gui-cs.Terminal.Gui/Terminal.Gui/View/ViewSubViews.cs

using System;
using System.Collections.Generic;

namespace Terminal.Gui; 

public partial class View {
	static readonly IList<View> _empty = new List<View> (0).AsReadOnly ();

	internal bool _addingView;

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

	View _superView;

	/// <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 virtual View SuperView {
		get => _superView;
		set => throw new NotImplementedException ();
	}

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

	// 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;

	/// <summary>
	/// Returns a value indicating if this View is currently on Top (Active)
	/// </summary>
	public bool IsCurrentTop => Application.Current == this;

	/// <summary>
	/// Indicates whether the view was added to <see cref="SuperView"/>.
	/// </summary>
	public bool IsAdded { get; private set; }

	/// <summary>
	/// Event fired when this view is added to another.
	/// </summary>
	public event EventHandler<SuperViewChangedEventArgs> Added;

	/// <summary>
	/// Adds a subview (child) to this view.
	/// </summary>
	/// <remarks>
	/// The Views that have been added to this view can be retrieved via the <see cref="Subviews"/> property.
	/// See also <seealso cref="Remove(View)"/> <seealso cref="RemoveAll"/>
	/// </remarks>
	public virtual void Add (View view)
	{
		if (view == null) {
			return;
		}
		if (_subviews == null) {
			_subviews = new List<View> ();
		}
		if (_tabIndexes == null) {
			_tabIndexes = new List<View> ();
		}
		_subviews.Add (view);
		_tabIndexes.Add (view);
		view._superView = this;
		if (view.CanFocus) {
			_addingView = true;
			if (SuperView?.CanFocus == false) {
				SuperView._addingView = true;
				SuperView.CanFocus    = true;
				SuperView._addingView = false;
			}
			CanFocus       = true;
			view._tabIndex = _tabIndexes.IndexOf (view);
			_addingView    = false;
		}
		if (view.Enabled && !Enabled) {
			view._oldEnabled = true;
			view.Enabled     = false;
		}

		OnAdded (new SuperViewChangedEventArgs (this, view));
		if (IsInitialized && !view.IsInitialized) {
			view.BeginInit ();
			view.EndInit ();
		}

		SetNeedsLayout ();
		SetNeedsDisplay ();
	}

	/// <summary>
	/// Adds the specified views (children) to the view.
	/// </summary>
	/// <param name="views">Array of one or more views (can be optional parameter).</param>
	/// <remarks>
	/// The Views that have been added to this view can be retrieved via the <see cref="Subviews"/> property.
	/// See also <seealso cref="Remove(View)"/> <seealso cref="RemoveAll"/>
	/// </remarks>
	public void Add (params View [] views)
	{
		if (views == null) {
			return;
		}
		foreach (var view in views) {
			Add (view);
		}
	}

	/// <summary>
	/// Method invoked when a subview is being added to this view.
	/// </summary>
	/// <param name="e">Event where <see cref="ViewEventArgs.View"/> is the subview being added.</param>
	public virtual void OnAdded (SuperViewChangedEventArgs e)
	{
		var view = e.Child;
		view.IsAdded = true;
		view.OnResizeNeeded ();
		view.Added?.Invoke (this, e);
	}

	/// <summary>
	/// Event fired when this view is removed from another.
	/// </summary>
	public event EventHandler<SuperViewChangedEventArgs> Removed;

	/// <summary>
	/// Removes all subviews (children) added via <see cref="Add(View)"/> or <see cref="Add(View[])"/> from this View.
	/// </summary>
	public virtual void RemoveAll ()
	{
		if (_subviews == null) {
			return;
		}

