Composite

UML class diagram

Participants
The classes and objects participating in this pattern are:
- Component (
DrawingElement
)- declares the interface for objects in the composition.
- implements default behavior for the interface common to all classes, as appropriate.
- declares an interface for accessing and managing its child components.
- (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.
- Leaf (
PrimitiveElement
)- represents leaf objects in the composition. A leaf has no children.
- defines behavior for primitive objects in the composition.
- Composite (
CompositeElement
)- defines behavior for components having children.
- stores child components.
- implements child-related operations in the Component interface.
- Client (
CompositeApp
)- manipulates objects in the composition through the Component interface.
Structural code in C#
This structural code demonstrates the Composite pattern which allows the creation of a tree structure in which individual nodes are accessed uniformly whether they are leaf nodes or branch (composite) nodes.
using System;
using System.Collections.Generic;
namespace DoFactory.GangOfFour.Composite.Structural
{
/// <summary>
/// MainApp startup class for Structural
/// Composite Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Create a tree structure
Composite root = new Composite("root");
root.Add(new Leaf("Leaf A"));
root.Add(new Leaf("Leaf B"));
Composite comp = new Composite("Composite X");
comp.Add(new Leaf("Leaf XA"));
comp.Add(new Leaf("Leaf XB"));
root.Add(comp);
root.Add(new Leaf("Leaf C"));
// Add and remove a leaf
Leaf leaf = new Leaf("Leaf D");
root.Add(leaf);
root.Remove(leaf);
// Recursively display tree
root.Display(1);
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Component' abstract class
/// </summary>
abstract class Component
{
protected string name;
// Constructor
public Component(string name)
{
this.name = name;
}
public abstract void Add(Component c);
public abstract void Remove(Component c);
public abstract void Display(int depth);
}
/// <summary>
/// The 'Composite' class
/// </summary>
class Composite : Component
{
private List<Component> _children = new List<Component>();
// Constructor
public Composite(string name)
: base(name)
{
}
public override void Add(Component component)
{
_children.Add(component);
}
public override void Remove(Component component)
{
_children.Remove(component);
}
public override void Display(int depth)
{
Console.WriteLine(new String('-', depth) + name);
// Recursively display child nodes
foreach (Component component in _children)
{
component.Display(depth + 2);
}
}
}
/// <summary>
/// The 'Leaf' class
/// </summary>
class Leaf : Component
{
// Constructor
public Leaf(string name)
: base(name)
{
}
public override void Add(Component c)
{
Console.WriteLine("Cannot add to a leaf");
}
public override void Remove(Component c)
{
Console.WriteLine("Cannot remove from a leaf");
}
public override void Display(int depth)
{
Console.WriteLine(new String('-', depth) + name);
}
}
}
Output
---Leaf A
---Leaf B
---Composite X
-----Leaf XA
-----Leaf XB
---Leaf C
Real-world code in C#
This real-world code demonstrates the Composite pattern used in building a graphical tree structure made up of primitive nodes (lines, circles, etc) and composite nodes (groups of drawing elements that make up more complex elements).
using System;
using System.Collections.Generic;
namespace DoFactory.GangOfFour.Composite.RealWorld
{
/// <summary>
/// MainApp startup class for Real-World
/// Composite Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Create a tree structure
CompositeElement root =
new CompositeElement("Picture");
root.Add(new PrimitiveElement("Red Line"));
root.Add(new PrimitiveElement("Blue Circle"));
root.Add(new PrimitiveElement("Green Box"));
// Create a branch
CompositeElement comp =
new CompositeElement("Two Circles");
comp.Add(new PrimitiveElement("Black Circle"));
comp.Add(new PrimitiveElement("White Circle"));
root.Add(comp);
// Add and remove a PrimitiveElement
PrimitiveElement pe =
new PrimitiveElement("Yellow Line");
root.Add(pe);
root.Remove(pe);
// Recursively display nodes
root.Display(1);
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Component' Treenode
/// </summary>
abstract class DrawingElement
{
protected string _name;
// Constructor
public DrawingElement(string name)
{
this._name = name;
}
public abstract void Add(DrawingElement d);
public abstract void Remove(DrawingElement d);
public abstract void Display(int indent);
}
/// <summary>
/// The 'Leaf' class
/// </summary>
class PrimitiveElement : DrawingElement
{
// Constructor
public PrimitiveElement(string name)
: base(name)
{
}
public override void Add(DrawingElement c)
{
Console.WriteLine(
"Cannot add to a PrimitiveElement");
}
public override void Remove(DrawingElement c)
{
Console.WriteLine(
"Cannot remove from a PrimitiveElement");
}
public override void Display(int indent)
{
Console.WriteLine(
new String('-', indent) + " " + _name);
}
}
/// <summary>
/// The 'Composite' class
/// </summary>
class CompositeElement : DrawingElement
{
private List<DrawingElement> elements =
new List<DrawingElement>();
// Constructor
public CompositeElement(string name)
: base(name)
{
}
public override void Add(DrawingElement d)
{
elements.Add(d);
}
public override void Remove(DrawingElement d)
{
elements.Remove(d);
}
public override void Display(int indent)
{
Console.WriteLine(new String('-', indent) +
"+ " + _name);
// Display each child element on this node
foreach (DrawingElement d in elements)
{
d.Display(indent + 2);
}
}
}
}
Output
--- Red Line
--- Blue Circle
--- Green Box
---+ Two Circles
----- Black Circle
----- White Circle
.NET Optimized code in C#
The .NET optimized code demonstrates the
same real-world situation as above but uses modern, built-in .NET features,
such as, generics, reflection, LINQ, lambda functions, and more.
You can find an example on our Singleton pattern page.
All other patterns, and so much more, are available in our Dofactory .NET product.
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