//
// Core.cs: The core engine for gui.cs
//
// Authors:
//   Miguel de Icaza (miguel@gnome.org)
//
// Pending:
//   - Check for NeedDisplay on the hierarchy and repaint
//   - Layout support
//   - "Colors" type or "Attributes" type?
//   - What to surface as "BackgroundCOlor" when clearing a window, an attribute or colors?
//
// Optimziations
//   - Add rendering limitation to the exposed area
using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;
using System.Linq;
using NStack;
using System.ComponentModel;

namespace Terminal.Gui {

	/// <summary>
	/// Responder base class implemented by objects that want to participate on keyboard and mouse input.
	/// </summary>
	public class Responder {
		/// <summary>
		/// Gets or sets a value indicating whether this <see cref="T:Terminal.Gui.Responder"/> can focus.
		/// </summary>
		/// <value><c>true</c> if can focus; otherwise, <c>false</c>.</value>
		public virtual bool CanFocus { get; set; }

		/// <summary>
		/// Gets or sets a value indicating whether this <see cref="T:Terminal.Gui.Responder"/> has focus.
		/// </summary>
		/// <value><c>true</c> if has focus; otherwise, <c>false</c>.</value>
		public virtual bool HasFocus { get; internal set; }

		// Key handling
		/// <summary>
		///   This method can be overwritten by view that
		///     want to provide accelerator functionality
		///     (Alt-key for example).
		/// </summary>
		/// <remarks>
		///   <para>
		///     Before keys are sent to the subview on the
		///     current view, all the views are
		///     processed and the key is passed to the widgets
		///     to allow some of them to process the keystroke
		///     as a hot-key. </para>
		///  <para>
		///     For example, if you implement a button that
		///     has a hotkey ok "o", you would catch the
		///     combination Alt-o here.  If the event is
		///     caught, you must return true to stop the
		///     keystroke from being dispatched to other
		///     views.
		///  </para>
		/// </remarks>

		public virtual bool ProcessHotKey (KeyEvent kb)
		{
			return false;
		}

		/// <summary>
		///   If the view is focused, gives the view a
		///   chance to process the keystroke. 
		/// </summary>
		/// <remarks>
		///   <para>
		///     Views can override this method if they are
		///     interested in processing the given keystroke.
		///     If they consume the keystroke, they must
		///     return true to stop the keystroke from being
		///     processed by other widgets or consumed by the
		///     widget engine.    If they return false, the
		///     keystroke will be passed using the ProcessColdKey
		///     method to other views to process.
		///   </para>
		///   <para>
		///     The View implementation does nothing but return false,
		///     so it is not necessary to call base.ProcessKey if you 
		///     derive directly from View, but you should if you derive
		///     other View subclasses.
		///   </para>
		/// </remarks>
		/// <param name="keyEvent">Contains the details about the key that produced the event.</param>
		public virtual bool ProcessKey (KeyEvent keyEvent)
		{
			return false;
		}

		/// <summary>
		///   This method can be overwritten by views that
		///     want to provide accelerator functionality
		///     (Alt-key for example), but without
		///     interefering with normal ProcessKey behavior.
		/// </summary>
		/// <remarks>
		///   <para>
		///     After keys are sent to the subviews on the
		///     current view, all the view are
		///     processed and the key is passed to the views
		///     to allow some of them to process the keystroke
		///     as a cold-key. </para>
		///  <para>
		///    This functionality is used, for example, by
		///    default buttons to act on the enter key.
		///    Processing this as a hot-key would prevent
		///    non-default buttons from consuming the enter
		///    keypress when they have the focus.
		///  </para>
		/// </remarks>
		/// <param name="keyEvent">Contains the details about the key that produced the event.</param>
		public virtual bool ProcessColdKey (KeyEvent keyEvent)
		{
			return false;
		}

		/// <summary>
		/// Method invoked when a mouse event is generated
		/// </summary>
		/// <returns><c>true</c>, if the event was handled, <c>false</c> otherwise.</returns>
		/// <param name="mouseEvent">Contains the details about the mouse event.</param>
		public virtual bool MouseEvent (MouseEvent mouseEvent)
		{
			return false;
		}
	}

	/// <summary>
	/// Determines the LayoutStyle for a view, if Absolute, during LayoutSubviews, the
	/// value from the Frame will be used, if the value is Computer, then the Frame 
	/// will be updated from the X, Y Pos objets and the Width and Heigh Dim objects.
	/// </summary>
	public enum LayoutStyle {
		/// <summary>
		/// The position and size of the view are based on the Frame value.
		/// </summary>
		Absolute,

		/// <summary>
		/// The position and size of the view will be computed based on the
		/// X, Y, Width and Height properties and set on the Frame.
		/// </summary>
		Computed
	}

	/// <summary>
	/// View is the base class for all views on the screen and represents a visible element that can render itself and contains zero or more nested views.
	/// </summary>
	/// <remarks>
	/// <para>
	///    The View defines the base functionality for user interface elements in Terminal/gui.cs.  Views
	///    can contain one or more subviews, can respond to user input and render themselves on the screen.
	/// </para>
	/// <para>
	///    Views can either be created with an absolute position, by calling the constructor that takes a
	///    Rect parameter to specify the absolute position and size (the Frame of the View) or by setting the
	///    X, Y, Width and Height properties on the view.    Both approaches use coordinates that are relative 
	///    to the container they are being added to.
	/// </para>
	/// <para>
	///    When you do not specify a Rect frame you can use the more flexible 
	///    Dim and Pos objects that can dynamically update the position of a view.   
	///    The X and Y properties are of type <see cref="T:Terminal.Gui.Pos"/>
	///    and you can use either absolute positions, percentages or anchor
	///    points.   The Width and Height properties are of type 
	///    <see cref="T:Terminal.Gui.Dim"/> and can use absolute position, 
	///    percentages and anchors.  These are useful as they will take
	///    care of repositioning your views if your view's frames are resized
	///    or if the terminal size changes.
	/// </para>
	/// <para>
	///    When you specify the Rect parameter to a view, you are setting the LayoutStyle to Absolute, and the 
	///    view will always stay in the position that you placed it.   To change the position change the 
	///    Frame property to the new position.
	/// </para>
	/// <para>
	///    Subviews can be added to a View by calling the Add method.   The container of a view is the 
	///    Superview.
	/// </para>
	/// <para>
	///    Developers can call the SetNeedsDisplay method on the view to flag a region or the entire view
	///    as requiring to be redrawn.
	/// </para>
	/// <para>
	///    Views have a ColorScheme property that defines the default colors that subviews
	///    should use for rendering.   This ensures that the views fit in the context where
	///    they are being used, and allows for themes to be plugged in.   For example, the
	///    default colors for windows and toplevels uses a blue background, while it uses 
	///    a white background for dialog boxes and a red background for errors.
	/// </para>
	/// <para>
	///    If a ColorScheme is not set on a view, the result of the ColorScheme is the
	///    value of the SuperView and the value might only be valid once a view has been
	///    added to a SuperView, so your subclasses should not rely on ColorScheme being
	///    set at construction time.
	/// </para>
	/// <para>
	///    Using ColorSchemes has the advantage that your application will work both
	///    in color as well as black and white displays.
	/// </para>
	/// <para>
	///    Views that are focusable should implement the PositionCursor to make sure that
	///    the cursor is placed in a location that makes sense.   Unix terminals do not have
	///    a way of hiding the cursor, so it can be distracting to have the cursor left at 
	///    the last focused view.   So views should make sure that they place the cursor
	///    in a visually sensible place.
	/// </para>
	/// <para>
	///    The metnod LayoutSubviews is invoked when the size or layout of a view has
	///    changed.   The default processing system will keep the size and dimensions
	///    for views that use the LayoutKind.Absolute, and will recompute the
	///    frames for the vies that use LayoutKind.Computed.
	/// </para>
	/// </remarks>
	public class View : Responder, IEnumerable {
		internal enum Direction {
			Forward,
			Backward
		}

