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iOS Fabric: fontFamily (PostScript name) + explicit fontWeight resolves to the heaviest face — elvis-operator typo in RCTFontUtils.mm

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評估

難度
1/5
預估耗時
1 小時以內
新手友好度
85/100
Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
活躍
技術堆疊
ios, objective-c, react-native
領域
mobile-dev

研究方向

從 ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTFontUtils.mm 第 364 行附近開始，檢查明確指定的字型粗細如何選取。使用 Poppins PostScript-name 範例重現問題，並檢查轉譯後的 NSAttributedString 字型 run。完成標準是：明確指定的非預設粗細解析為要求的字型，而不是最粗的字型。

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描述

Needs: Author Feedback Needs: Repro

Description

On iOS/Fabric in React Native 0.86.2, any <Text> style that combines a PostScript-named fontFamily with an explicit non-default fontWeight renders the heaviest face in the family instead of the requested weight.

// Renders Poppins-Medium (correct)
<Text style={{ fontFamily: 'Poppins-Medium' }} />

// Renders Poppins-Bold (bug — expected Poppins-Medium)
<Text style={{ fontFamily: 'Poppins-Medium', fontWeight: '500' }} />

// Renders Poppins-Bold (bug — expected Poppins-SemiBold)
<Text style={{ fontFamily: 'Poppins-SemiBold', fontWeight: '600' }} />

// Bare family name + weight resolves correctly (different code path)
<Text style={{ fontFamily: 'Poppins', fontWeight: '500' }} />  // correct

Verified via the rendered NSAttributedString font runs (not UIFont lookups): the fonts are registered and loadable; the resolver selects the wrong face.

Root cause

ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTFontUtils.mm (~line 364):

fontWeight = (fontWeight != 0.0) ?: RCTGetFontWeight(font);

The GNU ?: (elvis) operator assigns the boolean result of the comparison — 1.0 — for any explicit nonzero weight, rather than preserving the original fontWeight value. UIFontWeight 1.0 is the heaviest weight, so the subsequent family search selects the boldest face.

Suggested fix

fontWeight = (fontWeight != 0.0) ? fontWeight : RCTGetFontWeight(font);

(Note: this still cannot distinguish an explicit fontWeight: '400' from "no weight," since UIFontWeightRegular == 0.0 — a separate, pre-existing limitation.)

Impact

Any app pairing custom-font PostScript names with explicit weights (a common pattern, and the style many older codebases carry from pre-Fabric versions where the named face won) renders bold text across the board after upgrading. We hit this migrating a production app from 0.81.5 to 0.86.2 — every fontFamily: 'Poppins-<Face>' + matching fontWeight pair (~450 style sites) collapsed to Poppins-Bold. We are carrying a one-line patch-package fix of the ternary, which restores correct resolution for all pairings.

Steps to reproduce

  1. Bundle a multi-weight custom font family (e.g. Poppins Regular/Medium/SemiBold/Bold) via UIAppFonts.
  2. Render <Text style={{ fontFamily: 'Poppins-Medium', fontWeight: '500' }}>test</Text> on 0.86.2 with Fabric.
  3. Inspect the rendered attributed string: the resolved font is Poppins-Bold.

Environment

React Native 0.86.2 (Fabric / new architecture), iOS 26.5 simulator + device, Expo SDK 57 prebuild (bare workflow equivalent). Regression vs 0.81.5 behavior.

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