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

	/// <summary>
	/// Removes a subview added via <see cref="Add(View)"/> or <see cref="Add(View[])"/> from this View.
	/// </summary>
	/// <remarks>
	/// </remarks>
	public virtual void Remove (View view)
	{
		if (view == null || _subviews == null) {
			return;
		}

		var touched = view.Frame;
		_subviews.Remove (view);
		_tabIndexes.Remove (view);
		view._superView = null;
		view._tabIndex  = -1;
		SetNeedsLayout ();
		SetNeedsDisplay ();

		foreach (var v in _subviews) {
			if (v.Frame.IntersectsWith (touched)) {
				view.SetNeedsDisplay ();
			}
		}
		OnRemoved (new SuperViewChangedEventArgs (this, view));
		if (Focused == view) {
			Focused = null;
		}
	}

	/// <summary>
	/// Method invoked when a subview is being removed from this view.
	/// </summary>
	/// <param name="e">Event args describing the subview being removed.</param>
	public virtual void OnRemoved (SuperViewChangedEventArgs e)
	{
		var view = e.Child;
		view.IsAdded = false;
		view.Removed?.Invoke (this, e);
	}


	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>
	/// Get the top superview of a given <see cref="View"/>.
	/// </summary>
	/// <returns>The superview view.</returns>
	public View GetTopSuperView (View view = null, View superview = null)
	{
		var top = superview ?? Application.Top;
		for (var v = view?.SuperView ?? this?.SuperView; v != null; v = v.SuperView) {
			top = v;
			if (top == superview) {
				break;
			}
		}

		return top;
	}



	#region Focus
	internal enum Direction {
		Forward,
		Backward
	}

	/// <summary>
	/// Event fired when the view gets focus.
	/// </summary>
	public event EventHandler<FocusEventArgs> Enter;

	/// <summary>
	/// Event fired when the view looses focus.
	/// </summary>
	public event EventHandler<FocusEventArgs> Leave;

	Direction _focusDirection;

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


	// BUGBUG: v2 - Seems weird that this is in View and not Responder.
	bool _hasFocus;

	/// <inheritdoc/>
	public override bool HasFocus => _hasFocus;

	void SetHasFocus (bool value, View view, bool force = false)
	{
		if (_hasFocus != value || force) {
			_hasFocus = value;
			if (value) {
				OnEnter (view);
			} else {
				OnLeave (view);
			}
			SetNeedsDisplay ();
		}

		// Remove focus down the chain of subviews if focus is removed
		if (!value && Focused != null) {
			var f = Focused;
			f.OnLeave (view);
			f.SetHasFocus (false, view);
			Focused = null;
		}
	}

	/// <summary>
	/// Event fired when the <see cref="CanFocus"/> value is being changed.
	/// </summary>
	public event EventHandler CanFocusChanged;

	/// <inheritdoc/>
	public override void OnCanFocusChanged () => CanFocusChanged?.Invoke (this, EventArgs.Empty);

	bool _oldCanFocus;

	/// <inheritdoc/>
	public override bool CanFocus {
		get => base.CanFocus;
		set {
			if (!_addingView && IsInitialized && SuperView?.CanFocus == false && value) {
				throw new InvalidOperationException ("Cannot set CanFocus to true if the SuperView CanFocus is false!");
			}
			if (base.CanFocus != value) {
				base.CanFocus = value;

				switch (value) {
				case false when _tabIndex > -1:
					TabIndex = -1;
					break;
				case true when SuperView?.CanFocus == false && _addingView:
					SuperView.CanFocus = true;
					break;
				}

				if (value && _tabIndex == -1) {
					TabIndex = SuperView != null ? SuperView._tabIndexes.IndexOf (this) : -1;
				}
				TabStop = value;

				if (!value && SuperView?.Focused == this) {
					SuperView.Focused = null;
				}
				if (!value && HasFocus) {
					SetHasFocus (false, this);
					SuperView?.EnsureFocus ();
					if (SuperView != null && SuperView.Focused == null) {
						SuperView.FocusNext ();
						if (SuperView.Focused == null && Application.Current != null) {
							Application.Current.FocusNext ();
						}
						Application.BringOverlappedTopToFront ();
					}
				}
				if (_subviews != null && IsInitialized) {
					foreach (var view in _subviews) {
						if (view.CanFocus != value) {
							if (!value) {
								view._oldCanFocus = view.CanFocus;
								view._oldTabIndex = view._tabIndex;
								view.CanFocus     = false;
								view._tabIndex    = -1;
							} else {
								if (_addingView) {
									view._addingView = true;
								}
								view.CanFocus    = view._oldCanFocus;
								view._tabIndex   = view._oldTabIndex;
								view._addingView = false;
							}
						}
					}
				}
				OnCanFocusChanged ();
				SetNeedsDisplay ();
			}
		}
	}