		View container = null;
		View focused = null;
		Direction focusDirection;

		internal Direction FocusDirection {
			get => SuperView?.FocusDirection ?? focusDirection;
			set {
				if (SuperView != null)
					SuperView.FocusDirection = value;
				else
					focusDirection = value;
			}
		}

		/// <summary>
		/// Points to the current driver in use by the view, it is a convenience property
		/// for simplifying the development of new views.
		/// </summary>
		public static ConsoleDriver Driver { get { return Application.Driver; } }

		static IList<View> empty = new List<View> (0).AsReadOnly ();

		// This is null, and allocated on demand.  
		List<View> subviews;

		/// <summary>
		/// This returns a list of the subviews contained by this view.
		/// </summary>
		/// <value>The subviews.</value>
		public IList<View> Subviews => subviews == null ? empty : subviews.AsReadOnly ();

		// Internally, we use InternalSubviews rather than subviews, as we do not expect us
		// to make the same mistakes our users make when they poke at the Subviews.
		internal IList<View> InternalSubviews => subviews ?? empty;

		internal Rect NeedDisplay { get; private set; } = Rect.Empty;

		// The frame for the object
		Rect frame;

		/// <summary>
		/// Gets or sets an identifier for the view;
		/// </summary>
		/// <value>The identifier.</value>
		public ustring Id { get; set; } = "";

		/// <summary>
		/// Gets or sets a value indicating whether this <see cref="T:Terminal.Gui.View"/> want mouse position reports.
		/// </summary>
		/// <value><c>true</c> if want mouse position reports; otherwise, <c>false</c>.</value>
		public virtual bool WantMousePositionReports { get; set; } = false;

		/// <summary>
		/// Gets or sets the frame for the view.
		/// </summary>
		/// <value>The frame.</value>
		/// <remarks>
		///    Altering the Frame of a view will trigger the redrawing of the 
		///    view as well as the redrawing of the affected regions in the superview.
		/// </remarks>
		public virtual Rect Frame {
			get => frame;
			set {
				if (SuperView != null) {
					SuperView.SetNeedsDisplay (frame);
					SuperView.SetNeedsDisplay (value);
				}
				frame = value;

				SetNeedsLayout ();
				SetNeedsDisplay (frame);
			}
		}

		/// <summary>
		/// Gets an enumerator that enumerates the subviews in this view.
		/// </summary>
		/// <returns>The enumerator.</returns>
		public IEnumerator GetEnumerator ()
		{
			foreach (var v in InternalSubviews)
				yield return v;
		}

		LayoutStyle layoutStyle;

		/// <summary>
		/// Controls how the view's Frame is computed during the LayoutSubviews method, if Absolute, then
		/// LayoutSubviews does not change the Frame properties, otherwise the Frame is updated from the
		/// values in X, Y, Width and Height properties.
		/// </summary>
		/// <value>The layout style.</value>
		public LayoutStyle LayoutStyle {
			get => layoutStyle;
			set {
				layoutStyle = value;
				SetNeedsLayout ();
			}
		}

		/// <summary>
		/// The bounds represent the View-relative rectangle used for this view.   Updates to the Bounds update the Frame, and has the same side effects as updating the frame.
		/// </summary>
		/// <value>The bounds.</value>
		public Rect Bounds {
			get => new Rect (Point.Empty, Frame.Size);
			set {
				Frame = new Rect (frame.Location, value.Size);
			}
		}

		Pos x, y;
		/// <summary>
		/// Gets or sets the X position for the view (the column).  This is only used when the LayoutStyle is Computed, if the
		/// LayoutStyle is set to Absolute, this value is ignored.
		/// </summary>
		/// <value>The X Position.</value>
		public Pos X {
			get => x;
			set {
				x = value;
				SetNeedsLayout ();
			}
		}

		/// <summary>
		/// Gets or sets the Y position for the view (line).  This is only used when the LayoutStyle is Computed, if the
		/// LayoutStyle is set to Absolute, this value is ignored.
		/// </summary>
		/// <value>The y position (line).</value>
		public Pos Y {
			get => y;
			set {
				y = value;
				SetNeedsLayout ();
			}
		}

		Dim width, height;

		/// <summary>
		/// Gets or sets the width for the view. This is only used when the LayoutStyle is Computed, if the
		/// LayoutStyle is set to Absolute, this value is ignored.
		/// </summary>
		/// <value>The width.</value>
		public Dim Width {
			get => width;
			set {
				width = value;
				SetNeedsLayout ();
			}
		}

		/// <summary>
		/// Gets or sets the height for the view. This is only used when the LayoutStyle is Computed, if the
		/// LayoutStyle is set to Absolute, this value is ignored.
		/// </summary>
		/// <value>The height.</value>
		public Dim Height {
			get => height;
			set {
				height = value;
				SetNeedsLayout ();
			}
		}

		/// <summary>
		/// Returns the container for this view, or null if this view has not been added to a container.
		/// </summary>
		/// <value>The super view.</value>
		public View SuperView => container;

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.View"/> class with the absolute
		/// dimensions specified in the frame.   If you want to have Views that can be positioned with
		/// Pos and Dim properties on X, Y, Width and Height, use the empty constructor.
		/// </summary>
		/// <param name="frame">The region covered by this view.</param>
		public View (Rect frame)
		{
			this.Frame = frame;
			CanFocus = false;
			LayoutStyle = LayoutStyle.Absolute;
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.View"/> class and sets the
		/// view up for Computed layout, which will use the values in X, Y, Width and Height to 
		/// compute the View's Frame.
		/// </summary>
		public View ()
		{
			CanFocus = false;
			LayoutStyle = LayoutStyle.Computed;
		}

		/// <summary>
		/// Invoke to flag that this view needs to be redisplayed, by any code
		/// that alters the state of the view.
		/// </summary>
		public void SetNeedsDisplay ()
		{
			SetNeedsDisplay (Bounds);
		}

		bool layoutNeeded = true;

		internal void SetNeedsLayout ()
		{
			if (layoutNeeded)
				return;
			layoutNeeded = true;
			if (SuperView == null)
				return;
			SuperView.layoutNeeded = true;
		}

		/// <summary>
		/// Flags the specified rectangle region on this view as needing to be repainted.
		/// </summary>
		/// <param name="region">The region that must be flagged for repaint.</param>
		public void SetNeedsDisplay (Rect region)
		{
			if (NeedDisplay.IsEmpty)
				NeedDisplay = region;
			else {
				var x = Math.Min (NeedDisplay.X, region.X);
				var y = Math.Min (NeedDisplay.Y, region.Y);
				var w = Math.Max (NeedDisplay.Width, region.Width);
				var h = Math.Max (NeedDisplay.Height, region.Height);
				NeedDisplay = new Rect (x, y, w, h);
			}
			if (container != null)
				container.ChildNeedsDisplay ();
			if (subviews == null)
				return;
			foreach (var view in subviews)
				if (view.Frame.IntersectsWith (region)) {
					var childRegion = Rect.Intersect (view.Frame, region);
					childRegion.X -= view.Frame.X;
					childRegion.Y -= view.Frame.Y;
					view.SetNeedsDisplay (childRegion);
				}
		}

		internal bool childNeedsDisplay;

		/// <summary>
		/// Flags this view for requiring the children views to be repainted.
		/// </summary>
		public void ChildNeedsDisplay ()
		{
			childNeedsDisplay = true;
			if (container != null)
				container.ChildNeedsDisplay ();
		}

		/// <summary>
		///   Adds a subview to this view.
		/// </summary>
		/// <remarks>
		/// </remarks>
		public virtual void Add (View view)
		{
			if (view == null)
				return;
			if (subviews == null)
				subviews = new List<View> ();
			subviews.Add (view);
			view.container = this;
			if (view.CanFocus)
				CanFocus = true;
			SetNeedsLayout ();
			SetNeedsDisplay ();
		}

