Design System Structure

design-system-structure

A design system is code that is incapable of holding a stray opinion: every aesthetic value lives in one directory, and every widget reads it back through a named slot. This skill owns how tokens, themes, components, and shapes are structured and consumed — not which colors or radii to pick (that belongs to your design source of truth). Getting the structure right makes a reskin a diff lib/theme/ and turns a stray Color(0xFF…) in feature code into a build failure instead of a style nit.

Read the reference for the task at hand:

• references/token-tiers-and-themeextension.md — two-tier token naming, ThemeExtension mechanics, the asserting of(), hand-authored ColorScheme vs fromSeed, honest lerp.
• references/motion-and-reduced-motion.md — the three animations Material mounts by default, why NoSplash is not enough, reduced-motion-means-zero, pumpAndSettle bans in tests.
• references/typography-and-fonts.md — bundling fonts, LicenseRegistry, FontWeight drives wght, silent FontVariation no-ops, subsetting without instancing, per-script fallback cascades.
• references/contrast-and-redundant-encoding.md — AA as a unit test, ≥3 grayscale-legible signals per meaning, reading a11y flags from MediaQuery.

Run scripts/check_raw_values.sh and scripts/check_font_bundling.sh before a PR.

Non-negotiable rules

1. No raw aesthetic value outside lib/theme/** — the gate is the law. A Color(0x…) / Colors.* (except transparent) / Curves.* / Duration(milliseconds:|seconds: (except Duration.zero) / literal BorderRadius.circular(n) / fontSize: n in feature or shared UI code fails scripts/check_raw_values.sh. A legitimate new need is a new token slot, never a // ignore — one place to diff is the whole point.
2. Two token tiers; widgets read only the semantic tier. Tier 1 = primitives named by the measured value (neutral12 = OKLCH lightness ×100), never by rank (grey700), appearance (darkGrey), or brand (brandPrimary). Tier 2 = semantic slots named by role (surface, onSurface, hairline, accent). A widget that reaches a Tier-1 primitive has hardcoded one theme. WHY: rank scales have no room to insert and lie in dark mode; appearance names invert catastrophically; brand names die with the brand.
3. Read tokens through a ThemeExtension and an asserting of(context). Never a static const class, never an InheritedWidget side channel. of() asserts on a missing extension — it does not ?? fallback. WHY: MaterialApp drives light/dark switching and correct-by-construction invalidation; a fallback silently ships a theme no test verified, and loud-in-debug beats wrong-in-field.
4. Hand-author ColorScheme; never ColorScheme.fromSeed (or dynamic_color) for identity. fromSeed's per-role overrides do not propagate — override surface and surfaceContainerHigh stays seed-derived. State every role you consume, keep the M3 role names so Material widgets theme themselves, and let unstated roles take ColorScheme's own defaults, not a seed's opinion. WHY: "seed plus a few overrides" means owning the ~9 roles you chose and every seam of the ~40 you didn't.
5. Layer the system: tokens → theme → modifiers/components → shapes. Author light and dark by hand (dark is never an auto-flip), each passing AA independently, and attach every extension to both ThemeDatas. Inject the theme exactly once, at the composition root. WHY: theming the app must touch one directory; a palette-only dark theme ships light's ratios into the dark.
6. lerp honestly or snap deliberately; copyWith is mandatory. Interpolate colors and doubles; snap non-interpolables (font families, ShapeBorder) at t < 0.5. If your app bans theme animation and you snap everything, leave a comment — a bare step-lerp reads as unfinished and the next reader will "fix" it. WHY: a new field silently forgotten in lerp is the classic design-system rot.
7. Color is a derived token, computed LAST — never a status's only channel. Color is a pure function of a canonical value object (an enum/small class), resolved through a semantic slot at the end; status is never read from color, and the supporting color is a slot read, not a literal. The never-color-alone floor itself — ≥3 non-color signals (glyph, label, weight/position) per stateful meaning — is owned by accessibility-as-code; this skill only enforces that the color reinforcing them is a derived slot. WHY: a color not derived from the value object drifts out of sync, and a widget that leans on color alone dies in grayscale.
8. Reduced motion collapses to zero, not "gentler." Read MediaQuery.disableAnimationsOf(context) and resolve to Duration.zero / a cross-fade to the end state — never a shorter duration or a softer curve. Motion is never the only signal for a state change. WHY: a user who asked the OS to stop animations asked for stop.
9. Restore the persisted theme before first paint. Load the saved theme mode / variant before runApp, from the same versioned settings the rest of the app uses (not an async shared_preferences read that lands a frame late). An unknown or corrupt stored value falls back explicitly and visibly, never to a null that leaves of() asserting on a device with no debugger. WHY: a flash of the wrong theme is a sudden luminance change the user did not cause.
10. Bundle fonts; never google_fonts at runtime. Declare fonts in pubspec.yaml, ship the license text and register it via LicenseRegistry, and drive weight with FontWeight (which drives the wght axis) — not a redundant FontVariation('wght', …). Declare a per-script fontFamilyFallback cascade that ends in a known-good face. WHY: google_fonts ships an HTTP code path by default; FontVariation('opsz'|'ital', …) silently no-ops on a font that lacks the axis, so a "working" italic is invisible in review.
11. Painters snapshot tokens; paint() never reads BuildContext. Resolve Theme.of(context).extension<T>() once at the widget layer and pass the values into the CustomPainter/Scene fields. WHY: keeps paint() allocation-free and testable without a MaterialApp; see custom-canvas-and-gestures.
12. The reduced-motion flag is read from MediaQuery (disableAnimationsOf), never app state — it feeds resolveMotion (this skill's helper, below). Reading the other platform a11y flags (boldTextOf, highContrastOf, textScaler) from MediaQuery rather than a stale app-state copy is owned by accessibility-as-code.

