Font-fallback implementation

QuestPDF.Examples/TextExamples.cs

@@ -2,6 +2,7 @@
 using System.Linq;
 using System.Text;
 using NUnit.Framework;
+using QuestPDF.Elements.Text;
 using QuestPDF.Examples.Engine;
 using QuestPDF.Fluent;
 using QuestPDF.Helpers;
@@ -618,5 +619,40 @@ namespace QuestPDF.Examples
                         .FontSize(20);
                 });
         }
+        
+        [Test]
+        public void FontFallback()
+        {
+            RenderingTest
+                .Create()
+                .ProducePdf()
+                .ShowResults()
+                .RenderDocument(container =>
+                {
+                    container.Page(page =>
+                    {
+                        page.Margin(50);
+                        page.PageColor(Colors.White);
+
+                        page.Size(PageSizes.A4);
+
+                        page.Content().Text(t =>
+                        {
+                            t.Line("This is normal text.");
+                            t.EmptyLine();
+
+                            t.Line("Following line should use font fallback:");
+                            t.Line("中文文本");
+                            t.EmptyLine();
+
+                            t.Line("The following line contains a mix of known and unknown characters.");
+                            t.Line("Mixed line: This 中文 is 文文 a mixed 本 本 line 本 中文文本!");
+                            t.EmptyLine();
+
+                            t.Line("Emojis work out of the box because of font fallback: 😊😅🥳👍❤😍👌");
+                        });
+                    });
+                });
+        }
     }
 }

QuestPDF/Drawing/Exceptions/DocumentDrawingException.cs

@@ -4,6 +4,11 @@ namespace QuestPDF.Drawing.Exceptions
 {
     public class DocumentDrawingException : Exception
     {
+        internal DocumentDrawingException(string message) : base(message)
+        {
+            
+        }
+        
         internal DocumentDrawingException(string message, Exception inner) : base(message, inner)
         {
             

QuestPDF/Elements/Text/FontFallback.cs

@@ -0,0 +1,84 @@
+using System.Collections.Generic;
+using QuestPDF.Drawing;
+using QuestPDF.Drawing.Exceptions;
+using QuestPDF.Elements.Text.Items;
+using QuestPDF.Fluent;
+using QuestPDF.Infrastructure;
+using SkiaSharp;
+
+namespace QuestPDF.Elements.Text
+{
+    internal static class FontFallback
+    {
+        public struct TextRun
+        {
+            public string Content { get; set; }
+            public TextStyle Style { get; set; }
+        }
+
+        private static SKFontManager FontManager => SKFontManager.Default;
+
+        public static IEnumerable<TextRun> SplitWithFontFallback(this string text, TextStyle textStyle)
+        {
+            var partStartIndex = 0;
+            var partTextStyle = textStyle;
+
+            for (var i = 0; i < text.Length; i += char.IsSurrogatePair(text, i) ? 2 : 1)
+            {
+                var codepoint = char.ConvertToUtf32(text, i);
+                var font = partTextStyle.ToFont();
+                var typeface = font.Typeface;
+
+                if (font.ContainsGlyph(codepoint))
+                    continue;
+                
+                var fallbackTypeface = FontManager.MatchCharacter(typeface.FamilyName, typeface.FontWeight, typeface.FontWidth, typeface.FontSlant, null, codepoint);
+
+                if (fallbackTypeface == null)
+                    throw new DocumentDrawingException($"Could not find an appropriate font fallback for text: '{text}'");
+
+                yield return new TextRun
+                {
+                    Content = text.Substring(partStartIndex, i - partStartIndex),
+                    Style = partTextStyle
+                };
+
+                partStartIndex = i;
+                partTextStyle = textStyle.FontFamily(fallbackTypeface.FamilyName).Weight((FontWeight)fallbackTypeface.FontWeight);
+            }
+            
+            if (partStartIndex > text.Length)
+                yield break;
+            
+            yield return new TextRun
+            {
+                Content = text.Substring(partStartIndex, text.Length - partStartIndex),
+                Style = partTextStyle
+            };
+        }
+
+        public static IEnumerable<ITextBlockItem> ApplyFontFallback(this ICollection<ITextBlockItem> textBlockItems)
+        {
+            foreach (var textBlockItem in textBlockItems)
+            {
+                if (textBlockItem is TextBlockSpan textBlockSpan)
+                {
+                    var textRuns = textBlockSpan.Text.SplitWithFontFallback(textBlockSpan.Style);
+
+                    foreach (var textRun in textRuns)
+                    {
+                        yield return new TextBlockSpan
+                        {
+                            Text = textRun.Content,
+                            Style = textRun.Style
+                        };
+                    }
+                }
+                else
+                {
+                    yield return textBlockItem;
+                }
+            }
+        }
+    }
+}

QuestPDF/Elements/Text/Items/TextBlockSpan.cs

@@ -15,7 +15,7 @@ namespace QuestPDF.Elements.Text.Items
     {
         public string Text { get; set; }
         public TextStyle Style { get; set; } = new();
-        public TextShapingResult? TextShapingResult { get; set; }
+        private TextShapingResult? TextShapingResult { get; set; }
 
         private Dictionary<(int startIndex, float availableWidth), TextMeasurementResult?> MeasureCache = new ();
         protected virtual bool EnableTextCache => true; 

QuestPDF/Elements/Text/TextBlock.cs

@@ -18,8 +18,11 @@ namespace QuestPDF.Elements.Text
         private Queue<ITextBlockItem> RenderingQueue { get; set; }
         private int CurrentElementIndex { get; set; }
 
+        private bool FontFallbackApplied { get; set; } = false;
+        
         public void ResetState()
         {
+            ApplyFontFallback();
             InitializeQueue();
             CurrentElementIndex = 0;
 
@@ -37,6 +40,15 @@ namespace QuestPDF.Elements.Text
                 foreach (var item in Items)
                     RenderingQueue.Enqueue(item);
             }
+
+            void ApplyFontFallback()
+            {
+                if (FontFallbackApplied)
+                    return;
+
+                Items = Items.ApplyFontFallback().ToList();
+                FontFallbackApplied = true;
+            }
         }
 
         internal override SpacePlan Measure(Size availableSpace)

QuestPDF/Infrastructure/TextStyle.cs

@@ -19,12 +19,10 @@ namespace QuestPDF.Infrastructure
         internal bool? HasUnderline { get; set; }
         internal bool? WrapAnywhere { get; set; }
 
-        internal object PaintKey { get; private set; }
-        internal object FontMetricsKey { get; private set; }
+        // TODO: without cache, this may be an expensive operation
+        internal object PaintKey => (FontFamily, Size, FontWeight, FontPosition, IsItalic, Color);
+        internal object FontMetricsKey => (FontFamily, Size, FontWeight, IsItalic);
         
-        // REVIEW: Should this be a method call that news up a TextStyle,
-        // or can it be a static variable?
-        // (style mutations seem to create a clone anyway)
         internal static readonly TextStyle LibraryDefault = new TextStyle
         {
             Color = Colors.Black,
@@ -49,10 +47,7 @@ namespace QuestPDF.Infrastructure
                 return;
             
             HasGlobalStyleApplied = true;
-
             ApplyParentStyle(globalStyle);
-            PaintKey ??= (FontFamily, Size, FontWeight, FontPosition, IsItalic, Color);
-            FontMetricsKey ??= (FontFamily, Size, FontWeight, IsItalic);
         }
         
         internal void ApplyParentStyle(TextStyle parentStyle)