDecorator
max_filtered = MaxDecorator
min_filtered = MinDecorator
final = VisibilityDecorator
final.draw

Note:
Please do not confuse the Decorator Pattern with Python Decorators, a Python language feature. They are different things.
Second to the Python Wiki:
The Decorator Pattern is a pattern described in the Design Patterns Book. It is a way of apparently modifying an object's behavior, by enclosing it inside a decorating object with a similar interface.
This is not to be confused with Python Decorators, which is a language feature for dynamically modifying a function or class.

Crystal


abstract class Coffee
abstract def cost
abstract def ingredients
end
  1. Extension of a simple coffee
class SimpleCoffee < Coffee
def cost
1.0
end
def ingredients
"Coffee"
end
end
  1. Abstract decorator
class CoffeeDecorator < Coffee
protected getter decorated_coffee : Coffee
def initialize
end
def cost
decorated_coffee.cost
end
def ingredients
decorated_coffee.ingredients
end
end
class WithMilk < CoffeeDecorator
def cost
super + 0.5
end
def ingredients
super + ", Milk"
end
end
class WithSprinkles < CoffeeDecorator
def cost
super + 0.2
end
def ingredients
super + ", Sprinkles"
end
end
class Program
def print
puts "Cost: #; Ingredients: #"
end
def initialize
coffee = SimpleCoffee.new
print
coffee = WithMilk.new
print
coffee = WithSprinkles.new
print
end
end
Program.new

Output:

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

C#


namespace WikiDesignPatterns

Output:

Bike: 'Aluminium Bike + Sport Package + Security Package' Cost: 111