#region License
//
// The Open Toolkit Library License
//
// Copyright (c) 2006 - 2009 the Open Toolkit library.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights to 
// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
// the Software, and to permit persons to whom the Software is furnished to do
// so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
#endregion

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Drawing;
using System.Threading;
using OpenTK.Graphics;
using OpenTK.Input;
using OpenTK.Platform;

namespace OpenTK
{
    /// <summary>
    /// The GameWindow class contains cross-platform methods to create and render on an OpenGL
    /// window, handle input and load resources.
    /// </summary>
    /// <remarks>
    /// GameWindow contains several events you can hook or override to add your custom logic:
    /// <list>
    /// <item>
    /// OnLoad: Occurs after creating the OpenGL context, but before entering the main loop.
    /// Override to load resources.
    /// </item>
    /// <item>
    /// OnUnload: Occurs after exiting the main loop, but before deleting the OpenGL context.
    /// Override to unload resources.
    /// </item>
    /// <item>
    /// OnResize: Occurs whenever GameWindow is resized. You should update the OpenGL Viewport
    /// and Projection Matrix here.
    /// </item>
    /// <item>
    /// OnUpdateFrame: Occurs at the specified logic update rate. Override to add your game
    /// logic.
    /// </item>
    /// <item>
    /// OnRenderFrame: Occurs at the specified frame render rate. Override to add your
    /// rendering code.
    /// </item>
    /// </list>
    /// Call the Run() method to start the application's main loop. Run(double, double) takes two
    /// parameters that
    /// specify the logic update rate, and the render update rate.
    /// </remarks>
    public class GameWindow : NativeWindow0, IGameWindow, IDisposable
    {
        #region --- Fields ---

        //DisplayMode mode; // TODO: Removable?

        object exit_lock = new object();

        IGraphicsContext glContext;

        bool hasMainLoop;
        bool isExiting = false;

        int main_loop_thread_id;

        double update_period, render_period;
        double target_update_period, target_render_period;
        // TODO: Implement these:
        double update_time, render_time;//, event_time;
        //bool allow_sleep = true;    // If true, GameWindow will call Timer.Sleep() if there is enough time.
        VSyncMode vsync;

        //InputDriver input_driver;

        #endregion

        #region --- Contructors ---

        #region public GameWindow()

        /// <summary>Constructs a new GameWindow with sensible default attributes.</summary>
        public GameWindow()
            : this(640, 480, GraphicsMode.Default, "OpenTK Game Window", 0, DisplayDevice.Default) { }

        #endregion

        #region public GameWindow(int width, int height)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        public GameWindow(int width, int height)
            : this(width, height, GraphicsMode.Default, "OpenTK Game Window", 0, DisplayDevice.Default) { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        public GameWindow(int width, int height, GraphicsMode mode)
            : this(width, height, mode, "OpenTK Game Window", 0, DisplayDevice.Default) { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode, string title)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        /// <param name="title">The title of the GameWindow.</param>
        public GameWindow(int width, int height, GraphicsMode mode, string title)
            : this(width, height, mode, title, 0, DisplayDevice.Default) { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        /// <param name="title">The title of the GameWindow.</param>
        /// <param name="options">GameWindow options regarding window appearance and behavior.</param>
        public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options)
            : this(width, height, mode, title, options, DisplayDevice.Default) { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        /// <param name="title">The title of the GameWindow.</param>
        /// <param name="options">GameWindow options regarding window appearance and behavior.</param>
        /// <param name="device">The OpenTK.Graphics.DisplayDevice to construct the GameWindow in.</param>
        public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device)
            : this(width, height, mode, title, options, device, 1, 0, GraphicsContextFlags.Default)
        { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device, int major, int minor, GraphicsContextFlags flags)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        /// <param name="title">The title of the GameWindow.</param>
        /// <param name="options">GameWindow options regarding window appearance and behavior.</param>
        /// <param name="device">The OpenTK.Graphics.DisplayDevice to construct the GameWindow in.</param>
        /// <param name="major">The major version for the OpenGL GraphicsContext.</param>
        /// <param name="minor">The minor version for the OpenGL GraphicsContext.</param>
        /// <param name="flags">The GraphicsContextFlags version for the OpenGL GraphicsContext.</param>
        public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device,
            int major, int minor, GraphicsContextFlags flags)
            : this(width, height, mode, title, options, device, major, minor, flags, null)
        { }

