Avalonia/src/Avalonia.Controls/Primitives/AccessText.cs

using Avalonia.Automation.Peers;
using Avalonia.Input;
using Avalonia.Reactive;
using Avalonia.Media;
using Avalonia.Media.TextFormatting;

namespace Avalonia.Controls.Primitives
{
    /// <summary>
    /// A text block that displays a character prefixed with an underscore as an access key.
    /// </summary>
    public class AccessText : TextBlock
    {
        /// <summary>
        /// Defines the <see cref="ShowAccessKey"/> attached property.
        /// </summary>
        public static readonly AttachedProperty<bool> ShowAccessKeyProperty =
            AvaloniaProperty.RegisterAttached<AccessText, Control, bool>("ShowAccessKey", inherits: true);

        /// <summary>
        /// The access key handler for the current window.
        /// </summary>
        private IAccessKeyHandler? _accessKeys;

        /// <summary>
        /// Initializes static members of the <see cref="AccessText"/> class.
        /// </summary>
        static AccessText()
        {
            AffectsRender<AccessText>(ShowAccessKeyProperty);
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="AccessText"/> class.
        /// </summary>
        public AccessText()
        {
            this.GetObservable(TextProperty).Subscribe(TextChanged);
        }

        /// <summary>
        /// Gets the access key.
        /// </summary>
        public char AccessKey
        {
            get;
            private set;
        }

        /// <summary>
        /// Gets or sets a value indicating whether the access key should be underlined.
        /// </summary>
        public bool ShowAccessKey
        {
            get { return GetValue(ShowAccessKeyProperty); }
            set { SetValue(ShowAccessKeyProperty, value); }
        }

        /// <summary>
        /// Renders the <see cref="AccessText"/> to a drawing context.
        /// </summary>
        /// <param name="context">The drawing context.</param>
        private protected override void RenderCore(DrawingContext context)
        {
            base.RenderCore(context);
            int underscore = Text?.IndexOf('_') ?? -1;

            if (underscore != -1 && ShowAccessKey)
            {
                var rect = TextLayout!.HitTestTextPosition(underscore);
                var offset = new Vector(0, -1.5);
                context.DrawLine(
                    new Pen(Foreground, 1),
                    rect.BottomLeft + offset,
                    rect.BottomRight + offset);
            }
        }

        /// <inheritdoc/>
        protected override TextLayout CreateTextLayout(string? text)
        {
            return base.CreateTextLayout(RemoveAccessKeyMarker(text));
        }

        /// <inheritdoc/>
        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
        {
            base.OnAttachedToVisualTree(e);
            _accessKeys = (e.Root as TopLevel)?.AccessKeyHandler;

            if (_accessKeys != null && AccessKey != 0)
            {
                _accessKeys.Register(AccessKey, this);
            }
        }

        /// <inheritdoc/>
        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
        {
            base.OnDetachedFromVisualTree(e);

            if (_accessKeys != null && AccessKey != 0)
            {
                _accessKeys.Unregister(this);
                _accessKeys = null;
            }
        }

        protected override AutomationPeer OnCreateAutomationPeer()
        {
            return new NoneAutomationPeer(this);
        }

        internal static string? RemoveAccessKeyMarker(string? text)
        {
            if (!string.IsNullOrEmpty(text))
            {
                var accessKeyMarker = "_";
                var doubleAccessKeyMarker = accessKeyMarker + accessKeyMarker;
                int index = FindAccessKeyMarker(text);
                if (index >= 0 && index < text.Length - 1)
                    text = text.Remove(index, 1);
                text = text.Replace(doubleAccessKeyMarker, accessKeyMarker);
            }
            return text;
        }

        private static int FindAccessKeyMarker(string text)
        {
            var length = text.Length;
            var startIndex = 0;
            while (startIndex < length)
            {
                int index = text.IndexOf('_', startIndex);
                if (index == -1)
                    return -1;
                if (index + 1 < length && text[index + 1] != '_')
                    return index;
                startIndex = index + 2;
            }

            return -1;
        }

        /// <summary>
        /// Called when the <see cref="TextBlock.Text"/> property changes.
        /// </summary>
        /// <param name="text">The new text.</param>
        private void TextChanged(string? text)
        {
            var key = (char)0;

            if (text != null)
            {
                int underscore = text.IndexOf('_');

                if (underscore != -1 && underscore < text.Length - 1)
                {
                    key = text[underscore + 1];
                }
            }

            AccessKey = key;

            if (_accessKeys != null && AccessKey != 0)
            {
                _accessKeys.Register(AccessKey, this);
            }
        }
    }
}