//******************************************************************************************************
//  ShutdownHandler.cs - Gbtc
//
//  Copyright © 2016, Grid Protection Alliance.  All Rights Reserved.
//
//  Licensed to the Grid Protection Alliance (GPA) under one or more contributor license agreements. See
//  the NOTICE file distributed with this work for additional information regarding copyright ownership.
//  The GPA licenses this file to you under the MIT License (MIT), the "License"; you may
//  not use this file except in compliance with the License. You may obtain a copy of the License at:
//
//      http://www.opensource.org/licenses/MIT
//
//  Unless agreed to in writing, the subject software distributed under the License is distributed on an
//  "AS-IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. Refer to the
//  License for the specific language governing permissions and limitations.
//
//  Code Modification History:
//  ----------------------------------------------------------------------------------------------------
//  11/03/2016 - Steven E. Chisholm
//       Generated original version of source code. 
//
//******************************************************************************************************

using System;
using System.Collections.Generic;
#if !SQLCLR
using System.ComponentModel;
#endif
using GSF.Collections;
using GSF.Diagnostics;

namespace GSF.Threading
{
    /// <summary>
    /// The order in which the specified callback should occur when shutting down.
    /// </summary>
    public enum ShutdownHandlerOrder
    {
        /// <summary>
        /// This queue is processed first. Unless there is a compelling reason to execute first, select the Default one. 
        /// </summary>
        First,

        /// <summary>
        /// This shutdown order occurs after First, but before Last. 
        /// </summary>
        Default,

        /// <summary>
        /// This queue is processed last. Items such as flushing application logs should go here.
        /// </summary>
        Last
    }

    /// <summary>
    /// This class will monitor the state to the application and raise events when it detects that the application is about to shutdown.
    /// </summary>
    public static class ShutdownHandler
    {
        private static readonly LogPublisher Log;

        /// <summary>
        /// Gets if this process is shutting down.
        /// </summary>
        public static bool IsShuttingDown { get; private set; }

        /// <summary>
        /// Gets if this process has already shut down.
        /// </summary>
        public static bool HasShutdown { get; private set; }

        private static readonly List<WeakAction> s_onShutdownCallbackFirst;
        private static readonly List<WeakAction> s_onShutdownCallbackDefault;
        private static readonly List<WeakAction> s_onShutdownCallbackLast;
        private static readonly object s_syncRoot;

        static ShutdownHandler()
        {
            s_syncRoot = new object();
            s_onShutdownCallbackFirst = new List<WeakAction>();
            s_onShutdownCallbackDefault = new List<WeakAction>();
            s_onShutdownCallbackLast = new List<WeakAction>();
            Logger.Initialize();
            Log = Logger.CreatePublisher(typeof(ShutdownHandler), MessageClass.Component);

            if (AppDomain.CurrentDomain.IsDefaultAppDomain())
                AppDomain.CurrentDomain.ProcessExit += InitiateSafeShutdown;
            else
                AppDomain.CurrentDomain.DomainUnload += InitiateSafeShutdown;

        }

        /// <summary>
        /// Initializes the shutdown handler. This is recommended to put in main loop of the program, but it is not critical.
        /// </summary>
        public static void Initialize()
        {
            //This is handled through the static constructor.
        }

        /// <summary>
        /// Attempts Registers a callback that will be called
        /// when the application is shutdown.
        /// </summary>
        /// <param name="callback">the callback when the shutdown occurs</param>
        /// <param name="shutdownOrder">the order that the callback will occur.</param>
        /// <returns></returns>
        public static bool TryRegisterCallback(Action callback, ShutdownHandlerOrder shutdownOrder = ShutdownHandlerOrder.Default)
        {
            List<WeakAction> list;
            switch (shutdownOrder)
            {
                case ShutdownHandlerOrder.First:
                    list = s_onShutdownCallbackFirst;
                    break;
                case ShutdownHandlerOrder.Default:
                    list = s_onShutdownCallbackDefault;
                    break;
                case ShutdownHandlerOrder.Last:
                    list = s_onShutdownCallbackLast;
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(shutdownOrder), shutdownOrder, null);
            }

            if (IsShuttingDown)
                return false;

            lock (s_syncRoot)
            {
                if (IsShuttingDown)
                    return false;

                list.RemoveWhere(x => !x.IsAlive);
                list.Add(new WeakAction(callback));

                return true;
            }
        }

        private static void InitiateSafeShutdown(object sender, EventArgs e)
        {
            List<WeakAction> shutdownList = new List<WeakAction>();

            lock (s_syncRoot)
            {
                if (IsShuttingDown)
                    return;

                IsShuttingDown = true;
                shutdownList.AddRange(s_onShutdownCallbackFirst);
                shutdownList.AddRange(s_onShutdownCallbackDefault);
                shutdownList.AddRange(s_onShutdownCallbackLast);
            }

            Log.Publish(MessageLevel.Info, MessageFlags.SystemHealth, "Shutting Down", $"Sending shutdown notification to {shutdownList.Count} objects");

            foreach (WeakAction weakAction in shutdownList)
            {
                try
                {
                    weakAction.TryInvoke();
                }
                catch (Exception ex)
                {
                    Log.Publish(MessageLevel.Warning, "Application Shutdown Generated an Error", null, null, ex);
                }
            }

            Logger.Shutdown();

            HasShutdown = true;
        }

        /// <summary>
        /// Requests that certain components initiate a safe shutdown.
        /// </summary>
        /// <remarks>
        /// This method should only be called when the main thread exits. Calling this outside
        /// of the application exiting could result in unpredictable behavior.
        /// </remarks>
#if !SQLCLR
        [EditorBrowsable(EditorBrowsableState.Advanced)]
#endif
        public static void InitiateSafeShutdown()
        {
            InitiateSafeShutdown(null, null);
        }
    }
}