        #endregion

        #region public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device, int major, int minor, GraphicsContextFlags flags, IGraphicsContext sharedContext)

        /// <summary>Constructs a new GameWindow with the specified attributes.</summary>
        /// <param name="width">The width of the GameWindow in pixels.</param>
        /// <param name="height">The height of the GameWindow in pixels.</param>
        /// <param name="mode">The OpenTK.Graphics.GraphicsMode of the GameWindow.</param>
        /// <param name="title">The title of the GameWindow.</param>
        /// <param name="options">GameWindow options regarding window appearance and behavior.</param>
        /// <param name="device">The OpenTK.Graphics.DisplayDevice to construct the GameWindow in.</param>
        /// <param name="major">The major version for the OpenGL GraphicsContext.</param>
        /// <param name="minor">The minor version for the OpenGL GraphicsContext.</param>
        /// <param name="flags">The GraphicsContextFlags version for the OpenGL GraphicsContext.</param>
        /// <param name="sharedContext">An IGraphicsContext to share resources with.</param>
        public GameWindow(int width, int height, GraphicsMode mode, string title, GameWindowFlags options, DisplayDevice device,
                          int major, int minor, GraphicsContextFlags flags, IGraphicsContext sharedContext)
            : base(width, height, title, options,
                   mode == null ? GraphicsMode.Default : mode,
                   device == null ? DisplayDevice.Default : device)
        {
            try
            {
                glContext = new GraphicsContext(mode == null ? GraphicsMode.Default : mode, WindowInfo, major, minor, flags);
                glContext.MakeCurrent(WindowInfo);
                (glContext as IGraphicsContextInternal).LoadAll();

                VSync = VSyncMode.On;

                //glWindow.WindowInfoChanged += delegate(object sender, EventArgs e) { OnWindowInfoChangedInternal(e); };
                EnableEvents();
            }
            catch (Exception e)
            {
                Debug.Print(e.ToString());
                base.Dispose();
                throw;
            }
        }

        #endregion

        #endregion

        #region --- Public Members ---

        #region Methods

        #region Dispose

        /// <summary>
        /// Disposes of the GameWindow, releasing all resources consumed by it.
        /// </summary>
        public new void Dispose()
        {
            try
            {
                Dispose(true);
            }
            finally
            {
                if (!IsDisposed())
                {
                    if (glContext != null)
                    {
                        glContext.Dispose();
                        glContext = null;
                    }

                    base.Dispose();
                }
            }
            GC.SuppressFinalize(this);
        }

        #endregion

        #region Exit

        /// <summary>
        /// Gracefully exits the GameWindow. May be called from any thread.
        /// </summary>
        /// <remarks>
        /// <para>Override if you are not using <see cref="GameWindow.Run()"/>.</para>
        /// <para>If you override this method, place a call to base.Exit(), to ensure proper OpenTK shutdown.</para>
        /// </remarks>
        public virtual void Exit()
        {
            lock (exit_lock)
            {
                EnsureUndisposed();

                if (!IsExiting && Exists)
                {
                    CancelEventArgs e = new CancelEventArgs();
                    OnClosing(e);
                    if (e.Cancel)
                        return;

                    isExiting = true;

                    if (HasMainLoop)
                    {
                        if (main_loop_thread_id == Thread.CurrentThread.ManagedThreadId)
                            ExitInternal();
                        else
                            ExitAsync();
                    }
                }
            }
        }

        #endregion

        #region MakeCurrent

        /// <summary>
        /// Makes the GraphicsContext current on the calling thread.
        /// </summary>
        public void MakeCurrent()
        {
            EnsureUndisposed();
            Context.MakeCurrent(WindowInfo);
        }

        #endregion

        #region OnLoad

        /// <summary>
        /// Occurs after establishing an OpenGL context, but before entering the main loop.
        /// Override to load resources that should be maintained for the lifetime of the application.
        /// </summary>
        /// <param name="e">Not used.</param>
        public virtual void OnLoad(EventArgs e)
        {
            EnsureUndisposed(); //if (disposed) throw new ObjectDisposedException("GameWindow"); // What is the exact purpose?
        }