	/// <inheritdoc/>
	public override bool OnEnter (View view)
	{
		var args = new FocusEventArgs (view);
		Enter?.Invoke (this, args);
		if (args.Handled) {
			return true;
		}
		if (base.OnEnter (view)) {
			return true;
		}

		return false;
	}

	/// <inheritdoc/>
	public override bool OnLeave (View view)
	{
		var args = new FocusEventArgs (view);
		Leave?.Invoke (this, args);
		if (args.Handled) {
			return true;
		}
		if (base.OnLeave (view)) {
			return true;
		}

		Driver?.SetCursorVisibility (CursorVisibility.Invisible);
		return false;
	}

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

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

	/// <summary>
	/// Causes the specified subview to have focus.
	/// </summary>
	/// <param name="view">View.</param>
	void SetFocus (View view)
	{
		if (view == null) {
			return;
		}
		//Console.WriteLine ($"Request to focus {view}");
		if (!view.CanFocus || !view.Visible || !view.Enabled) {
			return;
		}
		if (Focused?._hasFocus == true && Focused == view) {
			return;
		}
		if (Focused?._hasFocus == true && Focused?.SuperView == view || view == this) {

			if (!view._hasFocus) {
				view._hasFocus = true;
			}
			return;
		}
		// Make sure that this view is a subview
		View c;
		for (c = view._superView; c != null; c = c._superView) {
			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.SetHasFocus (false, view);
		}

		var f = Focused;
		Focused = view;
		Focused.SetHasFocus (true, f);
		Focused.EnsureFocus ();

		// Send focus upwards
		if (SuperView != null) {
			SuperView.SetFocus (this);
		} else {
			SetFocus (this);
		}
	}

	/// <summary>
	/// Causes the specified view and the entire parent hierarchy to have the focused order updated.
	/// </summary>
	public void SetFocus ()
	{
		if (!CanBeVisible (this) || !Enabled) {
			if (HasFocus) {
				SetHasFocus (false, this);
			}
			return;
		}

		if (SuperView != null) {
			SuperView.SetFocus (this);
		} else {
			SetFocus (this);
		}
	}

	/// <summary>
	/// Finds the first view in the hierarchy that wants to get the focus if nothing is currently focused, otherwise, does
	/// nothing.
	/// </summary>
	public void EnsureFocus ()
	{
		if (Focused == null && _subviews?.Count > 0) {
			if (FocusDirection == Direction.Forward) {
				FocusFirst ();
			} else {
				FocusLast ();
			}
		}
	}

	/// <summary>
	/// Focuses the first focusable subview if one exists.
	/// </summary>
	public void FocusFirst ()
	{
		if (!CanBeVisible (this)) {
			return;
		}

		if (_tabIndexes == null) {
			SuperView?.SetFocus (this);
			return;
		}

		foreach (var view in _tabIndexes) {
			if (view.CanFocus && view._tabStop && view.Visible && view.Enabled) {
				SetFocus (view);
				return;
			}
		}
	}

	/// <summary>
	/// Focuses the last focusable subview if one exists.
	/// </summary>
	public void FocusLast ()
	{
		if (!CanBeVisible (this)) {
			return;
		}

		if (_tabIndexes == null) {
			SuperView?.SetFocus (this);
			return;
		}

		for (var i = _tabIndexes.Count; i > 0;) {
			i--;

			var v = _tabIndexes [i];
			if (v.CanFocus && v._tabStop && v.Visible && v.Enabled) {
				SetFocus (v);
				return;
			}
		}
	}

