Set the font weight variation axis based on the text style's FontWeight #175771

jason-simmons · 2025-09-22T15:10:05Z

This makes it possible for applications to set a FontWeight and get the expected result for both variable fonts and fonts that provide separate assets for each weight.

See #148026

gemini-code-assist

Code Review

This pull request refactors FontWeight to be based on an integer value rather than a discrete index, enabling support for variable font weights. The changes are consistently applied across the Dart and C++ layers, including the CanvasKit and Skwasm backends, by setting the wght font variation axis from TextStyle.fontWeight. New tests are also included to validate this functionality. My review includes suggestions to enhance API robustness by adding assertions to the FontWeight constructor and to refactor a section of the font variation logic for improved performance and readability.

engine/src/flutter/lib/ui/text.dart

engine/src/flutter/lib/web_ui/lib/src/engine/skwasm/skwasm_impl/paragraph.dart

engine/src/flutter/lib/web_ui/lib/text.dart

A proposed engine PR will allow construction of arbitrary FontWeight instances and thus needs to overload the FontWeight equality operator (see flutter/flutter#175771) With that change it will no longer be possible to use FontWeight as the key in a const map.

eyebrowsoffire

Skwasm changes look good!

flutter-dashboard · 2025-10-01T23:30:29Z

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Changes reported for pull request #175771 at sha d411900

engine/src/flutter/lib/ui/text.dart

harryterkelsen · 2025-10-02T23:07:57Z

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/text.dart

      skTextStyle.fontStyle = toSkFontStyle(fontWeight, fontStyle);
    }

+    final int weightValue = fontWeight?.value ?? ui.FontWeight.normal.value;


We want to avoid adding fontVariations to the text style when we can because it requires us to malloc an array on the WASM side.

Unfortunately I don't know of a way to work around this.

Some variable fonts may not have the default value of the wght attribute configured to match the value of FontWeight.normal. If the engine does not explicitly set the wght value in fontVariations, then these fonts will not render as intended at the normal weight.

That makes sense. I guess we can live with it, given that it seems that browsers apparently always set axes according to CSS, it makes sense for us to set them according to Flutter text style properties (source: https://groups.google.com/g/skia-discuss/c/eFFDObvJyQ8). But let's keep an eye on text benchmarks after this lands.

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/text.dart

harryterkelsen

LGTM for web_ui parts

LongCatIsLooong

LGTM with nits.

LongCatIsLooong · 2025-10-07T07:41:08Z

engine/src/flutter/lib/ui/text.dart

  ///
-  /// Varies the stroke thickness of the font, similar to [FontWeight] but on a
-  /// continuous axis.
+  /// Applications should avoid using this and should instead declare font


Should this be marked as deprecated then?

Or do we want to keep this in case people want to use weight for selecting a specific type face they have in mind and wght for the actual weight used for rendering?

FontVariation.weight is being kept for backward compatibility and for scenarios like the one you described where the app wants direct control over wght (see #148026 (comment))

LongCatIsLooong · 2025-10-07T20:24:50Z

engine/src/flutter/lib/ui/text.dart

+/// constructed using values other than the predefined values.
 class FontWeight {
-  const FontWeight._(this.index, this.value);
+  /// Create a [FontWeight] object, which can be added to a [TextStyle] to


Suggested change

/// Create a [FontWeight] object, which can be added to a [TextStyle] to

/// Creates a [FontWeight] object, which can be added to a [TextStyle] to

LongCatIsLooong · 2025-10-07T20:26:09Z

engine/src/flutter/lib/ui/text.dart


  /// The encoded integer value of this font weight.
-  final int index;
+  int get index => (value ~/ 100 - 1).clamp(0, 8);


Do we still need this? I think this should be deprecated / discouraged in the docs?

index is a public API that is being retained for backward compatibility. Marked it as deprecated.

LongCatIsLooong · 2025-10-07T20:28:53Z

engine/src/flutter/lib/ui/text.dart

    }
-    return values[_lerpInt((a ?? normal).index, (b ?? normal).index, t).round().clamp(0, 8)];
+    return FontWeight(
+      _lerpInt((a ?? normal).value, (b ?? normal).value, t).round().clamp(100, 900),


Is there a reason for still clamping the weight value to the [100, 900] range?