		/// <summary>
		/// Adds the specified views to the view.
		/// </summary>
		/// <param name="views">Array of one or more views (can be optional parameter).</param>
		public void Add (params View [] views)
		{
			if (views == null)
				return;
			foreach (var view in views)
				Add (view);
		}

		/// <summary>
		///   Removes all the widgets from this container.
		/// </summary>
		/// <remarks>
		/// </remarks>
		public virtual void RemoveAll ()
		{
			if (subviews == null)
				return;

			while (subviews.Count > 0) {
				Remove (subviews[0]);
			}
		}

		/// <summary>
		///   Removes a widget from this container.
		/// </summary>
		/// <remarks>
		/// </remarks>
		public virtual void Remove (View view)
		{
			if (view == null || subviews == null)
				return;

			SetNeedsLayout ();
			SetNeedsDisplay ();
			var touched = view.Frame;
			subviews.Remove (view);
			view.container = null;

			if (subviews.Count < 1)
				this.CanFocus = false;

			foreach (var v in subviews) {
				if (v.Frame.IntersectsWith (touched))
					view.SetNeedsDisplay ();
			}
		}

		void PerformActionForSubview (View subview, Action<View> action)
		{
			if (subviews.Contains (subview)) {
				action (subview);
			}

			SetNeedsDisplay ();
			subview.SetNeedsDisplay ();
		}

		/// <summary>
		/// Brings the specified subview to the front so it is drawn on top of any other views.
		/// </summary>
		/// <param name="subview">The subview to send to the front</param>
		/// <remarks>
		///   <seealso cref="SendSubviewToBack"/>.
		/// </remarks>
		public void BringSubviewToFront (View subview)
		{
			PerformActionForSubview (subview, x => {
				subviews.Remove (x);
				subviews.Add (x);
			});
		}

		/// <summary>
		/// Sends the specified subview to the front so it is the first view drawn
		/// </summary>
		/// <param name="subview">The subview to send to the front</param>
		/// <remarks>
		///   <seealso cref="BringSubviewToFront(View)"/>.
		/// </remarks>
		public void SendSubviewToBack (View subview)
		{
			PerformActionForSubview (subview, x => {
				subviews.Remove (x);
				subviews.Insert (0, subview);
			});
		}

		/// <summary>
		/// Moves the subview backwards in the hierarchy, only one step
		/// </summary>
		/// <param name="subview">The subview to send backwards</param>
		/// <remarks>
		/// If you want to send the view all the way to the back use SendSubviewToBack.
		/// </remarks>
		public void SendSubviewBackwards (View subview)
		{
			PerformActionForSubview (subview, x => {
				var idx = subviews.IndexOf (x);
				if (idx > 0) {
					subviews.Remove (x);
					subviews.Insert (idx - 1, x);
				}
			});
		}

		/// <summary>
		/// Moves the subview backwards in the hierarchy, only one step
		/// </summary>
		/// <param name="subview">The subview to send backwards</param>
		/// <remarks>
		/// If you want to send the view all the way to the back use SendSubviewToBack.
		/// </remarks>
		public void BringSubviewForward (View subview)
		{
			PerformActionForSubview (subview, x => {
				var idx = subviews.IndexOf (x);
				if (idx+1 < subviews.Count) {
					subviews.Remove (x);
					subviews.Insert (idx+1, x);
				}
			});
		}

		/// <summary>
		///   Clears the view region with the current color.
		/// </summary>
		/// <remarks>
		///   <para>
		///     This clears the entire region used by this view.
		///   </para>
		/// </remarks>
		public void Clear ()
		{
			var h = Frame.Height;
			var w = Frame.Width;
			for (int line = 0; line < h; line++) {
				Move (0, line);
				for (int col = 0; col < w; col++)
					Driver.AddRune (' ');
			}
		}

		/// <summary>
		///   Clears the specfied rectangular region with the current color
		/// </summary>
		public void Clear (Rect r)
		{
			var h = r.Height;
			var w = r.Width;
			for (int line = 0; line < h; line++) {
				Move (0, line);
				for (int col = 0; col < w; col++)
					Driver.AddRune (' ');
			}
		}

		/// <summary>
		/// Converts the (col,row) position from the view into a screen (col,row).  The values are clamped to (0..ScreenDim-1)
		/// </summary>
		/// <param name="col">View-based column.</param>
		/// <param name="row">View-based row.</param>
		/// <param name="rcol">Absolute column, display relative.</param>
		/// <param name="rrow">Absolute row, display relative.</param>
		/// <param name="clipped">Whether to clip the result of the ViewToScreen method, if set to true, the rcol, rrow values are clamped to the screen dimensions.</param>
		internal void ViewToScreen (int col, int row, out int rcol, out int rrow, bool clipped = true)
		{
			// Computes the real row, col relative to the screen.
			rrow = row + frame.Y;
			rcol = col + frame.X;
			var ccontainer = container;
			while (ccontainer != null) {
				rrow += ccontainer.frame.Y;
				rcol += ccontainer.frame.X;
				ccontainer = ccontainer.container;
			}

			// The following ensures that the cursor is always in the screen boundaries.
			if (clipped) {
				rrow = Math.Max (0, Math.Min (rrow, Driver.Rows - 1));
				rcol = Math.Max (0, Math.Min (rcol, Driver.Cols - 1));
			}
		}

		/// <summary>
		/// Converts a point from screen coordinates into the view coordinate space.
		/// </summary>
		/// <returns>The mapped point.</returns>
		/// <param name="x">X screen-coordinate point.</param>
		/// <param name="y">Y screen-coordinate point.</param>
		public Point ScreenToView (int x, int y)
		{
			if (SuperView == null) {
				return new Point (x - Frame.X, y - frame.Y);
			} else {
				var parent = SuperView.ScreenToView (x, y);
				return new Point (parent.X - frame.X, parent.Y - frame.Y);
			}
		}

		// Converts a rectangle in view coordinates to screen coordinates.
		Rect RectToScreen (Rect rect)
		{
			ViewToScreen (rect.X, rect.Y, out var x, out var y, clipped: false);
			return new Rect (x, y, rect.Width, rect.Height);
		}

		// Clips a rectangle in screen coordinates to the dimensions currently available on the screen
		Rect ScreenClip (Rect rect)
		{
			var x = rect.X < 0 ? 0 : rect.X;
			var y = rect.Y < 0 ? 0 : rect.Y;
			var w = rect.X + rect.Width >= Driver.Cols ? Driver.Cols - rect.X : rect.Width;
			var h = rect.Y + rect.Height >= Driver.Rows ? Driver.Rows - rect.Y : rect.Height;

			return new Rect (x, y, w, h);
		}

		/// <summary>
		/// Sets the Console driver's clip region to the current View's Bounds.
		/// </summary>
		/// <returns>The existing driver's Clip region, which can be then set by setting the Driver.Clip property.</returns>
		public Rect ClipToBounds ()
		{
			return SetClip (Bounds);
		}

		/// <summary>
		/// Sets the clipping region to the specified region, the region is view-relative
		/// </summary>
		/// <returns>The previous clip region.</returns>
		/// <param name="rect">Rectangle region to clip into, the region is view-relative.</param>
		public Rect SetClip (Rect rect)
		{
			var bscreen = RectToScreen (rect);
			var previous = Driver.Clip;
			Driver.Clip = ScreenClip (RectToScreen (Bounds));
			return previous;
		}

		/// <summary>
		/// Draws a frame in the current view, clipped by the boundary of this view
		/// </summary>
		/// <param name="rect">Rectangular region for the frame to be drawn.</param>
		/// <param name="padding">The padding to add to the drawn frame.</param>
		/// <param name="fill">If set to <c>true</c> it fill will the contents.</param>
		public void DrawFrame (Rect rect, int padding = 0, bool fill = false)
		{
			var scrRect = RectToScreen (rect);
			var savedClip = Driver.Clip;
			Driver.Clip = ScreenClip (RectToScreen (Bounds));
			Driver.DrawFrame (scrRect, padding, fill);
			Driver.Clip = savedClip;
		}

