Avalonia/src/Avalonia.Controls/TextBlock.cs

using System.Reactive.Linq;
using Avalonia.LogicalTree;
using Avalonia.Media;
using Avalonia.Media.TextFormatting;
using Avalonia.Metadata;

namespace Avalonia.Controls
{
    /// <summary>
    /// A control that displays a block of text.
    /// </summary>
    public class TextBlock : Control
    {
        /// <summary>
        /// Defines the <see cref="Background"/> property.
        /// </summary>
        public static readonly StyledProperty<IBrush> BackgroundProperty =
            Border.BackgroundProperty.AddOwner<TextBlock>();

        /// <summary>
        /// Defines the <see cref="Padding"/> property.
        /// </summary>
        public static readonly StyledProperty<Thickness> PaddingProperty =
            Decorator.PaddingProperty.AddOwner<TextBlock>();

        // TODO: Define these attached properties elsewhere (e.g. on a Text class) and AddOwner
        // them into TextBlock.

        /// <summary>
        /// Defines the <see cref="FontFamily"/> property.
        /// </summary>
        public static readonly AttachedProperty<FontFamily> FontFamilyProperty =
            AvaloniaProperty.RegisterAttached<TextBlock, Control, FontFamily>(
                nameof(FontFamily),
                defaultValue: FontFamily.Default,
                inherits: true);

        /// <summary>
        /// Defines the <see cref="FontSize"/> property.
        /// </summary>
        public static readonly AttachedProperty<double> FontSizeProperty =
            AvaloniaProperty.RegisterAttached<TextBlock, Control, double>(
                nameof(FontSize),
                defaultValue: 12,
                inherits: true);

        /// <summary>
        /// Defines the <see cref="FontStyle"/> property.
        /// </summary>
        public static readonly AttachedProperty<FontStyle> FontStyleProperty =
            AvaloniaProperty.RegisterAttached<TextBlock, Control, FontStyle>(
                nameof(FontStyle),
                inherits: true);

        /// <summary>
        /// Defines the <see cref="FontWeight"/> property.
        /// </summary>
        public static readonly AttachedProperty<FontWeight> FontWeightProperty =
            AvaloniaProperty.RegisterAttached<TextBlock, Control, FontWeight>(
                nameof(FontWeight),
                inherits: true,
                defaultValue: FontWeight.Normal);

        /// <summary>
        /// Defines the <see cref="Foreground"/> property.
        /// </summary>
        public static readonly AttachedProperty<IBrush> ForegroundProperty =
            AvaloniaProperty.RegisterAttached<TextBlock, Control, IBrush>(
                nameof(Foreground),
                Brushes.Black,
                inherits: true);

        /// <summary>
        /// Defines the <see cref="LineHeight"/> property.
        /// </summary>
        public static readonly StyledProperty<double> LineHeightProperty =
            AvaloniaProperty.Register<TextBlock, double>(
                nameof(LineHeight),
                double.NaN,
                validate: IsValidLineHeight);

        /// <summary>
        /// Defines the <see cref="MaxLines"/> property.
        /// </summary>
        public static readonly StyledProperty<int> MaxLinesProperty =
            AvaloniaProperty.Register<TextBlock, int>(
                nameof(MaxLines),
                validate: IsValidMaxLines);

        /// <summary>
        /// Defines the <see cref="Text"/> property.
        /// </summary>
        public static readonly DirectProperty<TextBlock, string> TextProperty =
            AvaloniaProperty.RegisterDirect<TextBlock, string>(
                nameof(Text),
                o => o.Text,
                (o, v) => o.Text = v);

        /// <summary>
        /// Defines the <see cref="TextAlignment"/> property.
        /// </summary>
        public static readonly StyledProperty<TextAlignment> TextAlignmentProperty =
            AvaloniaProperty.Register<TextBlock, TextAlignment>(nameof(TextAlignment));

        /// <summary>
        /// Defines the <see cref="TextWrapping"/> property.
        /// </summary>
        public static readonly StyledProperty<TextWrapping> TextWrappingProperty =
            AvaloniaProperty.Register<TextBlock, TextWrapping>(nameof(TextWrapping));

        /// <summary>
        /// Defines the <see cref="TextTrimming"/> property.
        /// </summary>
        public static readonly StyledProperty<TextTrimming> TextTrimmingProperty =
            AvaloniaProperty.Register<TextBlock, TextTrimming>(nameof(TextTrimming));

        /// <summary>
        /// Defines the <see cref="TextDecorations"/> property.
        /// </summary>
        public static readonly StyledProperty<TextDecorationCollection> TextDecorationsProperty =
            AvaloniaProperty.Register<TextBlock, TextDecorationCollection>(nameof(TextDecorations));

        private string _text;
        private TextLayout _textLayout;
        private Size _constraint;

        /// <summary>
        /// Initializes static members of the <see cref="TextBlock"/> class.
        /// </summary>
        static TextBlock()
        {
            ClipToBoundsProperty.OverrideDefaultValue<TextBlock>(true);

