Composite

Composite Pattern

The intent of composite pattern is to compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Problem

• A part-whole hierarchy should be represented so that clients can treat part and whole objects uniformly.
• A part-whole hierarchy should be represented as tree structure.

Solution

• Define a unified Component interface for both part (Leaf) objects and whole (Composite) objects.
• Individual Leaf objects implement the Component interface directly, and Composite objects forward requests to their child components.

Common Structure

• Component (IFileSystemComponent)
  • declares an interface for objects in the composition.
  • implements default behaviour for the interface common to all classes, as appropriate.
• Leaf (File)
  • A leaf has no children.
  • defines behavior for primitive objects in the composition.
• Composite (Folder)
  • stores child components
  • defines/implements behaviour for components having children.
• Client (FileSystem)
  • manipulates objects in the composition through the Component interface.

Collaboration

• Clients use the Component class interface to interact with objects in the composite structure.
  • If the recipient is a Leaf, then the request is handled directly.
  • If the recipient is a Composite, then it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding.

Benefits

• Makes the client simple.
  • It can apply the same operations over both composites and individual objects (leaves).
• In most cases we can ignore the differences between the composition of objects and leaves.
• Makes it easier to add new kinds of components
  • Newly defined Composite or Leaf subclasses work automatically with existing structures and client code.

Drawbacks

• In specific cases, it is difficult to restrict the components of the tree to only particular types. Therefore, to enforce such contraint, the program must rely on run-time checks, since it cannot use the type system of programming language.

Definition

Component

 interface IFileSystemComponent
{
   void PrintName(string prefix);
}

Leaf

class File : IFileSystemComponent
{
    public string Name { get; set; }
    public File(string name)
    {
        this.Name = name;
    }

    public void PrintName(string prefix = "")
    {
        Console.WriteLine("{0} {1}", prefix, Name);
    }
}

Composite

class Folder : IFileSystemComponent
{
    private readonly List<IFileSystemComponent> _fsComponents;
    public string Name { get; set; }

    public Folder(string name)
    {
        this.Name = name;
        this._fsComponents = new List<IFileSystemComponent>();
    }

    ...

    public void PrintName(string prefix = "")
    {
        Console.WriteLine("{0} {1}", prefix, Name);
        foreach (var fileSystemComponent in _fsComponents)
        {
            fileSystemComponent.PrintName(prefix + "\t");
        }
    }
}

Usage

Folder mainFolder = new Folder("Main Folder");
Folder subFolder1 = new Folder("Sub Folder 1");
Folder subFolder2 = new Folder("Sub Folder 2");

mainFolder.Add(subFolder1);
mainFolder.Add(subFolder2);
mainFolder.Add(new File("File 1 in Main Folder"));

subFolder1.Add(new File("File 1 in Sub Folder 1"));
subFolder1.Add(new File("File 2 in Sub Folder 1"));

subFolder2.Add(new File("File 1 in Sub Folder 2"));
subFolder2.Add(new Folder("Empty folder in Sub Folder 2"));

mainFolder.PrintName();

Output

 Main Folder
         Sub Folder 1
                 File 1 in Sub Folder 1
                 File 2 in Sub Folder 1
         Sub Folder 2
                 File 1 in Sub Folder 2
                 Empty folder in Sub Folder 2
         File 1 in Main Folder

Relations with Other Patterns

• Chain of Responsibility - Often the component-parent link is used for a Chain of Responsibility.
• Decorator is often used with Composite. When decorators and composites are used together, they will usually have a common parent class. So decorators will have to support the Component interface with operations like Add, Remove, and GetChild.
• Flyweight lets you share components, but they can no longer refer to their parents.
• Iterator can be used to traverse composites.
• Visitor localizes operations and behavior that would otherwise be distributed across Composite and Leaf classes.