State Pattern Example

StatePatternExample.cs

/*
 * THE STATE PATTERN
 * - Allows an object to alter its behavior when its internal state changes
 * - The object "transforms" and appears to be completely different
 */

using System;

class Program
{
    static void Main(string[] args)
    {
        var mp3Player = new MP3PlayerContext();
        mp3Player.Play();
        Console.WriteLine(string.Format("The MP3 player state is {0}.", mp3Player.CurrentState));
        mp3Player.Play();
        Console.WriteLine(string.Format("The MP3 player state is {0}.", mp3Player.CurrentState));
    }
}

public class MP3PlayerContext
{
    public MP3PlayerContext()
    {
        this.CurrentState = new StandbyState();
    }
    
    public MP3PlayerContext(IState state)
    {
       this.CurrentState = state; 
    }
    
    public IState CurrentState { get; set; }
    
    public void Play()
    {
        this.CurrentState.PressPlay(this);
    }
}

public interface IState
{
    void PressPlay(MP3PlayerContext context);
}

public class StandbyState : IState
{
    public void PressPlay(MP3PlayerContext context)
    {
        context.CurrentState = new PlayingState();
    }
}

public class PlayingState : IState
{
    public void PressPlay(MP3PlayerContext context)
    {
        context.CurrentState = new StandbyState();
    }
}