What is the best practice for a hierarchical state machine using the state pattern?

Question

I'm about to implement a hierarchical state machine in C# using the state pattern. As a guide I'm using this example. The example doesn't provide an answer regarding hierarchical states though. Unfortunately, I can't seem to find good examples elsewhere. My first thought is to create nested classed for the hierarchical states. But is this considered best practice or are there better solutions?

Greets!

UPDATE:

I've been sitting all afternoon on trying to implement the state pattern as described above. The HSM is based on a very simple media player:

alt text http://www.freeimagehosting.net/uploads/e8d2d6486a.jpg

I thought I've done it but one thing I do not understand. First the code I've written (sorry, it is quite al lot):

public class MediaPlayer
{
    public MediaPlayerStates state;

    public MediaPlayer(MediaPlayerStates state)
    {
        this.state = state;
    }

    public void OnButtonPressed()
    {
        state.OnButtonPressed(this);
    }

    public void DeviceBooted()
    { 
        state. ?????
    }

    //Other Functions
}

//The 3 initial states (Start, On, End) know only 2 events.
public abstract class MediaPlayerStates
{
    public abstract void OnButtonPressed(MediaPlayer player);
    public abstract void OffButtonPressed(MediaPlayer player);
}

//The very beginpoint of the state machine
public class Start : MediaPlayerStates
{
    //When hitting the onbutton, the state changes to the OnState state
    public override void OnButtonPressed(MediaPlayer player)
    {
        player.state = new OnState(player);
    }

    //No need to implement this one
    public override void OffButtonPressed(MediaPlayer player)
    {
        throw new NotImplementedException();
    }
}

//OnState implements the 2 events from the MediaPlayerStates abstract class.
public class OnState : MediaPlayerStates
{
    //When entered the OnState state, a new entrypoint is creaeted: the Start state
    public OnState(MediaPlayer player)
    {
        player.state = new OnStartState();
    }

    //The OnState doesn't have a OnButtonPressed event so it doesn't need to be implemented
    public override void OnButtonPressed(MediaPlayer player)
    {
        throw new NotImplementedException();
    }

    //When hitting the offbutton in the OnState, the new state is End
    public override void OffButtonPressed(MediaPlayer player)
    {
        player.state = new End();
    }

    //The OnState itself containts 3 events, therefore these need to be implemented by every state whitin the OnState state
    public abstract class SubStates : MediaPlayerStates
    {
        public abstract void DeviceBooted(MediaPlayer player);
        public abstract void PlayButtonPressed(MediaPlayer player);
        public abstract void StopButtonPressed(MediaPlayer player);
    }

    //The OnStartState is the pseudoState where the On state starts
    public class OnStartState : SubStates
    {
        //When booted, the state of the player changes to the ShowMediaFileState state
        public override void DeviceBooted(MediaPlayer player)
        {
            player.state = new ShowMediaFileState();
        }

        //The events below don't need to be implemented since they don't exist. 
        public override void PlayButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        public override void StopButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        public override void OnButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        public override void OffButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }
    }

    public class ShowMediaFileState : SubStates
    {
        //This event doesn't exists for this state
        public override void DeviceBooted(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        //When hitting the play button in this state, play the mediafile
        public override void PlayButtonPressed(MediaPlayer player)
        {
            player.state = new PlayMediaFileState();
        }

        //These events also don't exist for this state
        public override void StopButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        public override void OnButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        public override void OffButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }
    }

    public class PlayMediaFileState : SubStates
    {
        //This event doesn't exist for this state
        public override void DeviceBooted(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        //This event doesn't exist for this state
        public override void PlayButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        //While playing a file and hitting the stopbutton, the state changes to the ShowMediaFileState state
        public override void StopButtonPressed(MediaPlayer player)
        {
            player.state = new ShowMediaFileState();
        }

        //This event doesn't exist for this state
        public override void OnButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }

        //This event doesn't exist for this state
        public override void OffButtonPressed(MediaPlayer player)
        {
            throw new NotImplementedException();
        }
    }
}

//The endstate doesn't need any implementation since there cannot occur a event while being off
public class End : MediaPlayerStates
{
    public override void OnButtonPressed(MediaPlayer player)
    {
        throw new NotImplementedException();
    }

    public override void OffButtonPressed(MediaPlayer player)
    {
        throw new NotImplementedException();
    }
}

When defining the events in the MediaPlayer class, I can't call any other functions then

• OnButtonPressed
• OffButtonPressed

So I wonder, is my implementation any good? What is wrong? I also tried to look at the suggestion of using the composite pattern but I don't understand how it should be used with the state pattern. Hope anybody can help!

Answer 1

I think you'll want Composite as well; that will allow you to link state machines together.