ScenePresenceStateMachine.cs

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using System;

namespace OpenSim.Region.Framework.Scenes
{
    public enum ScenePresenceState
    {
        Running,                // Normal operation state.  The scene presence is available.
        PreRemove,              // The presence is due to be removed but can still be returning to running.
        Removing,               // The presence is in the process of being removed from the scene via Scene.RemoveClient.
        Removed,                // The presence has been removed from the scene and is effectively dead.
                                // There is no exit from this state.
    }

    internal class ScenePresenceStateMachine
    {
        private ScenePresence m_sp;
        private ScenePresenceState m_state;

        internal ScenePresenceStateMachine(ScenePresence sp)
        {
            m_sp = sp;
            m_state = ScenePresenceState.Running;
        }

        internal ScenePresenceState GetState()
        {
            return m_state;
        }

        internal void SetState(ScenePresenceState newState)
        {
            bool transitionOkay = false;

            lock (this)
            {
                if (newState == m_state)
                    return;
                else if (newState == ScenePresenceState.Running && m_state == ScenePresenceState.PreRemove)
                    transitionOkay = true;
                else if (newState == ScenePresenceState.PreRemove && m_state == ScenePresenceState.Running)
                    transitionOkay = true;
                else if (newState == ScenePresenceState.Removing)
                {   
                    if (m_state == ScenePresenceState.Running || m_state == ScenePresenceState.PreRemove)
                        transitionOkay = true;
                }
                else if (newState == ScenePresenceState.Removed && m_state == ScenePresenceState.Removing)
                    transitionOkay = true;
            }           

            if (!transitionOkay)
            {
                throw new Exception(
                    string.Format(
                        "Scene presence {0} is not allowed to move from state {1} to new state {2} in {3}",
                        m_sp.Name, m_state, newState, m_sp.Scene.Name));
            }
            else
            {
                m_state = newState;
            }
        }
    }
}