        #endregion

        #region OnUnload

        /// <summary>
        /// Occurs after after calling GameWindow.Exit, but before destroying the OpenGL context.
        /// Override to unload application resources.
        /// </summary>
        /// <param name="e">Not used.</param>
        public virtual void OnUnload(EventArgs e)
        {
            EnsureUndisposed(); //if (disposed) throw new ObjectDisposedException("GameWindow"); // What is the exact purpose?
        }

        #endregion

        #region public void Run()

        /// <summary>
        /// Enters the game loop of the GameWindow using the maximum update rate.
        /// </summary>
        /// <seealso cref="Run(double)"/>
        public void Run()
        {
            Run(0.0, 0.0);
        }

        #endregion

        #region public void Run(double updateFrequency)

        /// <summary>
        /// Enters the game loop of the GameWindow using the specified update rate.
        /// maximum possible render frequency.
        /// </summary>
        public void Run(double updateRate)
        {
            Run(updateRate, 0.0);
        }

        #endregion

        #region public void Run(double updates_per_second, double frames_per_second)

        /// <summary>
        /// Enters the game loop of the GameWindow updating and rendering at the specified frequency.
        /// </summary>
        /// <remarks>
        /// When overriding the default game loop you should call ProcessEvents()
        /// to ensure that your GameWindow responds to operating system events.
        /// <para>
        /// Once ProcessEvents() returns, it is time to call update and render the next frame.
        /// </para>
        /// </remarks>
        /// <param name="updates_per_second">The frequency of UpdateFrame events.</param>
        /// <param name="frames_per_second">The frequency of RenderFrame events.</param>
        public void Run(double updates_per_second, double frames_per_second)
        {
            EnsureUndisposed();

            try
            {
                // Necessary to be here, otherwise Exit() wouldn't work correctly when called inside OnLoad().
                hasMainLoop = true;
                main_loop_thread_id = Thread.CurrentThread.ManagedThreadId;

                if (updates_per_second < 0.0 || updates_per_second > 200.0)
                    throw new ArgumentOutOfRangeException("updates_per_second", updates_per_second,
                                                          "Parameter should be inside the range [0.0, 200.0]");
                if (frames_per_second < 0.0 || frames_per_second > 200.0)
                    throw new ArgumentOutOfRangeException("frames_per_second", frames_per_second,
                                                          "Parameter should be inside the range [0.0, 200.0]");

                TargetUpdateFrequency = updates_per_second;
                TargetRenderFrequency = frames_per_second;

                Stopwatch update_watch = new Stopwatch(), render_watch = new Stopwatch();
                double time, next_render = 0.0, next_update = 0.0, update_time_counter = 0.0;
                int num_updates = 0;
                FrameEventArgs update_args = new FrameEventArgs();
                FrameEventArgs render_args = new FrameEventArgs();

                update_watch.Reset();
                render_watch.Reset();

                OnLoadInternal(EventArgs.Empty);

                Debug.Print("Entering main loop.");
                while (!IsExiting && HasMainLoop)
                {
                    ProcessEvents();

                    // Raise UpdateFrame events
                    time = update_watch.Elapsed.TotalSeconds;
                    if (time > 1.0)
                        time = 1.0;
                    while (next_update - time <= 0.0)
                    {
                        next_update = next_update - time + TargetUpdatePeriod;
                        if (next_update < -1.0)       // Cap the maximum time drift, to avoid lengthy catch-up games.
                            next_update = -1.0;

                        update_time_counter += time;
                        ++num_updates;

                        update_watch.Reset();
                        update_watch.Start();

                        if (time > 0)
                        {
                            update_args.Time = time;
                            OnUpdateFrameInternal(update_args);
                            update_time = update_watch.Elapsed.TotalSeconds;
                        }
                        if (TargetUpdateFrequency == 0.0)
                            break;

                        time = update_watch.Elapsed.TotalSeconds;
                        next_update -= time;
                        update_time_counter += time;

