Sophisticated font fallback #22240

ramezgerges · 2026-01-05T12:22:20Z

GitHub Issue: closes #20292

PR Type:

What is the current behavior? 🤔

What is the new behavior? 🚀

PR Checklist ✅

Please check if your PR fulfills the following requirements:

📝 Commits must be following the Conventional Commits specification.
🧪 Added Runtime tests, UI tests, or a manual test sample for the changes have been added (for bug fixes / features) (if applicable)
📚 Docs have been added/updated which fit documentation template (for bug fixes / features)
🖼️ Validated PR Screenshots Compare Test Run results.
❗ Contains NO breaking changes

Other information ℹ️

unodevops · 2026-01-05T12:44:11Z

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-22240/docs/index.html

unodevops · 2026-01-05T13:11:55Z

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-22240/docs/index.html

Copilot

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

ramezgerges · 2026-01-07T20:48:56Z

/azp run

azure-pipelines · 2026-01-07T20:49:13Z

Azure Pipelines successfully started running 1 pipeline(s), but failed to run 1 pipeline(s).

unodevops · 2026-01-07T21:34:47Z

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-22240/docs/index.html

unodevops · 2026-01-08T14:20:08Z

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-22240/docs/index.html

Copilot

Copilot reviewed 16 out of 17 changed files in this pull request and generated 8 comments.

Copilot · 2026-01-09T17:42:03Z

src/FontFallbackPreprocessor/FontFallbackPreprocessor.csproj

+    </PropertyGroup>
+
+	<ItemGroup>
+		<PackageReference Include="System.CommandLine" Version="2.0.0-beta4.22272.1" />


The System.CommandLine package version 2.0.0-beta4.22272.1 is a pre-release beta version from 2022. Consider using a stable release version if available, as beta versions may have bugs or breaking changes. Check if a stable 2.0+ version has been released since this beta.

Suggested change

<PackageReference Include="System.CommandLine" Version="2.0.0-beta4.22272.1" />

<PackageReference Include="System.CommandLine" Version="2.0.0" />

Copilot · 2026-01-09T17:42:04Z

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBlock.cs

+			((CompositionTarget)expected.Visual.CompositionTarget)!.FrameRendered += async () =>
+			{
+				var screenshot1 = await UITestHelper.ScreenShot(SUT);
+				var screenshot2 = await UITestHelper.ScreenShot(expected);
+
+				var rect = ImageAssert.GetColorBounds(screenshot2, ((SolidColorBrush)DefaultBrushes.TextForegroundBrush).Color);
+
+				// we tolerate a 2 pixels difference between the bitmaps due to font differences
+				matched = rect is { Width: > 0, Height: > 0 } && await ImageAssert.AreRenderTargetBitmapsEqualAsync(screenshot1.Bitmap, screenshot2.Bitmap);
+			};


The FrameRendered event handler is async void, which can lead to unhandled exceptions and test flakiness. The event handler performs async operations (UITestHelper.ScreenShot and ImageAssert.AreRenderTargetBitmapsEqualAsync) but any exceptions thrown won't be properly caught by the test framework. Consider using a TaskCompletionSource or similar mechanism to properly handle async operations in this event handler.

Copilot · 2026-01-09T17:42:04Z

src/Uno.UI.Runtime.Skia.Android/UI/Xaml/Controls/TextBox/AndroidSkiaFontFallbackService.cs

+		_fonts = Task.Factory.StartNew(() =>
+		{
+			return Directory.EnumerateFiles("/system/fonts")
+				.Select(f => (Path.GetFileName(f), SKTypeface.FromStream(new MemoryStream(File.ReadAllBytes(f)))))


Loading all font files from /system/fonts into memory at once could cause significant memory pressure, especially on memory-constrained Android devices. The entire contents of each font file are read into a MemoryStream via File.ReadAllBytes. Consider lazy-loading fonts on-demand or using file streams directly with SKTypeface.FromFile if available.

Suggested change

.Select(f => (Path.GetFileName(f), SKTypeface.FromStream(new MemoryStream(File.ReadAllBytes(f)))))

.Select(f => (Path.GetFileName(f), SKTypeface.FromFile(f)))

Copilot · 2026-01-09T17:42:04Z

src/Uno.UI.Runtime.Skia.Android/UI/Xaml/Controls/TextBox/AndroidSkiaFontFallbackService.cs

+	public async Task<string?> GetFontNameForCodepoint(int codepoint)
+	{
+		foreach (var (fontName, typeface) in await _fonts)
+		{
+			if (typeface.ContainsGlyph(codepoint))
+			{
+				return fontName;
+			}
+		}
+		return null;


The GetFontNameForCodepoint method performs a linear search through all loaded fonts for each codepoint. This could be slow when many fonts are loaded. Consider building an index/cache mapping codepoint ranges to fonts after loading to improve lookup performance.