            AffectsRender<TextBlock>(BackgroundProperty, ForegroundProperty,
                TextAlignmentProperty, TextDecorationsProperty);

            AffectsMeasure<TextBlock>(FontSizeProperty, FontWeightProperty,
                FontStyleProperty, TextWrappingProperty, FontFamilyProperty,
                TextTrimmingProperty, TextProperty, PaddingProperty, LineHeightProperty, MaxLinesProperty);

            Observable.Merge<AvaloniaPropertyChangedEventArgs>(TextProperty.Changed, ForegroundProperty.Changed,
                TextAlignmentProperty.Changed, TextWrappingProperty.Changed,
                TextTrimmingProperty.Changed, FontSizeProperty.Changed,
                FontStyleProperty.Changed, FontWeightProperty.Changed,
                FontFamilyProperty.Changed, TextDecorationsProperty.Changed,
                PaddingProperty.Changed, MaxLinesProperty.Changed, LineHeightProperty.Changed
            ).AddClassHandler<TextBlock>((x, _) => x.InvalidateTextLayout());
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="TextBlock"/> class.
        /// </summary>
        public TextBlock()
        {
            _text = string.Empty;
        }

        /// <summary>
        /// Gets the <see cref="TextLayout"/> used to render the text.
        /// </summary>
        public TextLayout TextLayout
        {
            get
            {
                return _textLayout ?? (_textLayout = CreateTextLayout(_constraint, Text));
            }
        }

        /// <summary>
        /// Gets or sets the padding to place around the <see cref="Text"/>.
        /// </summary>
        public Thickness Padding
        {
            get { return GetValue(PaddingProperty); }
            set { SetValue(PaddingProperty, value); }
        }

        /// <summary>
        /// Gets or sets a brush used to paint the control's background.
        /// </summary>
        public IBrush Background
        {
            get { return GetValue(BackgroundProperty); }
            set { SetValue(BackgroundProperty, value); }
        }

        /// <summary>
        /// Gets or sets the text.
        /// </summary>
        [Content]
        public string Text
        {
            get { return _text; }
            set { SetAndRaise(TextProperty, ref _text, value); }
        }

        /// <summary>
        /// Gets or sets the font family.
        /// </summary>
        public FontFamily FontFamily
        {
            get { return GetValue(FontFamilyProperty); }
            set { SetValue(FontFamilyProperty, value); }
        }

        /// <summary>
        /// Gets or sets the font size.
        /// </summary>
        public double FontSize
        {
            get { return GetValue(FontSizeProperty); }
            set { SetValue(FontSizeProperty, value); }
        }

        /// <summary>
        /// Gets or sets the font style.
        /// </summary>
        public FontStyle FontStyle
        {
            get { return GetValue(FontStyleProperty); }
            set { SetValue(FontStyleProperty, value); }
        }

        /// <summary>
        /// Gets or sets the font weight.
        /// </summary>
        public FontWeight FontWeight
        {
            get { return GetValue(FontWeightProperty); }
            set { SetValue(FontWeightProperty, value); }
        }

        /// <summary>
        /// Gets or sets a brush used to paint the text.
        /// </summary>
        public IBrush Foreground
        {
            get { return GetValue(ForegroundProperty); }
            set { SetValue(ForegroundProperty, value); }
        }

        /// <summary>
        /// Gets or sets the height of each line of content.
        /// </summary>
        public double LineHeight
        {
            get => GetValue(LineHeightProperty);
            set => SetValue(LineHeightProperty, value);
        }

        /// <summary>
        /// Gets or sets the maximum number of text lines.
        /// </summary>
        public int MaxLines
        {
            get => GetValue(MaxLinesProperty);
            set => SetValue(MaxLinesProperty, value);
        }

        /// <summary>
        /// Gets or sets the control's text wrapping mode.
        /// </summary>
        public TextWrapping TextWrapping
        {
            get { return GetValue(TextWrappingProperty); }
            set { SetValue(TextWrappingProperty, value); }
        }

        /// <summary>
        /// Gets or sets the control's text trimming mode.
        /// </summary>
        public TextTrimming TextTrimming
        {
            get { return GetValue(TextTrimmingProperty); }
            set { SetValue(TextTrimmingProperty, value); }
        }

        /// <summary>
        /// Gets or sets the text alignment.
        /// </summary>
        public TextAlignment TextAlignment
        {
            get { return GetValue(TextAlignmentProperty); }
            set { SetValue(TextAlignmentProperty, value); }
        }

        /// <summary>
        /// Gets or sets the text decorations.
        /// </summary>
        public TextDecorationCollection TextDecorations
        {
            get => GetValue(TextDecorationsProperty);
            set => SetValue(TextDecorationsProperty, value);
        }

        /// <summary>
        /// Gets the value of the attached <see cref="FontFamilyProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <returns>The font family.</returns>
        public static FontFamily GetFontFamily(Control control)
        {
            return control.GetValue(FontFamilyProperty);
        }

        /// <summary>
        /// Gets the value of the attached <see cref="FontSizeProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <returns>The font family.</returns>
        public static double GetFontSize(Control control)
        {
            return control.GetValue(FontSizeProperty);
        }