                        // Allow up to 10 frames to be dropped. Prevents the application from hanging
                        // with very high update frequencies.
                        if (num_updates >= 10)
                            break;
                    }
                    if (num_updates > 0)
                    {
                        update_period = update_time_counter / (double)num_updates;
                        num_updates = 0;
                        update_time_counter = 0.0;
                    }

                    // Raise RenderFrame event
                    time = render_watch.Elapsed.TotalSeconds;
                    if (time > 1.0)
                        time = 1.0;
                    double time_left = next_render - time;
                    if (VSync == VSyncMode.Adaptive)
                    {
                        // Check if we have enough time for a vsync
                        if (TargetRenderPeriod != 0 && RenderTime > 2.0 * TargetRenderPeriod)
                            Context.VSync = false;
                        else
                            Context.VSync = true;
                    }

                    if (time_left <= 0.0)
                    {
                        next_render = time_left + TargetRenderPeriod;
                        if (next_render < -1.0)       // Cap the maximum time drift, to avoid lengthy catch-up games.
                            next_render = -1.0;

                        render_watch.Reset();
                        render_watch.Start();

                        if (time > 0)
                        {
                            render_period = render_args.Time = time;
                            OnRenderFrameInternal(render_args);
                            render_time = render_watch.Elapsed.TotalSeconds;
                        }
                    }
                }
            }
            catch (GameWindowExitException)
            {
                Debug.WriteLine("GameWindowExitException caught - exiting main loop.");
            }
            finally
            {
                Debug.Print("Restoring priority.");
                Thread.CurrentThread.Priority = ThreadPriority.Normal;

                OnUnloadInternal(EventArgs.Empty);

                if (Exists)
                {
                    Dispose();
                    //while (this.Exists) ProcessEvents(); // TODO: Should similar behaviour be retained, possibly on native window level?
                }
            }
        }

        #endregion

        #region SwapBuffers

        /// <summary>
        /// Swaps the front and back buffer, presenting the rendered scene to the user.
        /// </summary>
        public void SwapBuffers()
        {
            EnsureUndisposed();
            this.Context.SwapBuffers();
        }

        #endregion

        #endregion

        #region Properties

        #region Context

        /// <summary>
        /// Returns the opengl IGraphicsContext associated with the current GameWindow.
        /// </summary>
        public IGraphicsContext Context
        {
            get
            {
                EnsureUndisposed();
                return glContext;
            }
        }

        #endregion

        #region IsExiting

        /// <summary>
        /// Gets a value indicating whether the shutdown sequence has been initiated
        /// for this window, by calling GameWindow.Exit() or hitting the 'close' button.
        /// If this property is true, it is no longer safe to use any OpenTK.Input or
        /// OpenTK.Graphics.OpenGL functions or properties.
        /// </summary>
        public bool IsExiting
        {
            get
            {
                EnsureUndisposed();
                return isExiting;
            }
        }

        #endregion

        #region Joysticks

        /// <summary>
        /// Gets a readonly IList containing all available OpenTK.Input.JoystickDevices.
        /// </summary>
        [Obsolete]
        public IList<JoystickDevice> Joysticks
        {
            get { return InputDriver.Joysticks; }
        }

        #endregion

        #region Keyboard

        /// <summary>
        /// Gets the primary Keyboard device, or null if no Keyboard exists.
        /// </summary>
        [Obsolete]
        public KeyboardDevice Keyboard
        {
            get { return InputDriver.Keyboard.Count > 0 ? InputDriver.Keyboard[0] : null; }
        }

        #endregion

        #region Mouse

        /// <summary>
        /// Gets the primary Mouse device, or null if no Mouse exists.
        /// </summary>
        [Obsolete]
        public MouseDevice Mouse
        {
            get { return InputDriver.Mouse.Count > 0 ? InputDriver.Mouse[0] : null; }
        }

        #endregion

        #region --- GameWindow Timing ---

        // TODO: Disabled because it is not reliable enough. Use vsync as a workaround.