		/// <summary>
		/// Utility function to draw strings that contain a hotkey
		/// </summary>
		/// <param name="text">String to display, the underscoore before a letter flags the next letter as the hotkey.</param>
		/// <param name="hotColor">Hot color.</param>
		/// <param name="normalColor">Normal color.</param>
		public void DrawHotString (ustring text, Attribute hotColor, Attribute normalColor)
		{
			Driver.SetAttribute (normalColor);
			foreach (var rune in text) {
				if (rune == '_') {
					Driver.SetAttribute (hotColor);
					continue;
				}
				Driver.AddRune (rune);
				Driver.SetAttribute (normalColor);
			}
		}

		/// <summary>
		/// Utility function to draw strings that contains a hotkey using a colorscheme and the "focused" state.
		/// </summary>
		/// <param name="text">String to display, the underscoore before a letter flags the next letter as the hotkey.</param>
		/// <param name="focused">If set to <c>true</c> this uses the focused colors from the color scheme, otherwise the regular ones.</param>
		/// <param name="scheme">The color scheme to use.</param>
		public void DrawHotString (ustring text, bool focused, ColorScheme scheme)
		{
			if (focused)
				DrawHotString (text, scheme.HotFocus, scheme.Focus);
			else
				DrawHotString (text, scheme.HotNormal, scheme.Normal);
		}

		/// <summary>
		/// This moves the cursor to the specified column and row in the view.
		/// </summary>
		/// <returns>The move.</returns>
		/// <param name="col">Col.</param>
		/// <param name="row">Row.</param>
		public void Move (int col, int row)
		{
			ViewToScreen (col, row, out var rcol, out var rrow);
			Driver.Move (rcol, rrow);
		}

		/// <summary>
		///   Positions the cursor in the right position based on the currently focused view in the chain.
		/// </summary>
		public virtual void PositionCursor ()
		{
			if (focused != null)
				focused.PositionCursor ();
			else
				Move (frame.X, frame.Y);
		}

		/// <summary>
		/// Gets or sets a value indicating whether this <see cref="T:Terminal.Gui.View"/> has focus.
		/// </summary>
		/// <value><c>true</c> if has focus; otherwise, <c>false</c>.</value>
		public override bool HasFocus {
			get {
				return base.HasFocus;
			}
			internal set {
				if (base.HasFocus != value)
					SetNeedsDisplay ();
				base.HasFocus = value;

				// Remove focus down the chain of subviews if focus is removed
				if (value == false && focused != null) {
					focused.HasFocus = false;
					focused = null;
				}
			}
		}

		/// <summary>
		/// Returns the currently focused view inside this view, or null if nothing is focused.
		/// </summary>
		/// <value>The focused.</value>
		public View Focused => focused;

		/// <summary>
		/// Returns the most focused view in the chain of subviews (the leaf view that has the focus).
		/// </summary>
		/// <value>The most focused.</value>
		public View MostFocused {
			get {
				if (Focused == null)
					return null;
				var most = Focused.MostFocused;
				if (most != null)
					return most;
				return Focused;
			}
		}

		/// <summary>
		/// The color scheme for this view, if it is not defined, it returns the parent's
		/// color scheme.
		/// </summary>
		public ColorScheme ColorScheme {
			get {
				if (colorScheme == null)
					return SuperView?.ColorScheme;
				return colorScheme;
			}
			set {
				colorScheme = value;
			}
		}

		ColorScheme colorScheme;

		/// <summary>
		/// Displays the specified character in the specified column and row.
		/// </summary>
		/// <param name="col">Col.</param>
		/// <param name="row">Row.</param>
		/// <param name="ch">Ch.</param>
		public void AddRune (int col, int row, Rune ch)
		{
			if (row < 0 || col < 0)
				return;
			if (row > frame.Height - 1 || col > frame.Width - 1)
				return;
			Move (col, row);
			Driver.AddRune (ch);
		}

		/// <summary>
		/// Removes the SetNeedsDisplay and the ChildNeedsDisplay setting on this view.
		/// </summary>
		protected void ClearNeedsDisplay ()
		{
			NeedDisplay = Rect.Empty;
			childNeedsDisplay = false;
		}

		/// <summary>
		/// Performs a redraw of this view and its subviews, only redraws the views that have been flagged for a re-display.
		/// </summary>
		/// <param name="region">The region to redraw, this is relative to the view itself.</param>
		/// <remarks>
		/// <para>
		///    Views should set the color that they want to use on entry, as otherwise this will inherit
		///    the last color that was set globaly on the driver.
		/// </para>
		/// </remarks>
		public virtual void Redraw (Rect region)
		{
			var clipRect = new Rect (Point.Empty, frame.Size);

			if (subviews != null) {
				foreach (var view in subviews) {
					if (!view.NeedDisplay.IsEmpty || view.childNeedsDisplay) {
						if (view.Frame.IntersectsWith (clipRect) && view.Frame.IntersectsWith (region)) {

							// TODO: optimize this by computing the intersection of region and view.Bounds
							view.Redraw (view.Bounds);
						}
						view.NeedDisplay = Rect.Empty;
						view.childNeedsDisplay = false;
					}
				}
			}
			ClearNeedsDisplay ();
		}

		/// <summary>
		/// Focuses the specified sub-view.
		/// </summary>
		/// <param name="view">View.</param>
		public void SetFocus (View view)
		{
			if (view == null)
				return;
			//Console.WriteLine ($"Request to focus {view}");
			if (!view.CanFocus)
				return;
			if (focused == view)
				return;

			// Make sure that this view is a subview
			View c;
			for (c = view.container; c != null; c = c.container)
				if (c == this)
					break;
			if (c == null)
				throw new ArgumentException ("the specified view is not part of the hierarchy of this view");

			if (focused != null)
				focused.HasFocus = false;

			focused = view;
			focused.HasFocus = true;
			focused.EnsureFocus ();

			// Send focus upwards
			SuperView?.SetFocus(this);
		}

		/// <param name="keyEvent">Contains the details about the key that produced the event.</param>
		public override bool ProcessKey (KeyEvent keyEvent)
		{
			if (Focused?.ProcessKey (keyEvent) == true)
				return true;

			return false;
		}

		/// <param name="keyEvent">Contains the details about the key that produced the event.</param>
		public override bool ProcessHotKey (KeyEvent keyEvent)
		{
			if (subviews == null || subviews.Count == 0)
				return false;
			foreach (var view in subviews)
				if (view.ProcessHotKey (keyEvent))
					return true;
			return false;
		}

		/// <param name="keyEvent">Contains the details about the key that produced the event.</param>
		public override bool ProcessColdKey (KeyEvent keyEvent)
		{
			if (subviews == null || subviews.Count == 0)
				return false;
			foreach (var view in subviews)
				if (view.ProcessColdKey (keyEvent))
					return true;
			return false;
		}

		/// <summary>
		/// Finds the first view in the hierarchy that wants to get the focus if nothing is currently focused, otherwise, it does nothing.
		/// </summary>
		public void EnsureFocus ()
		{
			if (focused == null)
				if (FocusDirection == Direction.Forward)
					FocusFirst ();
				else
					FocusLast ();
		}

		/// <summary>
		/// Focuses the first focusable subview if one exists.
		/// </summary>
		public void FocusFirst ()
		{
			if (subviews == null) {
				SuperView?.SetFocus (this);
				return;
			}

			foreach (var view in subviews) {
				if (view.CanFocus) {
					SetFocus (view);
					return;
				}
			}
		}

		/// <summary>
		/// Focuses the last focusable subview if one exists.
		/// </summary>
		public void FocusLast ()
		{
			if (subviews == null) {
				SuperView?.SetFocus(this);
				return;
			}

			for (int i = subviews.Count; i > 0;) {
				i--;

				View v = subviews [i];
				if (v.CanFocus) {
					SetFocus (v);
					return;
				}
			}
		}

