Definition
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
UML class diagram
Participants
The classes and/or objects participating in this pattern are:
- Component - defines the interface for objects that can have responsibilities added to them dynamically.
- ConcreteComponent - defines an object to which additional responsibilities can be attached.
- Decorator - maintains a reference to a Component object and defines an interface that conforms to Component's interface.
- ConcreteDecorator - adds responsibilities to the component.
Sample code in C#
///
/// MainApp startup class for Structural
/// Decorator Design Pattern.
///
class MainApp
{
///
/// Entry point into console application.
///
static void Main()
{
// Create ConcreteComponent and two Decorators
ConcreteComponent c = new ConcreteComponent();
ConcreteDecoratorA d1 = new ConcreteDecoratorA();
ConcreteDecoratorB d2 = new ConcreteDecoratorB();
// Link decorators
d1.SetComponent(c);
d2.SetComponent(d1);
d2.Operation();
// Wait for user
Console.ReadKey();
}
}
///
/// The 'Component' abstract class
///
abstract class Component
{
public abstract void Operation();
}
///
/// The 'ConcreteComponent' class
///
class ConcreteComponent : Component
{
public override void Operation()
{
Console.WriteLine("ConcreteComponent.Operation()");
}
}
///
/// The 'Decorator' abstract class
///
abstract class Decorator : Component
{
protected Component component;
public void SetComponent(Component component)
{
this.component = component;
}
public override void Operation()
{
if (component != null)
{
component.Operation();
}
}
}
///
/// The 'ConcreteDecoratorA' class
///
class ConcreteDecoratorA : Decorator
{
public override void Operation()
{
base.Operation();
Console.WriteLine("ConcreteDecoratorA.Operation()");
}
}
///
/// The 'ConcreteDecoratorB' class
///
class ConcreteDecoratorB : Decorator
{
public override void Operation()
{
base.Operation();
AddedBehavior();
Console.WriteLine("ConcreteDecoratorB.Operation()");
}
void AddedBehavior()
{
}
}
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