        //#region public bool AllowSleep

        //public bool AllowSleep
        //{
        //    get { return allow_sleep; }
        //    set { allow_sleep = value; }
        //}

        //#endregion

        #region RenderFrequency

        /// <summary>
        /// Gets a double representing the actual frequency of RenderFrame events, in Herz (i.e. FPS or Frames Per Second).
        /// </summary>
        public double RenderFrequency
        {
            get
            {
                EnsureUndisposed();
                if (render_period == 0.0)
                    return 1.0;
                return 1.0 / render_period;
            }
        }

        #endregion

        #region RenderPeriod

        /// <summary>
        /// Gets a double representing the period of RenderFrame events, in seconds.
        /// </summary>
        public double RenderPeriod
        {
            get
            {
                EnsureUndisposed();
                return render_period;
            }
        }

        #endregion

        #region RenderTime

        /// <summary>
        /// Gets a double representing the time spent in the RenderFrame function, in seconds.
        /// </summary>
        public double RenderTime
        {
            get
            {
                EnsureUndisposed();
                return render_time;
            }
            protected set
            {
                EnsureUndisposed();
                render_time = value;
            }
        }

        #endregion

        #region TargetRenderFrequency

        /// <summary>
        /// Gets or sets a double representing the target render frequency, in Herz.
        /// </summary>
        /// <remarks>
        /// <para>A value of 0.0 indicates that RenderFrame events are generated at the maximum possible frequency (i.e. only limited by the hardware's capabilities).</para>
        /// <para>Values lower than 1.0Hz are clamped to 1.0Hz. Values higher than 200.0Hz are clamped to 200.0Hz.</para>
        /// </remarks>
        public double TargetRenderFrequency
        {
            get
            {
                EnsureUndisposed();
                if (TargetRenderPeriod == 0.0)
                    return 0.0;
                return 1.0 / TargetRenderPeriod;
            }
            set
            {
                EnsureUndisposed();
                if (value < 1.0)
                {
                    TargetRenderPeriod = 0.0;
                }
                else if (value <= 200.0)
                {
                    TargetRenderPeriod = 1.0 / value;
                }
                else Debug.Print("Target render frequency clamped to 200.0Hz."); // TODO: Where is it actually performed?
            }
        }

        #endregion

        #region TargetRenderPeriod

        /// <summary>
        /// Gets or sets a double representing the target render period, in seconds.
        /// </summary>
        /// <remarks>
        /// <para>A value of 0.0 indicates that RenderFrame events are generated at the maximum possible frequency (i.e. only limited by the hardware's capabilities).</para>
        /// <para>Values lower than 0.005 seconds (200Hz) are clamped to 0.0. Values higher than 1.0 seconds (1Hz) are clamped to 1.0.</para>
        /// </remarks>
        public double TargetRenderPeriod
        {
            get
            {
                EnsureUndisposed();
                return target_render_period;
            }
            set
            {
                EnsureUndisposed();
                if (value <= 0.005)
                {
                    target_render_period = 0.0;
                }
                else if (value <= 1.0)
                {
                    target_render_period = value;
                }
                else Debug.Print("Target render period clamped to 1.0 seconds.");
            }
        }

        #endregion

        #region TargetUpdateFrequency

        /// <summary>
        /// Gets or sets a double representing the target update frequency, in Herz.
        /// </summary>
        /// <remarks>
        /// <para>A value of 0.0 indicates that UpdateFrame events are generated at the maximum possible frequency (i.e. only limited by the hardware's capabilities).</para>
        /// <para>Values lower than 1.0Hz are clamped to 1.0Hz. Values higher than 200.0Hz are clamped to 200.0Hz.</para>
        /// </remarks>
        public double TargetUpdateFrequency
        {
            get
            {
                EnsureUndisposed();
                if (TargetUpdatePeriod == 0.0)
                    return 0.0;
                return 1.0 / TargetUpdatePeriod;
            }
            set
            {
                EnsureUndisposed();
                if (value < 1.0)
                {
                    TargetUpdatePeriod = 0.0;
                }
                else if (value <= 200.0)
                {
                    TargetUpdatePeriod = 1.0 / value;
                }
                else Debug.Print("Target update frequency clamped to 200.0Hz."); // TODO: Where is it actually performed?
            }
        }

