Terminal.Gui/Terminal.Gui/View/View.ScrollBars.cs

#nullable enable
namespace Terminal.Gui;

public partial class View
{
    private Lazy<ScrollBar> _horizontalScrollBar;
    private Lazy<ScrollBar> _verticalScrollBar;

    /// <summary>
    ///     Initializes the ScrollBars of the View. Called by the View constructor.
    /// </summary>
    private void SetupScrollBars ()
    {
        if (this is Adornment)
        {
            return;
        }

        _verticalScrollBar = new (() => CreateScrollBar (Orientation.Vertical));
        _horizontalScrollBar = new (() => CreateScrollBar (Orientation.Horizontal));
    }

    private ScrollBar CreateScrollBar (Orientation orientation)
    {
        var scrollBar = new ScrollBar
        {
            Orientation = orientation,
            Visible = false // Initially hidden until needed
        };

        if (orientation == Orientation.Vertical)
        {
            ConfigureVerticalScrollBar (scrollBar);
        }
        else
        {
            ConfigureHorizontalScrollBar (scrollBar);
        }

        scrollBar.Initialized += OnScrollBarInitialized;

        // Add after setting Initialized event!
        Padding?.Add (scrollBar);

        return scrollBar;
    }

    private void ConfigureVerticalScrollBar (ScrollBar scrollBar)
    {
        scrollBar.X = Pos.AnchorEnd ();

        scrollBar.Height = Dim.Fill (
                                     Dim.Func (
                                               () =>
                                               {
                                                   if (_horizontalScrollBar.IsValueCreated)
                                                   {
                                                       return _horizontalScrollBar.Value.Visible ? 1 : 0;
                                                   }

                                                   return 0;
                                               }));
        scrollBar.ScrollableContentSize = GetContentSize ().Height;

        ViewportChanged += (_, _) =>
                           {
                               scrollBar.Position = Viewport.Y;
                           };

        ContentSizeChanged += (_, _) => { scrollBar.ScrollableContentSize = GetContentSize ().Height; };
    }

    private void ConfigureHorizontalScrollBar (ScrollBar scrollBar)
    {
        scrollBar.Y = Pos.AnchorEnd ();

        scrollBar.Width = Dim.Fill (
                                    Dim.Func (
                                              () =>
                                              {
                                                  if (_verticalScrollBar.IsValueCreated)
                                                  {
                                                      return _verticalScrollBar.Value.Visible ? 1 : 0;
                                                  }

                                                  return 0;
                                              }));
        scrollBar.ScrollableContentSize = GetContentSize ().Width;

        ViewportChanged += (_, _) =>
                           {
                               scrollBar.Position = Viewport.X;
                           };

        ContentSizeChanged += (_, _) => { scrollBar.ScrollableContentSize = GetContentSize ().Width; };
    }

    private void OnScrollBarInitialized (object? sender, EventArgs e)
    {
        var scrollBar = (ScrollBar)sender!;

        if (scrollBar.Orientation == Orientation.Vertical)
        {
            ConfigureVerticalScrollBarEvents (scrollBar);
        }
        else
        {
            ConfigureHorizontalScrollBarEvents (scrollBar);
        }
    }

    private void ConfigureVerticalScrollBarEvents (ScrollBar scrollBar)
    {
        Padding!.Thickness = Padding.Thickness with { Right = scrollBar.Visible ? Padding.Thickness.Right + 1 : 0 };

        scrollBar.PositionChanged += (_, args) =>
                                     {
                                         Viewport = Viewport with
                                         {
                                             Y = Math.Min (args.CurrentValue, scrollBar.ScrollableContentSize - scrollBar.VisibleContentSize)
                                         };
                                     };

        scrollBar.VisibleChanged += (_, _) =>
                                    {
                                        Padding.Thickness = Padding.Thickness with
                                        {
                                            Right = scrollBar.Visible ? Padding.Thickness.Right + 1 : Padding.Thickness.Right - 1
                                        };
                                    };
    }

    private void ConfigureHorizontalScrollBarEvents (ScrollBar scrollBar)
    {
        Padding!.Thickness = Padding.Thickness with { Bottom = scrollBar.Visible ? Padding.Thickness.Bottom + 1 : 0 };

        scrollBar.PositionChanged += (_, args) =>
                                     {
                                         Viewport = Viewport with
                                         {
                                             X = Math.Min (args.CurrentValue, scrollBar.ScrollableContentSize - scrollBar.VisibleContentSize)
                                         };
                                     };

        scrollBar.VisibleChanged += (_, _) =>
                                    {
                                        Padding.Thickness = Padding.Thickness with
                                        {
                                            Bottom = scrollBar.Visible ? Padding.Thickness.Bottom + 1 : Padding.Thickness.Bottom - 1
                                        };
                                    };
    }

    /// <summary>
    ///     Gets the horizontal <see cref="ScrollBar"/>. This property is lazy-loaded and will not be created until it is accessed.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         See <see cref="ScrollBar"/> for more information on how to use the ScrollBar.
    ///     </para>
    /// </remarks>
    public ScrollBar HorizontalScrollBar => _horizontalScrollBar.Value;

    /// <summary>
    ///     Gets the vertical <see cref="ScrollBar"/>. This property is lazy-loaded and will not be created until it is accessed.
    /// </summary>
    /// <remarks>
    ///     <para>
    ///         See <see cref="ScrollBar"/> for more information on how to use the ScrollBar.
    ///     </para>
    /// </remarks>
    public ScrollBar VerticalScrollBar => _verticalScrollBar.Value;

    /// <summary>
    ///     Clean up the ScrollBars of the View. Called by View.Dispose.
    /// </summary>
    private void DisposeScrollBars ()
    {
        if (this is Adornment)
        {
            return;
        }

        if (_horizontalScrollBar.IsValueCreated)
        {
            Padding?.Remove (_horizontalScrollBar.Value);
            _horizontalScrollBar.Value.Dispose ();
        }

        if (_verticalScrollBar.IsValueCreated)
        {
            Padding?.Remove (_verticalScrollBar.Value);
            _verticalScrollBar.Value.Dispose ();
        }
    }
}