		/// <summary>
		/// Focuses the previous view.
		/// </summary>
		/// <returns><c>true</c>, if previous was focused, <c>false</c> otherwise.</returns>
		public bool FocusPrev ()
		{
			FocusDirection = Direction.Backward;
			if (subviews == null || subviews.Count == 0)
				return false;

			if (focused == null) {
				FocusLast ();
				return focused != null;
			}
			int focused_idx = -1;
			for (int i = subviews.Count; i > 0;) {
				i--;
				View w = subviews [i];

				if (w.HasFocus) {
					if (w.FocusPrev ())
						return true;
					focused_idx = i;
					continue;
				}
				if (w.CanFocus && focused_idx != -1) {
					focused.HasFocus = false;

					if (w != null && w.CanFocus)
						w.FocusLast ();

					SetFocus (w);
				return true;
				}
			}
			if (focused != null) {
				focused.HasFocus = false;
				focused = null;
			}
			return false;
		}

		/// <summary>
		/// Focuses the next view.
		/// </summary>
		/// <returns><c>true</c>, if next was focused, <c>false</c> otherwise.</returns>
		public bool FocusNext ()
		{
			FocusDirection = Direction.Forward;
			if (subviews == null || subviews.Count == 0)
				return false;

			if (focused == null) {
				FocusFirst ();
				return focused != null;
			}
			int n = subviews.Count;
			int focused_idx = -1;
			for (int i = 0; i < n; i++) {
				View w = subviews [i];

				if (w.HasFocus) {
					if (w.FocusNext ())
						return true;
					focused_idx = i;
					continue;
				}
				if (w.CanFocus && focused_idx != -1) {
					focused.HasFocus = false;

					if (w != null && w.CanFocus)
						w.FocusFirst ();

					SetFocus (w);
					return true;
				}
			}
			if (focused != null) {
				focused.HasFocus = false;
				focused = null;
			}
			return false;
		}

		/// <summary>
		/// Computes the RelativeLayout for the view, given the frame for its container.
		/// </summary>
		/// <param name="hostFrame">The Frame for the host.</param>
		internal void RelativeLayout (Rect hostFrame)
		{
			int w, h, _x, _y;

			if (x is Pos.PosCenter) {
				if (width == null)
					w = hostFrame.Width;
				else
					w = width.Anchor (hostFrame.Width);
				_x = x.Anchor (hostFrame.Width - w);
			} else {
				if (x == null)
					_x = 0;
				else
					_x = x.Anchor (hostFrame.Width);
				if (width == null)
					w = hostFrame.Width;
				else
					w = width.Anchor (hostFrame.Width - _x);
			}

			if (y is Pos.PosCenter) {
				if (height == null)
					h = hostFrame.Height;
				else
					h = height.Anchor (hostFrame.Height);
				_y = y.Anchor (hostFrame.Height - h);
			} else {
				if (y == null)
					_y = 0;
				else
					_y = y.Anchor (hostFrame.Height);
				if (height == null)
					h = hostFrame.Height;
				else
					h = height.Anchor (hostFrame.Height - _y);
			}
			Frame = new Rect (_x, _y, w, h);
		}

		// https://en.wikipedia.org/wiki/Topological_sorting
		static List<View> TopologicalSort (HashSet<View> nodes, HashSet<(View, View)> edges)
		{
			var result = new List<View> ();

			// Set of all nodes with no incoming edges
			var S = new HashSet<View> (nodes.Where (n => edges.All (e => e.Item2.Equals (n) == false)));

			while (S.Any ()) {
				//  remove a node n from S
				var n = S.First ();
				S.Remove (n);

				// add n to tail of L
				result.Add (n);

				// for each node m with an edge e from n to m do
				foreach (var e in edges.Where (e => e.Item1.Equals (n)).ToList ()) {
					var m = e.Item2;

					// remove edge e from the graph
					edges.Remove (e);

					// if m has no other incoming edges then
					if (edges.All (me => me.Item2.Equals (m) == false)) {
						// insert m into S
						S.Add (m);
					}
				}
			}

			// if graph has edges then
			if (edges.Any ()) {
				// return error (graph has at least one cycle)
				return null;
			} else {
				// return L (a topologically sorted order)
				return result;
			}
		}

		/// <summary>
		/// This virtual method is invoked when a view starts executing or 
		/// when the dimensions of the view have changed, for example in 
		/// response to the container view or terminal resizing.
		/// </summary>
		public virtual void LayoutSubviews ()
		{
			if (!layoutNeeded)
				return;

			// Sort out the dependencies of the X, Y, Width, Height properties
			var nodes = new HashSet<View> ();
			var edges = new HashSet<(View, View)> ();

			foreach (var v in InternalSubviews) {
				nodes.Add (v);
				if (v.LayoutStyle == LayoutStyle.Computed) {
					if (v.X is Pos.PosView)
						edges.Add ((v, (v.X as Pos.PosView).Target));
					if (v.Y is Pos.PosView)
						edges.Add ((v, (v.Y as Pos.PosView).Target));
					if (v.Width is Dim.DimView)
						edges.Add ((v, (v.Width as Dim.DimView).Target));
					if (v.Height is Dim.DimView)
						edges.Add ((v, (v.Height as Dim.DimView).Target));
				}
			}

			var ordered = TopologicalSort (nodes, edges);
			ordered.Reverse ();
			if (ordered == null)
				throw new Exception ("There is a recursive cycle in the relative Pos/Dim in the views of " + this);

			foreach (var v in ordered) {
				if (v.LayoutStyle == LayoutStyle.Computed)
					v.RelativeLayout (Frame);

				v.LayoutSubviews ();
				v.layoutNeeded = false;
			}
			layoutNeeded = false;
		}

		/// <summary>
		/// Returns a <see cref="T:System.String"/> that represents the current <see cref="T:Terminal.Gui.View"/>.
		/// </summary>
		/// <returns>A <see cref="T:System.String"/> that represents the current <see cref="T:Terminal.Gui.View"/>.</returns>
		public override string ToString ()
		{
			return $"{GetType ().Name}({Id})({Frame})";
		}
	}

	/// <summary>
	/// Toplevel views can be modally executed.
	/// </summary>
	/// <remarks>
	///   <para>
	///     Toplevels can be modally executing views, and they return control
	///     to the caller when the "Running" property is set to false, or
	///     by calling <see cref="M:Terminal.Gui.Application.RequestStop()"/>
	///   </para>
	///   <para>
	///     There will be a toplevel created for you on the first time use
	///     and can be accessed from the property <see cref="P:Terminal.Gui.Application.Top"/>,
	///     but new toplevels can be created and ran on top of it.   To run, create the
	///     toplevel and then invoke <see cref="M:Terminal.Gui.Application.Run"/> with the
	///     new toplevel.
	///   </para>
	///   <para>
	///     TopLevels can also opt-in to more sophisticated initialization 
	///     by implementing <see cref="ISupportInitialize"/>. When they do 
	///     so, the <see cref="ISupportInitialize.BeginInit"/> and 
	///     <see cref="ISupportInitialize.EndInit"/> methods will be called 
	///     before running the view.
	///     If first-run-only initialization is preferred, the <see cref="ISupportInitializeNotification"/> 
	///     can be implemented too, in which case the <see cref="ISupportInitialize"/> 
	///     methods will only be called if <see cref="ISupportInitializeNotification.IsInitialized"/> 
	///     is <see langword="false"/>. This allows proper View inheritance hierarchies 
	///     to override base class layout code optimally by doing so only on first run, 
	///     instead of on every run.
	///   </para>
	/// </remarks>
	public class Toplevel : View {
		/// <summary>
		/// This flag is checked on each iteration of the mainloop and it continues
		/// running until this flag is set to false.   
		/// </summary>
		public bool Running;

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Toplevel"/> class with the specified absolute layout.
		/// </summary>
		/// <param name="frame">Frame.</param>
		public Toplevel (Rect frame) : base (frame)
		{
			ColorScheme = Colors.Base;
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Toplevel"/> class with Computed layout, defaulting to <see langword="async"/> full screen.
		/// </summary>
		public Toplevel () : base ()
		{
			ColorScheme = Colors.Base;
			Width = Dim.Fill ();
			Height = Dim.Fill ();
		}