        #endregion

        #region TargetUpdatePeriod

        /// <summary>
        /// Gets or sets a double representing the target update period, in seconds.
        /// </summary>
        /// <remarks>
        /// <para>A value of 0.0 indicates that UpdateFrame events are generated at the maximum possible frequency (i.e. only limited by the hardware's capabilities).</para>
        /// <para>Values lower than 0.005 seconds (200Hz) are clamped to 0.0. Values higher than 1.0 seconds (1Hz) are clamped to 1.0.</para>
        /// </remarks>
        public double TargetUpdatePeriod
        {
            get
            {
                EnsureUndisposed();
                return target_update_period;
            }
            set
            {
                EnsureUndisposed();
                if (value <= 0.005)
                {
                    target_update_period = 0.0;
                }
                else if (value <= 1.0)
                {
                    target_update_period = value;
                }
                else Debug.Print("Target update period clamped to 1.0 seconds."); // TODO: Where is it actually performed?
            }
        }

        #endregion

        #region UpdateFrequency

        /// <summary>
        /// Gets a double representing the frequency of UpdateFrame events, in Herz.
        /// </summary>
        public double UpdateFrequency
        {
            get
            {
                EnsureUndisposed();
                if (update_period == 0.0)
                    return 1.0;
                return 1.0 / update_period;
            }
        }

        #endregion

        #region UpdatePeriod

        /// <summary>
        /// Gets a double representing the period of UpdateFrame events, in seconds.
        /// </summary>
        public double UpdatePeriod
        {
            get
            {
                EnsureUndisposed();
                return update_period;
            }
        }

        #endregion

        #region UpdateTime

        /// <summary>
        /// Gets a double representing the time spent in the UpdateFrame function, in seconds.
        /// </summary>
        public double UpdateTime
        {
            get
            {
                EnsureUndisposed();
                return update_time;
            }
        }

        #endregion

        #endregion

        #region VSync

        /// <summary>
        /// Gets or sets the VSyncMode.
        /// </summary>
        public VSyncMode VSync
        {
            get
            {
                EnsureUndisposed();
                GraphicsContext.Assert();
                return vsync;
            }
            set
            {
                EnsureUndisposed();
                GraphicsContext.Assert();
                Context.VSync = (vsync = value) != VSyncMode.Off;
            }
        }

        #endregion

        #endregion

        #region Events

        /// <summary>
        /// Occurs before the window is displayed for the first time.
        /// </summary>
        public event EventHandler<EventArgs> Load;

        /// <summary>
        /// Occurs when it is time to render a frame.
        /// </summary>
        public event EventHandler<FrameEventArgs> RenderFrame;

        /// <summary>
        /// Occurs before the window is destroyed.
        /// </summary>
        public event EventHandler<EventArgs> Unload;

        /// <summary>
        /// Occurs when it is time to update a frame.
        /// </summary>
        public event EventHandler<FrameEventArgs> UpdateFrame;

        #endregion

        #endregion

        #region --- Protected Members ---

        #region Dispose

        /// <summary>
        /// Override to add custom cleanup logic.
        /// </summary>
        /// <param name="manual">True, if this method was called by the application; false if this was called by the finalizer thread.</param>
        protected virtual void Dispose(bool manual) { }

        #endregion

        #region OnRenderFrame

        /// <summary>
        /// Override in derived classes to render a frame.
        /// </summary>
        /// <param name="e">Contains information necessary for frame rendering.</param>
        /// <remarks>
        /// The base implementation (base.OnRenderFrame) is empty, there is no need to call it.
        /// </remarks>
        protected virtual void OnRenderFrame(FrameEventArgs e) { }

        #endregion

        #region OnUpdateFrame

        /// <summary>
        /// Override in derived classes to update a frame.
        /// </summary>
        /// <param name="e">Contains information necessary for frame updating.</param>
        /// <remarks>
        /// The base implementation (base.OnUpdateFrame) is empty, there is no need to call it.
        /// </remarks>
        protected virtual void OnUpdateFrame(FrameEventArgs e) { }

        #endregion

        #region OnWindowInfoChanged

        /// <summary>
        /// Called when the WindowInfo for this GameWindow has changed.
        /// </summary>
        /// <param name="e">Not used.</param>
        protected virtual void OnWindowInfoChanged(EventArgs e) { }

