using System;

namespace Chernobyl.StateMachine
{
    /// <summary>
    /// Extension methods for implementations of the <see cref="IState"/> 
    /// interface.
    /// </summary>
    public static class StateExtensions
    {
        /// <summary>
        /// Determines whether the specified <paramref name="state"/> has been
        /// entered and has been left (i.e. it is an active state).
        /// </summary>
        /// <param name="state">The <see cref="IState"/> to check to see if it
        /// is active.</param>
        /// <returns>True if <paramref name="state"/> has been entered and has
        /// not been left.</returns>
        public static bool IsActive(this IState state)
        {
            return state.ParentState != null;
        }

        /// <summary>
        /// Adds the specified <paramref name="eventHandler"/> to the 
        /// <see cref="IState.Entered"/> event of the passed in 
        /// <paramref name="state"/> and also invokes the passed in method if
        /// the <see cref="IState"/> is currently active.
        /// </summary>
        /// <param name="state">The <see cref="IState"/> whose 
        /// <see cref="IState.Entered"/> will have the
        /// <paramref name="eventHandler"/> added to it.</param>
        /// <param name="eventHandler">The <see cref="EventHandler"/> to add
        /// to the <see cref="IState.Entered"/> evernt of <paramref name="state"/>.</param>
        public static void CheckedEnteredAdd(this IState state, EventHandler eventHandler)
        {
            state.Entered += eventHandler;

            if (state.IsActive())
                eventHandler(state, EventArgs.Empty);
        }

        /// <summary>
        /// Adds the specified <paramref name="eventHandler"/> to the 
        /// <see cref="IState.Left"/> event of the passed in 
        /// <paramref name="state"/> and also invokes the passed in method if
        /// the <see cref="IState"/> is currently inactive.
        /// </summary>
        /// <param name="state">The <see cref="IState"/> whose 
        /// <see cref="IState.Entered"/> will have the
        /// <paramref name="eventHandler"/> added to it.</param>
        /// <param name="eventHandler">The <see cref="EventHandler"/> to add
        /// to the <see cref="IState.Entered"/> evernt of <paramref name="state"/>.</param>
        public static void CheckedLeftAdd(this IState state, EventHandler eventHandler)
        {
            state.Left += eventHandler;

            if (state.IsActive() == false)
                eventHandler(state, EventArgs.Empty);
        }

        /// <summary>
        /// This method will cause the <paramref name="child"/> 
        /// <see cref="IState"/> to immediately become the 
        /// <paramref name="parent"/> instance's child (and therefore be 
        /// considered an active or entered state) when the 
        /// <paramref name="parent"/> has become active (i.e. 
        /// <see cref="IState.Entered"/> is raised). This method can be used to 
        /// give an <see cref="IState"/> (the <paramref name="parent"/>) a 
        /// default sub-state (the <paramref name="child"/>) when it is made 
        /// active.
        /// </summary>
        /// <param name="parent">The <see cref="IState"/> that is to have its
        /// <see cref="IState.ChildState"/> set to the <paramref name="child"/>
        /// <see cref="IState"/> when it is activated (i.e. 
        /// <see cref="IState.Entered"/> is raised).</param>
        /// <param name="child">The <see cref="IState"/> that is to become the
        /// child <see cref="IState"/> of <paramref name="parent"/> when
        /// <paramref name="parent"/> has been activated (i.e. 
        /// <see cref="IState.Entered"/> is raised).</param>
        public static void EnterWith(this IState parent, IState child)
        {
            parent.CheckedEnteredAdd((sender, e) => parent.ChildState = child);
        }
    }
}