This is also for backward compatibility. The docs for FontWeight.lerp say that "The result is clamped to the range w100–w900" (https://api.flutter.dev/flutter/dart-ui/FontWeight/lerp.html)

The wght attribute can potentially range from 1 to 1000. But 100-900 covers most typical use cases.

~~Consider adding a quick comment as it's not immediately obvious why the clamp is necessary.~~

Edit: On second thought, that's not necessary as this is commented on the API already.

LongCatIsLooong · 2025-10-07T21:48:37Z

engine/src/flutter/lib/ui/text.dart

-/// See [FontVariation.weight] for details.
+/// When using these fonts, applications can specify [FontWeight] instances
+/// constructed using values other than the predefined values.
 class FontWeight {


I wonder if this can be an extension type now that it is just an int under the hood?

Ah there are FontWeight subclasses in the wild but since we were using index to communicate the weight I don't think subclassing does anything.

LongCatIsLooong · 2025-10-07T21:52:32Z

engine/src/flutter/lib/ui/text.dart

 /// Some modern fonts allow the weight to be adjusted in arbitrary increments.
-/// See [FontVariation.weight] for details.
+/// When using these fonts, applications can specify [FontWeight] instances
+/// constructed using values other than the predefined values.


nit: maybe cross ref FontVariation.weight and how it interacts with FontWeight?

This makes it possible for applications to set a FontWeight and get the expected result for both variable fonts and fonts that provide separate assets for each weight. See flutter#148026

…s FontWeight (flutter/flutter#175771)

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This adds documentation of the change made by flutter/flutter#175771

…12615) This adds documentation of the change made by flutter/flutter#175771 --------- Co-authored-by: Shams Zakhour <[email protected]> Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

…ht (#175771) This makes it possible for applications to set a FontWeight and get the expected result for both variable fonts and fonts that provide separate assets for each weight. See #148026

…ht (flutter#175771) This makes it possible for applications to set a FontWeight and get the expected result for both variable fonts and fonts that provide separate assets for each weight. See flutter#148026

jason-simmons requested review from chinmaygarde and eyebrowsoffire September 22, 2025 15:10

github-actions bot added a: text input Entering text in a text field or keyboard related problems engine flutter/engine related. See also e: labels. platform-web Web applications specifically e: impeller Impeller rendering backend issues and features requests labels Sep 22, 2025

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jason-simmons mentioned this pull request Sep 23, 2025

[google_fonts] Make a map keyed on FontWeight non-const flutter/packages#10058

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jason-simmons force-pushed the bug_148026 branch from 358c13b to 832d515 Compare October 7, 2025 22:50

jason-simmons added 8 commits October 27, 2025 05:21

Set the font weight variation axis based on the text style's FontWeight

49bd767

This makes it possible for applications to set a FontWeight and get the expected result for both variable fonts and fonts that provide separate assets for each weight. See flutter#148026

weight value asserts and simplificiation of skwasm implementation

bd654c6

analyzer fixes

bc15d49

cupertino/nav_bar_transition_test

6b36d3e

nav_bar_transition_test analyzer

1c3a99e

change kWeightAxisTag to constexpr string_view for Windows/MSVC builds

4ea2cea

set text color in test

68c1ebe

strut fixes

d4d12b1

docs and feedback

a49fef4

jason-simmons force-pushed the bug_148026 branch from f470ac9 to a49fef4 Compare October 27, 2025 12:21

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Merged via the queue into flutter:master with commit e090117 Oct 27, 2025
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Roll Flutter from 4c910983cbfd to 7cf0dc1be72e (31 revisions) flutter/packages#10311

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e090117bd Set the font weight variation axis based on the text style'…

3b38079

…s FontWeight (flutter/flutter#175771)

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Add a breaking change page for FontWeight handling of variable fonts

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This adds documentation of the change made by flutter/flutter#175771

jason-simmons mentioned this pull request Oct 30, 2025

Add a breaking change page for FontWeight handling of variable fonts flutter/website#12615

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chrisbobbe mentioned this pull request Oct 31, 2025

FontWeight should subsume wght in FontVariation #148026

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	/// Create a [FontWeight] object, which can be added to a [TextStyle] to
	/// Creates a [FontWeight] object, which can be added to a [TextStyle] to

Set the font weight variation axis based on the text style's FontWeight #175771

Set the font weight variation axis based on the text style's FontWeight #175771

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