        /// <summary>
        /// Gets the value of the attached <see cref="FontStyleProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <returns>The font family.</returns>
        public static FontStyle GetFontStyle(Control control)
        {
            return control.GetValue(FontStyleProperty);
        }

        /// <summary>
        /// Gets the value of the attached <see cref="FontWeightProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <returns>The font family.</returns>
        public static FontWeight GetFontWeight(Control control)
        {
            return control.GetValue(FontWeightProperty);
        }

        /// <summary>
        /// Gets the value of the attached <see cref="ForegroundProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <returns>The foreground.</returns>
        public static IBrush GetForeground(Control control)
        {
            return control.GetValue(ForegroundProperty);
        }

        /// <summary>
        /// Sets the value of the attached <see cref="FontFamilyProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="value">The property value to set.</param>
        /// <returns>The font family.</returns>
        public static void SetFontFamily(Control control, FontFamily value)
        {
            control.SetValue(FontFamilyProperty, value);
        }

        /// <summary>
        /// Sets the value of the attached <see cref="FontSizeProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="value">The property value to set.</param>
        /// <returns>The font family.</returns>
        public static void SetFontSize(Control control, double value)
        {
            control.SetValue(FontSizeProperty, value);
        }

        /// <summary>
        /// Sets the value of the attached <see cref="FontStyleProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="value">The property value to set.</param>
        /// <returns>The font family.</returns>
        public static void SetFontStyle(Control control, FontStyle value)
        {
            control.SetValue(FontStyleProperty, value);
        }

        /// <summary>
        /// Sets the value of the attached <see cref="FontWeightProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="value">The property value to set.</param>
        /// <returns>The font family.</returns>
        public static void SetFontWeight(Control control, FontWeight value)
        {
            control.SetValue(FontWeightProperty, value);
        }

        /// <summary>
        /// Sets the value of the attached <see cref="ForegroundProperty"/> on a control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="value">The property value to set.</param>
        /// <returns>The font family.</returns>
        public static void SetForeground(Control control, IBrush value)
        {
            control.SetValue(ForegroundProperty, value);
        }

        /// <summary>
        /// Renders the <see cref="TextBlock"/> to a drawing context.
        /// </summary>
        /// <param name="context">The drawing context.</param>
        public override void Render(DrawingContext context)
        {
            var background = Background;

            if (background != null)
            {
                context.FillRectangle(background, new Rect(Bounds.Size));
            }

            if (TextLayout is null)
            {
                return;
            }

            var textAlignment = TextAlignment;

            var width = Bounds.Size.Width;

            var offsetX = 0.0;

            switch (textAlignment)
            {
                case TextAlignment.Center:
                    offsetX = (width - TextLayout.Size.Width) / 2;
                    break;
                case TextAlignment.Right:
                    offsetX =  width - TextLayout.Size.Width;
                    break;
            }

            var padding = Padding;

            using (context.PushPostTransform(Matrix.CreateTranslation(padding.Left + offsetX, padding.Top)))
            {
                TextLayout.Draw(context);
            }
        }

        /// <summary>
        /// Creates the <see cref="TextLayout"/> used to render the text.
        /// </summary>
        /// <param name="constraint">The constraint of the text.</param>
        /// <param name="text">The text to format.</param>
        /// <returns>A <see cref="TextLayout"/> object.</returns>
        protected virtual TextLayout CreateTextLayout(Size constraint, string text)
        {
            if (constraint == Size.Empty)
            {
                return null;
            }

            return new TextLayout(
                text ?? string.Empty,
                new Typeface(FontFamily, FontStyle, FontWeight),
                FontSize,
                Foreground,
                TextAlignment,
                TextWrapping,
                TextTrimming,
                TextDecorations,
                constraint.Width,
                constraint.Height,
                maxLines: MaxLines,
                lineHeight: LineHeight);
        }

        /// <summary>
        /// Invalidates <see cref="TextLayout"/>.
        /// </summary>
        protected void InvalidateTextLayout()
        {
            _textLayout = null;
        }

        /// <summary>
        /// Measures the control.
        /// </summary>
        /// <param name="availableSize">The available size for the control.</param>
        /// <returns>The desired size.</returns>
        protected override Size MeasureOverride(Size availableSize)
        {
            if (string.IsNullOrEmpty(Text))
            {
                return new Size();
            }

            var padding = Padding;

            availableSize = availableSize.Deflate(padding);

            if (_constraint != availableSize)
            {
                _constraint = availableSize;

                InvalidateTextLayout();
            }

            var measuredSize = TextLayout?.Size ?? Size.Empty;

            return measuredSize.Inflate(padding);
        }

        protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
        {
            base.OnAttachedToLogicalTree(e);

            InvalidateTextLayout();

            InvalidateMeasure();
        }

        private static bool IsValidMaxLines(int maxLines) => maxLines >= 0;

        private static bool IsValidLineHeight(double lineHeight) => double.IsNaN(lineHeight) || lineHeight > 0;
    }
}