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Composite Pattern

Composite Pattern 


Gamma Categorization: Structural Design Pattern
Summary: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Sample Code

Problem: We want to store a trips photoset hierarchy.

Code

public abstract class Photo
{
    protected string name;

    public Photo(string name)
    {
        this.name = name;
    }

    public abstract void Add(Photo p);
    public abstract void Remove(Photo p);
    public abstract void Display(int depth);
}

public class SinglePhoto : Photo
{
    public SinglePhoto(string name) : base(name) { }

    public override void Add(Photo p) => Console.WriteLine("Cannot add to a leaf");

    public override void Remove(Photo p) => Console.WriteLine("Cannot remove from a leaf");

    public override void Display(int depth)
    {
        Console.WriteLine(new String('-', depth) + name);
    }
}

public class PhotoSet : Photo
{
    private List<Photo> _children = new List<Photo>();

    public PhotoSet(string name) : base(name) { }

    public override void Add(Photo p) => _children.Add(p);

    public override void Remove(Photo p) => _children.Remove(p);

    public override void Display(int depth)
    {
        Console.WriteLine(new String('-', depth) + name);
        foreach (Photo i in _children)
        {
            i.Display(depth + 2);
        }
    }
}

Usage

var lisbonPhotoSet = new PhotoSet("Trip to Lisbon");

lisbonPhotoSet.Add(new SinglePhoto("Belém Tower"));

var alfamaPhotoSet = new PhotoSet("Alfama area");
alfamaPhotoSet.Add(new SinglePhoto("Se Cathedral"));
alfamaPhotoSet.Add(new SinglePhoto("Portas do Sol"));
alfamaPhotoSet.Add(new SinglePhoto("Lisbon Castle"));
alfamaPhotoSet.Add(new SinglePhoto("Casa dos Bicos"));

lisbonPhotoSet.Add(alfamaPhotoSet);

lisbonPhotoSet.Add(new SinglePhoto("Jerónimos Monastery"));

lisbonPhotoSet.Display(0);


Output

 Trip to Lisbon
--Belém Tower
--Alfama area
----Se Cathedral
----Portas do Sol
----Lisbon Castle
----Casa dos Bicos
--Jerónimos Monastery

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