Copilot · 2026-01-09T17:42:05Z

src/Uno.UI.Runtime.Skia.Android/UI/Xaml/Controls/TextBox/AndroidSkiaFontFallbackService.cs

+	}
+
+	public async Task<SKTypeface?> GetTypefaceForFontName(string fontName, FontWeight weight, FontStretch stretch, FontStyle style)
+		=> (await _fonts).FirstOrDefault(f => f.fontName.Equals(fontName)).typeface;


Potential null reference exception when accessing default tuple element. If no font matches the fontName in the FirstOrDefault call, the tuple will have default values (null for both fontName and typeface), but the code directly accesses .typeface without checking if a match was found. This could return null when a font was actually found but has a null typeface, versus when no font was found at all.

Suggested change

=> (await _fonts).FirstOrDefault(f => f.fontName.Equals(fontName)).typeface;

{

var fonts = await _fonts;

var match = fonts.FirstOrDefault(f => string.Equals(f.fontName, fontName, StringComparison.Ordinal));

// If no matching font name was found, FirstOrDefault returns the default tuple (null, null).

// In that case, return null to indicate no matching typeface.

if (match.fontName is null)

{

return null;

}

return match.typeface;

}

Copilot · 2026-01-09T17:42:05Z

src/Uno.UI/UI/Xaml/Documents/FontFallbackService.skia.cs

+				font = locale switch
+				{
+					"zh-tw" or "zh-mo" => "TraditionalChinese",
+					"zh-hk" or "zh-hant-hk" => "TraditionalChineseHK",
+					_ when locale.Contains("hant") => "TraditionalChinese",
+					_ when locale.Contains("hans") => "SimplifiedChinese",
+					_ when locale.Contains("ja") => "Japanese",
+					_ when locale.Contains("ko") => "Korean",


The locale-based CJK font selection uses string.Contains on the locale which could lead to false matches. For example, a locale like "en-hansville" would incorrectly match the "hans" (Simplified Chinese) condition. Use more precise matching or check for specific locale patterns.

Suggested change

font = locale switch

{

"zh-tw" or "zh-mo" => "TraditionalChinese",

"zh-hk" or "zh-hant-hk" => "TraditionalChineseHK",

_ when locale.Contains("hant") => "TraditionalChinese",

_ when locale.Contains("hans") => "SimplifiedChinese",

_ when locale.Contains("ja") => "Japanese",

_ when locale.Contains("ko") => "Korean",

var localeParts = locale.Split('-', StringSplitOptions.RemoveEmptyEntries);

var languagePart = localeParts.Length > 0 ? localeParts[0] : string.Empty;

bool HasScript(string script) =>

localeParts.Any(p => string.Equals(p, script, StringComparison.Ordinal));

bool IsLanguage(string lang) =>

string.Equals(languagePart, lang, StringComparison.Ordinal);

font = locale switch

{

"zh-tw" or "zh-mo" => "TraditionalChinese",

"zh-hk" or "zh-hant-hk" => "TraditionalChineseHK",

_ when HasScript("hant") => "TraditionalChinese",

_ when HasScript("hans") => "SimplifiedChinese",

_ when IsLanguage("ja") => "Japanese",

_ when IsLanguage("ko") => "Korean",

Copilot · 2026-01-09T17:42:05Z

src/Uno.UI/UI/Xaml/Documents/FontFallbackService.skia.cs

+		if (_missingCodepoints.Count > 0)
+		{
+			await FetchFontsForMissingCodepoints();
+		}
+		else
+		{
+			_fetchTask = null;
+		}


Recursive call without a depth limit could cause stack overflow if _missingCodepoints keeps getting populated during font fetching. Consider adding a maximum recursion depth counter or restructuring to use iteration instead of recursion.

Copilot · 2026-01-09T17:42:06Z

src/Uno.UI/UI/Xaml/Documents/TextFormatting/FontDetailsCache.skia.cs

+		var (tempFont, task) = GetFont(name, fontSize, weight, stretch, style);
+		if (!task.IsCompleted)
+		{
+			_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.FromCurrentSynchronizationContext());


Potential exception if TaskScheduler.FromCurrentSynchronizationContext() returns null. When called from a thread without a SynchronizationContext (e.g., a background thread), FromCurrentSynchronizationContext() will throw an InvalidOperationException. The continuation should handle this case or ensure it's only called from a context with a valid SynchronizationContext.