		/// <summary>
		/// Convenience factory method that creates a new toplevel with the current terminal dimensions.
		/// </summary>
		/// <returns>The create.</returns>
		public static Toplevel Create ()
		{
			return new Toplevel (new Rect (0, 0, Driver.Cols, Driver.Rows));
		}

		public override bool CanFocus {
			get => true;
		}

		/// <summary>
		/// Determines whether the <see cref="Toplevel"/> is modal or not. 
		/// Causes <see cref="ProcessKey(KeyEvent)"/> to propagate keys upwards 
		/// by default unless set to <see langword="true"/>.
		/// </summary>
		public bool Modal { get; set; }

		public override bool ProcessKey (KeyEvent keyEvent)
		{
			if (base.ProcessKey (keyEvent))
				return true;

			switch (keyEvent.Key) {
			case Key.ControlC:
				// TODO: stop current execution of this container
				break;
			case Key.ControlZ:
				Driver.Suspend ();
				return true;

#if false
			case Key.F5:
				Application.DebugDrawBounds = !Application.DebugDrawBounds;
				SetNeedsDisplay ();
				return true;
#endif
			case Key.Tab:
			case Key.CursorRight:
			case Key.CursorDown:
				var old = Focused;
				if (!FocusNext ())
					FocusNext ();
				if (old != Focused) {
					old?.SetNeedsDisplay ();
					Focused?.SetNeedsDisplay ();
				}
				return true;
			case Key.CursorLeft:
			case Key.CursorUp:
			case Key.BackTab:
				old = Focused;
				if (!FocusPrev ())
					FocusPrev ();
				if (old != Focused) {
					old?.SetNeedsDisplay ();
					Focused?.SetNeedsDisplay ();
				}
				return true;

			case Key.ControlL:
				Application.Refresh ();
				return true;
			}
			return false;
		}

		/// <summary>
		/// This method is invoked by Application.Begin as part of the Application.Run after
		/// the views have been laid out, and before the views are drawn for the first time.
		/// </summary>
		public virtual void WillPresent ()
		{
			FocusFirst ();
		}
	}

	/// <summary>
	/// A toplevel view that draws a frame around its region and has a "ContentView" subview where the contents are added.
	/// </summary>
	public class Window : Toplevel, IEnumerable {
		View contentView;
		ustring title;

		/// <summary>
		/// The title to be displayed for this window.
		/// </summary>
		/// <value>The title.</value>
		public ustring Title {
			get => title;
			set {
				title = value;
				SetNeedsDisplay ();
			}
		}

		class ContentView : View {
			public ContentView (Rect frame) : base (frame) { }
			public ContentView () : base () { }
#if false
			public override void Redraw (Rect region)
			{
				Driver.SetAttribute (ColorScheme.Focus);

				for (int y = 0; y < Frame.Height; y++) {
					Move (0, y);
					for (int x = 0; x < Frame.Width; x++) {

						Driver.AddRune ('x');
					}
				}
			}
#endif
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Gui.Window"/> class with an optional title and a set frame.
		/// </summary>
		/// <param name="frame">Frame.</param>
		/// <param name="title">Title.</param>
		public Window (Rect frame, ustring title = null) : this (frame, title, padding: 0)
		{
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Window"/> class with an optional title.
		/// </summary>
		/// <param name="title">Title.</param>
		public Window (ustring title = null) : this (title, padding: 0)
		{
		}

		int padding;
		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Window"/> with
		/// the specified frame for its location, with the specified border 
		/// an optional title.
		/// </summary>
		/// <param name="frame">Frame.</param>
		/// <param name="padding">Number of characters to use for padding of the drawn frame.</param>
		/// <param name="title">Title.</param>
		public Window (Rect frame, ustring title = null, int padding = 0) : base (frame)
		{
			this.Title = title;
			int wb = 2 * (1 + padding);
			this.padding = padding;
			var cFrame = new Rect (1 + padding, 1 + padding, frame.Width - wb, frame.Height - wb);
			contentView = new ContentView (cFrame);
			base.Add (contentView);
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:Terminal.Gui.Window"/> with
		/// the specified frame for its location, with the specified border 
		/// an optional title.
		/// </summary>
		/// <param name="padding">Number of characters to use for padding of the drawn frame.</param>
		/// <param name="title">Title.</param>
		public Window (ustring title = null, int padding = 0) : base ()
		{
			this.Title = title;
			int wb = 1 + padding;
			this.padding = padding;
 			contentView = new ContentView () {
				X = wb,
				Y = wb,
				Width = Dim.Fill (wb),
				Height = Dim.Fill (wb)
			};
			base.Add (contentView);
		}

		/// <summary>
		/// Enumerates the various views in the ContentView.
		/// </summary>
		/// <returns>The enumerator.</returns>
		public new IEnumerator GetEnumerator ()
		{
			return contentView.GetEnumerator ();
		}

		void DrawFrame ()
		{
			DrawFrame (new Rect (0, 0, Frame.Width, Frame.Height), padding, fill: true);
		}

		/// <summary>
		/// Add the specified view to the ContentView.
		/// </summary>
		/// <param name="view">View to add to the window.</param>
		public override void Add (View view)
		{
			contentView.Add (view);
			if (view.CanFocus)
				CanFocus = true;
		}


		/// <summary>
		///   Removes a widget from this container.
		/// </summary>
		/// <remarks>
		/// </remarks>
		public override void Remove (View view)
		{
			if (view == null)
				return;

			SetNeedsDisplay ();
			var touched = view.Frame;
			contentView.Remove (view);

			if (contentView.InternalSubviews.Count < 1)
				this.CanFocus = false;
		}

		/// <summary>
		///   Removes all widgets from this container.
		/// </summary>
		/// <remarks>
		/// </remarks>
		public override void RemoveAll ()
		{
			contentView.RemoveAll ();
		}

		public override void Redraw (Rect bounds)
		{
			if (!NeedDisplay.IsEmpty) {
				Driver.SetAttribute (ColorScheme.Normal);
				DrawFrame ();
				if (HasFocus)
					Driver.SetAttribute (ColorScheme.Normal);
				var width = Frame.Width;
				if (Title != null && width > 4) {
					Move (1+padding, padding);
					Driver.AddRune (' ');
					var str = Title.Length > width ? Title [0, width-4] : Title;
					Driver.AddStr (str);
					Driver.AddRune (' ');
				}
				Driver.SetAttribute (ColorScheme.Normal);
			}
			contentView.Redraw (contentView.Bounds);
			ClearNeedsDisplay ();
		}

#if true
		// 
		// It does not look like the event is raised on clicked-drag
		// need to figure that out.
		//
		Point? dragPosition;
		public override bool MouseEvent(MouseEvent mouseEvent)
		{
			// The code is currently disabled, because the 
			// Driver.UncookMouse does not seem to have an effect if there is 
			// a pending mouse event activated.
			if (true)
				return false;
			
			if ((mouseEvent.Flags == MouseFlags.Button1Pressed|| mouseEvent.Flags == MouseFlags.Button4Pressed)){
				
				if (dragPosition.HasValue) {
					var dx = mouseEvent.X - dragPosition.Value.X;
					var dy = mouseEvent.Y - dragPosition.Value.Y;

					var nx = Frame.X + dx;
					var ny = Frame.Y + dy;
					if (nx < 0)
						nx = 0;
					if (ny < 0)
						ny = 0;

					//Demo.ml2.Text = $"{dx},{dy}";
					dragPosition = new Point (mouseEvent.X, mouseEvent.Y);