	/// <summary>
	/// Focuses the previous view.
	/// </summary>
	/// <returns><see langword="true"/> if previous was focused, <see langword="false"/> otherwise.</returns>
	public bool FocusPrev ()
	{
		if (!CanBeVisible (this)) {
			return false;
		}

		FocusDirection = Direction.Backward;
		if (_tabIndexes == null || _tabIndexes.Count == 0) {
			return false;
		}

		if (Focused == null) {
			FocusLast ();
			return Focused != null;
		}

		var focusedIdx = -1;
		for (var i = _tabIndexes.Count; i > 0;) {
			i--;
			var w = _tabIndexes [i];

			if (w.HasFocus) {
				if (w.FocusPrev ()) {
					return true;
				}
				focusedIdx = i;
				continue;
			}
			if (w.CanFocus && focusedIdx != -1 && w._tabStop && w.Visible && w.Enabled) {
				Focused.SetHasFocus (false, w);

				if (w.CanFocus && w._tabStop && w.Visible && w.Enabled) {
					w.FocusLast ();
				}

				SetFocus (w);
				return true;
			}
		}
		if (Focused != null) {
			Focused.SetHasFocus (false, this);
			Focused = null;
		}
		return false;
	}

	/// <summary>
	/// Focuses the next view.
	/// </summary>
	/// <returns><see langword="true"/> if next was focused, <see langword="false"/> otherwise.</returns>
	public bool FocusNext ()
	{
		if (!CanBeVisible (this)) {
			return false;
		}

		FocusDirection = Direction.Forward;
		if (_tabIndexes == null || _tabIndexes.Count == 0) {
			return false;
		}

		if (Focused == null) {
			FocusFirst ();
			return Focused != null;
		}
		var focusedIdx = -1;
		for (var i = 0; i < _tabIndexes.Count; i++) {
			var w = _tabIndexes [i];

			if (w.HasFocus) {
				if (w.FocusNext ()) {
					return true;
				}
				focusedIdx = i;
				continue;
			}
			if (w.CanFocus && focusedIdx != -1 && w._tabStop && w.Visible && w.Enabled) {
				Focused.SetHasFocus (false, w);

				if (w.CanFocus && w._tabStop && w.Visible && w.Enabled) {
					w.FocusFirst ();
				}

				SetFocus (w);
				return true;
			}
		}
		if (Focused != null) {
			Focused.SetHasFocus (false, this);
			Focused = null;
		}
		return false;
	}

	View GetMostFocused (View view)
	{
		if (view == null) {
			return null;
		}

		return view.Focused != null ? GetMostFocused (view.Focused) : view;
	}

	/// <summary>
	/// Positions the cursor in the right position based on the currently focused view in the chain.
	/// </summary>
	/// Views that are focusable should override
	/// <see cref="PositionCursor"/>
	/// to ensure
	/// 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. Views should make sure that they place the cursor
	/// in a visually sensible place.
	public virtual void PositionCursor ()
	{
		if (!CanBeVisible (this) || !Enabled) {
			return;
		}

		// BUGBUG: v2 - This needs to support children of Frames too

		if (Focused == null && SuperView != null) {
			SuperView.EnsureFocus ();
		} else if (Focused?.Visible == true && Focused?.Enabled == true && Focused?.Frame.Width > 0 && Focused.Frame.Height > 0) {
			Focused.PositionCursor ();
		} else if (Focused?.Visible == true && Focused?.Enabled == false) {
			Focused = null;
		} else if (CanFocus && HasFocus && Visible && Frame.Width > 0 && Frame.Height > 0) {
			Move (TextFormatter.HotKeyPos == -1 ? 0 : TextFormatter.CursorPosition, 0);
		} else {
			Move (_frame.X, _frame.Y);
		}
	}
	#endregion Focus
}