        #endregion

        #endregion

        #region --- Assembly Members ---

        #region Methods

        #region ExitAsync

        // Gracefully exits the GameWindow. May be called from any thread.
        void ExitAsync()
        {
            HasMainLoop = false;
            isExiting = true;
            //UpdateFrame += delegate
            //{
            //    ExitInternal();
            //};
        }

        #endregion

        #region ExitInternal

        // Stops the main loop, if one exists.
        void ExitInternal()
        {
            if (HasMainLoop)
            {
                throw new GameWindowExitException();
            }
        }

        #endregion

        #endregion

        #region Properties

        #region HasMainLoop

        bool HasMainLoop
        {
            get { return hasMainLoop; }
            set { hasMainLoop = value; }
        }

        #endregion

        #endregion

        #endregion

        #region --- Private Members ---

        #region OnLoadInternal

        /// <summary>
        /// Raises the Load event, and calls the user's OnLoad override.
        /// </summary>
        /// <param name="e">The event data.</param>
        private void OnLoadInternal(EventArgs e)
        {
            OnResize(EventArgs.Empty);

            if (Load != null) Load(this, e);

            OnLoad(e);
        }

        #endregion

        #region OnRenderFrameInternal

        private void OnRenderFrameInternal(FrameEventArgs e)
        {
            EnsureUndisposed();

            if (!this.Exists || this.IsExiting) return; // TODO: Redundant because of EnsureUndisposed.

            if (RenderFrame != null) RenderFrame(this, e);

            OnRenderFrame(e);
        }

        #endregion

        #region OnUnloadInternal

        /// <summary>
        /// Raises the Unload event, and calls the user's OnUnload override.
        /// </summary>
        /// <param name="e">The event data.</param>
        private void OnUnloadInternal(EventArgs e)
        {
            if (Unload != null) Unload(this, e);

            OnUnload(e);
        }

        #endregion

        #region OnUpdateFrameInternal

        private void OnUpdateFrameInternal(FrameEventArgs e)
        {
            EnsureUndisposed();

            if (!this.Exists || this.IsExiting) return; // TODO: Redundant because of EnsureUndisposed.

            if (UpdateFrame != null) UpdateFrame(this, e);

            OnUpdateFrame(e);
        }

        #endregion

        #region OnWindowInfoChangedInternal

        private void OnWindowInfoChangedInternal(EventArgs e)
        {
            glContext.MakeCurrent(WindowInfo);

            OnWindowInfoChanged(e);
        }

        #endregion

        #endregion

        ///// <summary>Finalizes unmanaged resources consumed by the GameWindow.</summary>
        //~GameWindow()
        //{
        //    Dispose(false);
        //    DisposeInternal(false);
        //}

    }

    #region public enum VSyncMode

    /// <summary>
    /// Enumerates available VSync modes.
    /// </summary>
    public enum VSyncMode
    {
        /// <summary>
        /// Vsync disabled.
        /// </summary>
        Off = 0,
        /// <summary>
        /// VSync enabled.
        /// </summary>
        On,
        /// <summary>
        /// VSync enabled, but automatically disabled if framerate falls below a specified limit.
        /// </summary>
        Adaptive,
    }

    #endregion

    #region --- GameWindow Exceptions ---

    [DebuggerNonUserCode]
    class GameWindowExitException : ApplicationException
    {
        public override string Message
        {
            get
            {
                return
@"This exception is a normal part of the GameWindow shutdown process and is completely harmless. While this warning will never be seen by end-users, Visual Studio reminds you that an exception is leaving your code unhandled, which can sometimes be a security breach.
You can disable this warning for this specific exception: select Debug->Exceptions from the menu bar and click ""Add"". Choose ""Common Language Runtime Exceptions"", type ""OpenTK.GameWindowExitException"" in the box below and click ""Ok"". Deselecting the ""User-unhandled"" checkbox from the newly created exception will disable this warning.
Alternatively, you can disable the ""Just my code"" debugging mode (""Tools->Options->Debugging->General"" and untick ""Enable Just my code (Managed only)"". This has the sideffect that it will allow you to step into OpenTK code if an error happens. Please, do this only if you are confident in your debugging skills.";
            }
        }
    }

    #endregion

}