Composite¶
- Composite
- UML class diagram
- Participants
- Structural code in C# .{10}
- Real-world code in C# .{10}
- .NET Optimized code in C# .{10}
Summary: 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
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# .{10}¶
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¶
Real-world code in C# .{10}¶
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¶
-+ Picture
--- Red Line
--- Blue Circle
--- Green Box
---+ Two Circles
----- Black Circle
----- White Circle
.NET Optimized code in C# .{10}¶
using System;
using System.Collections.Generic;
namespace DoFactory.GangOfFour.Composite.NETOptimized
{
/// <summary>
/// MainApp startup class for .NET optimized
/// Composite Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Build a tree of shapes
var root = new TreeNode<Shape> { Node = new Shape("Picture") };
root.Add(new Shape("Red Line"));
root.Add(new Shape("Blue Circle"));
root.Add(new Shape("Green Box"));
var branch = root.Add(new Shape("Two Circles"));
branch.Add(new Shape("Black Circle"));
branch.Add(new Shape("White Circle"));
// Add, remove, and add a shape
var shape = new Shape("Yellow Line");
root.Add(shape);
root.Remove(shape);
root.Add(shape);
// Display tree using static method
TreeNode<Shape>.Display(root, 1);
Console.ReadKey();
}
}
/// <summary>
/// Generic tree node class
/// </summary>
/// <typeparam name="T">Node type</typeparam>
class TreeNode<T> where T : IComparable<T>
{
private List<TreeNode<T>> _children = new List<TreeNode<T>>();
// Add a child tree node
public TreeNode<T> Add(T child)
{
var newNode = new TreeNode<T> { Node = child };
_children.Add(newNode);
return newNode;
}
// Remove a child tree node
public void Remove(T child)
{
foreach (var treeNode in _children)
{
if (treeNode.Node.CompareTo(child) == 0)
{
_children.Remove(treeNode);
return;
}
}
}
// Gets or sets the node
public T Node { get; set; }
// Gets treenode children
public List<TreeNode<T>> Children
{
get { return _children; }
}
// Recursively displays node and its children
public static void Display(TreeNode<T> node, int indentation)
{
string line = new String('-', indentation);
Console.WriteLine(line + " " + node.Node);
node.Children.ForEach(n => Display(n, indentation + 1));
}
}
/// <summary>
/// Shape class
/// <remarks>
/// Implements generic IComparable interface
/// </remarks>
/// </summary>
class Shape : IComparable<Shape>
{
private string _name;
// Constructor
public Shape(string name)
{
this._name = name;
}
public override string ToString()
{
return _name;
}
// IComparable<Shape> Member
public int CompareTo(Shape other)
{
return (this == other) ? 0 : -1;
}
}
}