Suggested change

_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.FromCurrentSynchronizationContext());

var synchronizationContext = SynchronizationContext.Current;

if (synchronizationContext is not null)

{

_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.FromCurrentSynchronizationContext());

}

else

{

_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.Current);

}

unodevops · 2026-01-09T19:14:52Z

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-22240/wasm-skia-net9/index.html

unodevops · 2026-01-09T20:09:22Z

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-22240/docs/index.html

unodevops · 2026-01-10T01:31:03Z

⚠️⚠️ The build 190710 has failed on Uno.UI - CI.

Copilot AI review requested due to automatic review settings January 5, 2026 12:22

github-actions bot added platform/wasm 🌐 Categorizes an issue or PR as relevant to the WebAssembly platform platform/android 🤖 Categorizes an issue or PR as relevant to the Android platform area/skia ✏️ Categorizes an issue or PR as relevant to Skia labels Jan 5, 2026

Copilot started reviewing on behalf of ramezgerges January 5, 2026 12:22 View session

Copilot AI reviewed Jan 5, 2026

View reviewed changes

ramezgerges added 14 commits January 8, 2026 07:48

chore: invalidate when fallback font task is resolved

9417a07

chore: minor fix and refactoring

6364e86

chore: initial attempt at font fallback service

8625ec7

chore: IFontFallbackService

dd21450

chore: fetching from github and more work

cd09e79

chore: avoid ContinueWith

60ba185

chore: cjk handling

ab9d0e1

chore: move win32 logic to Uno.WinUI and commit generated files

0289a8e

chore: add FontFallbackPreprocessor

903edc5

chore: add CI check for font fallback generation

87dcb71

chore: improved CJK font matching

78263e6

chore: add license

85c326e

test: add Check_FontFallback_Shaping2

66ecbfb

chore: move AndroidSkiaFontFallbackService work off the UI thread

5775d75

ramezgerges force-pushed the sophisticated_font_fallback branch from 8e0bbfa to 5775d75 Compare January 8, 2026 12:48

chore: azure script mistake

276d8e9

Copilot AI review requested due to automatic review settings January 9, 2026 17:31

Copilot started reviewing on behalf of ramezgerges January 9, 2026 17:31 View session

Copilot AI reviewed Jan 9, 2026

View reviewed changes

	<PackageReference Include="System.CommandLine" Version="2.0.0-beta4.22272.1" />
	<PackageReference Include="System.CommandLine" Version="2.0.0" />

	.Select(f => (Path.GetFileName(f), SKTypeface.FromStream(new MemoryStream(File.ReadAllBytes(f)))))
	.Select(f => (Path.GetFileName(f), SKTypeface.FromFile(f)))

-		=> (await _fonts).FirstOrDefault(f => f.fontName.Equals(fontName)).typeface;
+	{
+		var fonts = await _fonts;
+		var match = fonts.FirstOrDefault(f => string.Equals(f.fontName, fontName, StringComparison.Ordinal));
+		// If no matching font name was found, FirstOrDefault returns the default tuple (null, null).
+		// In that case, return null to indicate no matching typeface.
+		if (match.fontName is null)
+		{
+			return null;
+		}
+		return match.typeface;
+	}

-			_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.FromCurrentSynchronizationContext());
+			var synchronizationContext = SynchronizationContext.Current;
+			if (synchronizationContext is not null)
+			{
+				_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.FromCurrentSynchronizationContext());
+			}
+			else
+			{
+				_ = task.ContinueWith(_ => onFontLoaded(), TaskScheduler.Current);
+			}

Sophisticated font fallback #22240

Are you sure you want to change the base?

Sophisticated font fallback #22240

Conversation

ramezgerges commented Jan 5, 2026

PR Type:

What is the current behavior? 🤔

What is the new behavior? 🚀

PR Checklist ✅

Other information ℹ️

Uh oh!

unodevops commented Jan 5, 2026

Uh oh!

unodevops commented Jan 5, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Uh oh!

ramezgerges commented Jan 7, 2026

Uh oh!

azure-pipelines bot commented Jan 7, 2026

Uh oh!

unodevops commented Jan 7, 2026

Uh oh!

unodevops commented Jan 8, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 9, 2026

Choose a reason for hiding this comment

Uh oh!

unodevops commented Jan 9, 2026

Uh oh!

unodevops commented Jan 9, 2026

Uh oh!

unodevops commented Jan 10, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants