Composite.md

Composite

Definition

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Frequency of use:

Medium high

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UML class diagram

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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.

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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

-root\
---Leaf A\
---Leaf B\
---Composite X\
-----Leaf XA\
-----Leaf XB\
---Leaf C