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Command Pattern

Command Pattern 


Gamma Categorization: Behavioral Design Pattern
Summary: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Sample Code

Problem: We want to apply several commands on the digital camera.

Code

public interface ICommand
{
    void Execute();
}

public enum CameraCommands { ShutterSpeed, FocalDistance, ISO, Shoot }

public class ShutterSpeedCommand : ICommand
{
    private float shutterSpeed;

    public ShutterSpeedCommand(float shutterSpeed)
    {
        this.shutterSpeed = shutterSpeed;
    }

    public void Execute()
    {
        Console.WriteLine($"Setting shutter speed to {shutterSpeed} seconds.");
    }
}

public class FocalDistanceCommand : ICommand
{
    private int focalDistance;

    public FocalDistanceCommand(int focalDistance)
    {
        this.focalDistance = focalDistance;
    }

    public void Execute()
    {
        Console.WriteLine($"Setting focal distance to {focalDistance} mm.");
    }
}

public class ISOCommand : ICommand
{
    private int ISO;

    public ISOCommand(int ISO)
    {
        this.ISO = ISO;
    }

    public void Execute()
    {
        Console.WriteLine($"Setting ISO to {ISO}.");
    }
}

public class ShootCommand : ICommand
{
    public void Execute()
    {
        Console.WriteLine("Taking photo.");
    }
}

public class CameraInvoker
{
    public ICommand GetCommand(CameraCommands command, object parameter)
    {
        switch (command)
        {
            case CameraCommands.FocalDistance:
                return new FocalDistanceCommand((int)parameter);
            case CameraCommands.ShutterSpeed:
                return new ShutterSpeedCommand((float)parameter);
            case CameraCommands.ISO:
                return new ISOCommand((int)parameter);
            case CameraCommands.Shoot:
                return new ShootCommand();
            default:
                return null;
        }
    }
}

Usage

var cameraInvoker = new CameraInvoker();

var command = cameraInvoker.GetCommand(CameraCommands.FocalDistance, 28);
command.Execute();

command = cameraInvoker.GetCommand(CameraCommands.ShutterSpeed, 0.004f);
command.Execute();

command = cameraInvoker.GetCommand(CameraCommands.ISO, 100);
command.Execute();

command = cameraInvoker.GetCommand(CameraCommands.Shoot, null);
command.Execute();

Output

Setting focal distance to 28 mm.
Setting shutter speed to 0.004 seconds.
Setting ISO to 100.
Taking photo.

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