					// TODO: optimize, only SetNeedsDisplay on the before/after regions.
					if (SuperView == null)
						Application.Refresh ();
					else
						SuperView.SetNeedsDisplay ();
					Frame = new Rect (nx, ny, Frame.Width, Frame.Height);
					SetNeedsDisplay ();
					return true;
				} else {
					// Only start grabbing if the user clicks on the title bar.
					if (mouseEvent.Y == 0) {
						dragPosition = new Point (mouseEvent.X, mouseEvent.Y);
						Application.GrabMouse (this);
					}

					//Demo.ml2.Text = $"Starting at {dragPosition}";
					return true;
				}
			}

			if (mouseEvent.Flags == MouseFlags.Button1Released) {
				Application.UngrabMouse ();
				Driver.UncookMouse ();

				dragPosition = null;
				//Driver.StopReportingMouseMoves ();
			}

			//Demo.ml.Text = me.ToString ();
			return false;
		}
#endif
	}

	/// <summary>
	/// The application driver for gui.cs
	/// </summary>
	/// <remarks>
	///   <para>
	///     You can hook up to the Iteration event to have your method 
	///     invoked on each iteration of the mainloop.
	///   </para>
	///   <para>
	///     Creates a mainloop to process input events, handle timers and
	///     other sources of data.   It is accessible via the MainLoop property.
	///   </para>
	///   <para>
	///     When invoked sets the SynchronizationContext to one that is tied
	///     to the mainloop, allowing user code to use async/await.
	///   </para>
	/// </remarks>
	public static class Application {
		/// <summary>
		/// The current Console Driver in use.
		/// </summary>
		public static ConsoleDriver Driver;

		/// <summary>
		/// The Toplevel object used for the application on startup.
		/// </summary>
		/// <value>The top.</value>
		public static Toplevel Top { get; private set; }

		/// <summary>
		/// The current toplevel object.   This is updated when Application.Run enters and leaves and points to the current toplevel.
		/// </summary>
		/// <value>The current.</value>
		public static Toplevel Current { get; private set; }

		/// <summary>
		/// The mainloop driver for the applicaiton
		/// </summary>
		/// <value>The main loop.</value>
		public static Mono.Terminal.MainLoop MainLoop { get; private set; }

		static Stack<Toplevel> toplevels = new Stack<Toplevel> ();

		/// <summary>
		///   This event is raised on each iteration of the
		///   main loop. 
		/// </summary>
		/// <remarks>
		///   See also <see cref="Timeout"/>
		/// </remarks>
		static public event EventHandler Iteration;

		/// <summary>
		/// Returns a rectangle that is centered in the screen for the provided size.
		/// </summary>
		/// <returns>The centered rect.</returns>
		/// <param name="size">Size for the rectangle.</param>
		public static Rect MakeCenteredRect (Size size)
		{
			return new Rect (new Point ((Driver.Cols - size.Width) / 2, (Driver.Rows - size.Height) / 2), size);
		}

		//
		// provides the sync context set while executing code in gui.cs, to let
		// users use async/await on their code
		//
		class MainLoopSyncContext : SynchronizationContext {
			Mono.Terminal.MainLoop mainLoop;

			public MainLoopSyncContext (Mono.Terminal.MainLoop mainLoop)
			{
				this.mainLoop = mainLoop;
			}

			public override SynchronizationContext CreateCopy ()
			{
				return new MainLoopSyncContext (MainLoop);
			}

			public override void Post (SendOrPostCallback d, object state)
			{
				mainLoop.AddIdle (() => {
					d (state);
					return false;
				});
			}

			public override void Send (SendOrPostCallback d, object state)
			{
				mainLoop.Invoke (() => {
					d (state);
				});
			}
		}

		/// <summary>
		/// If set, it forces the use of the System.Console-based driver.
		/// </summary>
		public static bool UseSystemConsole;

		/// <summary>
		/// Initializes the Application
		/// </summary>
		public static void Init () => Init (() => Toplevel.Create ());

		static bool _initialized = false;

		/// <summary>
		/// Initializes the Application
		/// </summary>
		static void Init (Func<Toplevel> topLevelFactory)
		{
			if (_initialized) return;
			_initialized = true;

			var p = Environment.OSVersion.Platform;
			Mono.Terminal.IMainLoopDriver mainLoopDriver;

			if (UseSystemConsole) {
				mainLoopDriver = new Mono.Terminal.NetMainLoop ();
				Driver = new NetDriver ();
			} else if (p == PlatformID.Win32NT || p == PlatformID.Win32S || p == PlatformID.Win32Windows){
				var windowsDriver = new WindowsDriver ();
				mainLoopDriver = windowsDriver;
				Driver = windowsDriver;
			} else {
				mainLoopDriver = new Mono.Terminal.UnixMainLoop ();
				Driver = new CursesDriver ();
			}
			Driver.Init (TerminalResized);
			MainLoop = new Mono.Terminal.MainLoop (mainLoopDriver);
			SynchronizationContext.SetSynchronizationContext (new MainLoopSyncContext (MainLoop));
			Top = topLevelFactory ();
			Current = Top;
		}

		/// <summary>
		/// Captures the execution state for the provided TopLevel view.
		/// </summary>
		public class RunState : IDisposable {
			internal RunState (Toplevel view)
			{
				Toplevel = view;
			}
			internal Toplevel Toplevel;

			/// <summary>
			/// Releases alTop = l resource used by the <see cref="T:Terminal.Gui.Application.RunState"/> object.
			/// </summary>
			/// <remarks>Call <see cref="Dispose()"/> when you are finished using the <see cref="T:Terminal.Gui.Application.RunState"/>. The
			/// <see cref="Dispose()"/> method leaves the <see cref="T:Terminal.Gui.Application.RunState"/> in an unusable state. After
			/// calling <see cref="Dispose()"/>, you must release all references to the
			/// <see cref="T:Terminal.Gui.Application.RunState"/> so the garbage collector can reclaim the memory that the
			/// <see cref="T:Terminal.Gui.Application.RunState"/> was occupying.</remarks>
			public void Dispose ()
			{
				Dispose (true);
				GC.SuppressFinalize (this);
			}

			/// <summary>
			/// Dispose the specified disposing.
			/// </summary>
			/// <returns>The dispose.</returns>
			/// <param name="disposing">If set to <c>true</c> disposing.</param>
			protected virtual void Dispose (bool disposing)
			{
				if (Toplevel != null) {
					End (Toplevel);
					Toplevel = null;
				}
			}
		}

		static void ProcessKeyEvent (KeyEvent ke)
		{
			var chain = toplevels.ToList();
			foreach (var topLevel in chain) {
				if (topLevel.ProcessHotKey (ke))
					return;
				if (topLevel.Modal)
					break;
			}

			foreach (var topLevel in chain) {
				if (topLevel.ProcessKey (ke))
					return;
				if (topLevel.Modal)
					break;
			}

			foreach (var topLevel in chain) {
				// Process the key normally
				if (topLevel.ProcessColdKey (ke))
					return;
				if (topLevel.Modal)
					break;
			}
		}

		static View FindDeepestView (View start, int x, int y, out int resx, out int resy)
		{
			var startFrame = start.Frame;

			if (!startFrame.Contains (x, y)) {
				resx = 0;
				resy = 0;
				return null;
			}

			if (start.InternalSubviews != null){
				int count = start.InternalSubviews.Count;
				if (count > 0) {
					var rx = x - startFrame.X;
					var ry = y - startFrame.Y;
					for (int i = count - 1; i >= 0; i--) {
						View v = start.InternalSubviews [i];
						if (v.Frame.Contains (rx, ry)) {
							var deep = FindDeepestView (v, rx, ry, out resx, out resy);
							if (deep == null)
								return v;
							return deep;
						}
					}
				}
			}
			resx = x-startFrame.X;
			resy = y-startFrame.Y;
			return start;
		}

		static View mouseGrabView;

		/// <summary>
		/// Grabs the mouse, forcing all mouse events to be routed to the specified view until UngrabMouse is called.
		/// </summary>
		/// <returns>The grab.</returns>
		/// <param name="view">View that will receive all mouse events until UngrabMouse is invoked.</param>
		public static void GrabMouse (View view)
		{
			if (view == null)
				return;
			mouseGrabView = view;
			Driver.UncookMouse ();
		}

