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Visitor Pattern

Visitor Pattern 


Gamma Categorization: Behavioral Design Pattern
Summary: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Sample Code

Problem: We want to correct photo filters based on the camera brand

Code

public abstract class Element
{
    public abstract void Accept(IVisitor visitor);
}

public class Filter : Element
{
    public string Name { get; set; }
    public double Exposure { get; set; }
    public double Contrast { get; set; }
    public double Saturation { get; set; }

    public Filter(string name, double exposure, double contrast, double saturation)
    {
        this.Name = name;
        this.Exposure = exposure;
        this.Contrast = contrast;
        this.Saturation = saturation;
    }

    public override void Accept(IVisitor visitor)
    {
        visitor.Visit(this);
    }
}

public class Filters
{
    private List<Filter> _filters = new List<Filter>();

    public void Attach(Filter filter)
    {
        _filters.Add(filter);
    }

    public void Detach(Filter filter)
    {
        _filters.Remove(filter);
    }

    public void Accept(IVisitor visitor)
    {
        foreach (Filter f in _filters)
        {
            f.Accept(visitor);
        }
        Console.WriteLine();
    }
}

public class Impact : Filter
{
    public Impact() : base("Impact", 0, 1, 1) { }
}

public class Burn : Filter
{
    public Burn() : base("Brighten", 1, 0, 0) { }
}

public class Dodge : Filter
{
    public Dodge() : base("Impact", -1, 0, 0) { }
}

public interface IVisitor
{
    void Visit(Element element);
}

public class CanonCorrectionVisitor : IVisitor
{
    public void Visit(Element element)
    {
        Filter filter = element as Filter;
        filter.Exposure *= -1.01;
        filter.Contrast *= 1.01;
        filter.Saturation *= 1.02;

        Console.WriteLine("{0} {1}'s new configuration: exposure {2}, contrast {3}, saturation {4}", 
            filter.GetType().Name, filter.Name, filter.Exposure, filter.Contrast, filter.Saturation);
    }
}

public class NikonCorrectionVisitor : IVisitor
{
    public void Visit(Element element)
    {
        Filter filter = element as Filter;
        filter.Contrast *= 1.02;
        filter.Saturation *= 1.01;

        Console.WriteLine("{0} {1}'s new configuration: exposure {2}, contrast {3}, saturation {4}", 
            filter.GetType().Name, filter.Name, filter.Exposure, filter.Contrast, filter.Saturation);
    }
}

Usage

Filters f = new Filters();
f.Attach(new Impact());
f.Attach(new Burn());
f.Attach(new Dodge());

f.Accept(new CanonCorrectionVisitor());

Output

Impact Impact's new configuration: exposure -0, contrast 1,01, saturation 1,02
Burn Brighten's new configuration: exposure -1,01, contrast 0, saturation 0
Dodge Impact's new configuration: exposure 1,01, contrast 0, saturation 0

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