		/// <summary>
		/// Releases the mouse grab, so mouse events will be routed to the view on which the mouse is.
		/// </summary>
		public static void UngrabMouse ()
		{
			mouseGrabView = null;
			Driver.CookMouse ();
		}

		/// <summary>
		/// Merely a debugging aid to see the raw mouse events
		/// </summary>
		static public Action<MouseEvent> RootMouseEvent;

		static void ProcessMouseEvent (MouseEvent me)
		{
			RootMouseEvent?.Invoke (me);
			if (mouseGrabView != null) {
				var newxy = mouseGrabView.ScreenToView (me.X, me.Y);
				var nme = new MouseEvent () {
					X = newxy.X,
					Y = newxy.Y,
					Flags = me.Flags
				};
				mouseGrabView.MouseEvent (me);
				return;
			}

			int rx, ry;
			var view = FindDeepestView (Current, me.X, me.Y, out rx, out ry);
			if (view != null) {
				if (!view.WantMousePositionReports && me.Flags == MouseFlags.ReportMousePosition)
					return;

				var nme = new MouseEvent () {
					X = rx,
					Y = ry,
					Flags = me.Flags
				};
				// Should we bubbled up the event, if it is not handled?
				view.MouseEvent (nme);
			}
		}

		/// <summary>
		/// Building block API: Prepares the provided toplevel for execution.
		/// </summary>
		/// <returns>The runstate handle that needs to be passed to the End() method upon completion.</returns>
		/// <param name="toplevel">Toplevel to prepare execution for.</param>
		/// <remarks>
		///  This method prepares the provided toplevel for running with the focus,
		///  it adds this to the list of toplevels, sets up the mainloop to process the 
		///  event, lays out the subviews, focuses the first element, and draws the
		///  toplevel in the screen.   This is usually followed by executing
		///  the <see cref="RunLoop"/> method, and then the <see cref="End(RunState)"/> method upon termination which will
		///   undo these changes.
		/// </remarks>
		static public RunState Begin (Toplevel toplevel)
		{
			if (toplevel == null)
				throw new ArgumentNullException (nameof (toplevel));
			var rs = new RunState (toplevel);

			Init ();
			if (toplevel is ISupportInitializeNotification initializableNotification && 
			    !initializableNotification.IsInitialized) {
				initializableNotification.BeginInit();
				initializableNotification.EndInit();
			} else if (toplevel is ISupportInitialize initializable) {
				initializable.BeginInit();
				initializable.EndInit();
			}
			toplevels.Push (toplevel);
			Current = toplevel;
			Driver.PrepareToRun (MainLoop, ProcessKeyEvent, ProcessMouseEvent);
			if (toplevel.LayoutStyle == LayoutStyle.Computed)
				toplevel.RelativeLayout (new Rect (0, 0, Driver.Cols, Driver.Rows));
			toplevel.LayoutSubviews ();
			toplevel.WillPresent ();
			Redraw (toplevel);
			toplevel.PositionCursor ();
			Driver.Refresh ();

			return rs;
		}

		/// <summary>
		/// Building block API: completes the exection of a Toplevel that was started with Begin.
		/// </summary>
		/// <param name="runState">The runstate returned by the <see cref="Begin(Toplevel)"/> method.</param>
		static public void End (RunState runState)
		{
			if (runState == null)
				throw new ArgumentNullException (nameof (runState));

			runState.Dispose ();
		}

		public static void Shutdown ()
		{
			Driver.End ();
			_initialized = false;
		}

		static void Redraw (View view)
		{
			view.Redraw (view.Bounds);
			Driver.Refresh ();
		}

		static void Refresh (View view)
		{
			view.Redraw (view.Bounds);
			Driver.Refresh ();
		}

		/// <summary>
		/// Triggers a refresh of the entire display.
		/// </summary>
		public static void Refresh ()
		{
			Driver.UpdateScreen ();
			View last = null;
			foreach (var v in toplevels.Reverse ()) {
				v.SetNeedsDisplay ();
				v.Redraw (v.Bounds);
				last = v;
			}
			last?.PositionCursor ();
			Driver.Refresh ();
		}

		internal static void End (View view)
		{
			if (toplevels.Peek () != view)
				throw new ArgumentException ("The view that you end with must be balanced");
			toplevels.Pop ();
			if (toplevels.Count == 0)
				Shutdown ();
			else
			{
				Current = toplevels.Peek();
				Refresh ();
			}
		}

		/// <summary>
		///   Building block API: Runs the main loop for the created dialog
		/// </summary>
		/// <remarks>
		///   Use the wait parameter to control whether this is a
		///   blocking or non-blocking call.   
		/// </remarks>
		/// <param name="state">The state returned by the Begin method.</param>
		/// <param name="wait">By default this is true which will execute the runloop waiting for events, if you pass false, you can use this method to run a single iteration of the events.</param>
		public static void RunLoop (RunState state, bool wait = true)
		{
			if (state == null)
				throw new ArgumentNullException (nameof (state));
			if (state.Toplevel == null)
				throw new ObjectDisposedException ("state");

			for (state.Toplevel.Running = true; state.Toplevel.Running;) {
				if (MainLoop.EventsPending (wait)) {
					MainLoop.MainIteration ();
					if (Iteration != null)
						Iteration (null, EventArgs.Empty);
				} else if (wait == false)
					return;
				if (!state.Toplevel.NeedDisplay.IsEmpty || state.Toplevel.childNeedsDisplay) {
					state.Toplevel.Redraw (state.Toplevel.Bounds);
					if (DebugDrawBounds)
						DrawBounds (state.Toplevel);
					state.Toplevel.PositionCursor ();
					Driver.Refresh ();
				} else
					Driver.UpdateCursor ();
			}
		}

		internal static bool DebugDrawBounds;

		// Need to look into why this does not work properly.
		static void DrawBounds (View v)
		{
			v.DrawFrame (v.Frame, padding: 0, fill: false);
			if (v.InternalSubviews != null && v.InternalSubviews.Count > 0)
				foreach (var sub in v.InternalSubviews)
					DrawBounds (sub);
		}

		/// <summary>
		/// Runs the application with the built-in toplevel view
		/// </summary>
		public static void Run ()
		{
			Run (Top);
		}

		/// <summary>
		/// Runs the application with a new instance of the specified toplevel view
		/// </summary>
		public static void Run<T> () where T : Toplevel, new()
		{
			Init (() => new T());
			Run (Top);
		}

		/// <summary>
		///   Runs the main loop on the given container.
		/// </summary>
		/// <remarks>
		///   <para>
		///     This method is used to start processing events
		///     for the main application, but it is also used to
		///     run modal dialog boxes.
		///   </para>
		///   <para>
		///     To make a toplevel stop execution, set the "Running"
		///     property to false.
		///   </para>
		///   <para>
		///     This is equivalent to calling Begin on the toplevel view, followed by RunLoop with the
		///     returned value, and then calling end on the return value.
		///   </para>
		///   <para>
		///     Alternatively, if your program needs to control the main loop and needs to 
		///     process events manually, you can invoke Begin to set things up manually and then
		///     repeatedly call RunLoop with the wait parameter set to false.   By doing this
		///     the RunLoop method will only process any pending events, timers, idle handlers and
		///     then return control immediately.
		///   </para>
		/// </remarks>
		public static void Run (Toplevel view)
		{
			var runToken = Begin (view);
			RunLoop (runToken);
			End (runToken);
		}

		/// <summary>
		/// Stops running the most recent toplevel
		/// </summary>
		public static void RequestStop ()
		{
			Current.Running = false;
		}

		static void TerminalResized ()
		{
			var full = new Rect (0, 0, Driver.Cols, Driver.Rows);
			Driver.Clip = full;
			foreach (var t in toplevels) {
				t.RelativeLayout (full);
				t.LayoutSubviews ();
			}
			Refresh ();
		}
	}
}