The token layering

tokens → theme → modifiers → shapes. Primitives carry values; a ThemeExtension names semantic slots; a ThemeData builder folds slots into Material's ColorScheme and component themes; shape factories live on their own extension so a component asks for a silhouette, not a radius.

dartCopy code
// lib/theme/primitives.dart — TIER 1. The ONLY file allowed a raw color literal.
// Named by MEASURED value; values are placeholders owned by your design authority.
abstract final class Primitives {
  static const neutral08 = Color(0xFF141414); // L .08 — ink
  static const neutral96 = Color(0xFFF4F4F4); // L .96 — paper
  static const neutral40 = Color(0xFF636363);
  static const accent45  = Color(0xFF3A6FF0);
}

// lib/theme/app_colors.dart — TIER 2. Widgets read THESE, never a primitive.
@immutable
class AppColors extends ThemeExtension<AppColors> {
  const AppColors({required this.surface, required this.onSurface, required this.accent});
  final Color surface, onSurface, accent;

  static AppColors of(BuildContext context) {
    final ext = Theme.of(context).extension<AppColors>();
    assert(ext != null, 'AppColors missing. Build ThemeData via buildAppTheme().');
    return ext!; // assert, never `?? fallback` — a fallback ships an unverified theme.
  }
  // copyWith + lerp omitted here — see examples/app_theme.dart for the full class.
}

Full worked file (primitives, a shapes extension with a cardShape factory, the hand-authored ColorScheme, honest lerp, and one-shot injection): examples/app_theme.dart.

Hand-author ColorScheme, keep the M3 role names

dartCopy code
// WRONG — surfaceContainerHigh, surfaceDim, surfaceBright etc. stay seed-derived;
// your `surface` override does not propagate to them.
ColorScheme.fromSeed(seedColor: Primitives.accent45, surface: c.surface);

// RIGHT — state every role you read. Unstated roles get ColorScheme's defaults,
// not a seed's opinion, and the M3 names mean Material widgets theme themselves.
ColorScheme(
  brightness: brightness,
  primary: c.accent, onPrimary: c.surface,
  surface: c.surface, onSurface: c.onSurface,
  surfaceContainerHighest: c.surface, onSurfaceVariant: c.onSurfaceMuted,
  outline: c.hairline, error: c.danger, onError: c.surface,
);

A Material TextField reads surfaceContainerHighest, onSurfaceVariant, outline, and error on its own — name the roles right and it themes with zero per-widget InputDecoration patching that would drift out of sync on the next theme change.

Reduced motion: read the flag, collapse to zero

dartCopy code
// The one place a widget asks "should I animate?" — collapses under the OS flag.
Duration resolveMotion(BuildContext context, Duration full) =>
    MediaQuery.disableAnimationsOf(context) ? Duration.zero : full;

AnimatedContainer(
  duration: resolveMotion(context, AppMotion.of(context).medium), // token, not 300ms
  curve: AppMotion.of(context).enter,                              // token, not Curves.easeOut
  color: AppColors.of(context).surface,
  child: child,
);

NoSplash.splashFactory alone is not enough — the pressed highlight fade is a separate InkHighlight, and MaterialApp interpolates ThemeData over kThemeAnimationDuration unless you pass themeAnimationStyle: AnimationStyle.noAnimation. See references/motion-and-reduced-motion.md for all three switches.

Restore the theme before first paint

dartCopy code
// bootstrap: settings are loaded BEFORE runApp, so the first frame is correct.
Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  final settings = await Settings.load();               // synchronous by the time runApp runs
  final mode = ThemeVariant.tryParse(settings.themeName) // unknown/corrupt -> explicit default
      ?? ThemeVariant.system;
  runApp(App(initialVariant: mode));
}

Persisting the choice but reading it back asynchronously (a bare shared_preferences read) lands a frame late and flashes the wrong theme. state-management-riverpod covers threading the resolved variant through a themeModeProvider; app-startup-and-bootstrap covers the main() ordering.

Redundant encoding: derive color last

The ≥3-non-color-signals floor is owned by accessibility-as-code; the design-system contribution shown here is narrower — the supporting color is derived last from a canonical value object through a semantic slot, never read as the source of state:

dartCopy code
enum OrderStatus { pending, active, blocked }

extension OrderStatusView on OrderStatus {
  IconData get icon => switch (this) {          // signal 1: shape differs per status
        OrderStatus.pending => Icons.schedule,
        OrderStatus.active  => Icons.play_arrow,
        OrderStatus.blocked => Icons.error_outline,
      };
  String label(AppLocalizations l) => switch (this) { … }; // signal 2: the word
  double get emphasis => this == OrderStatus.blocked ? 2 : 1; // signal 3: monotonic weight
  Color color(AppColors c) => switch (this) {   // color is DECORATION, derived LAST
        OrderStatus.pending => c.onSurfaceMuted,
        OrderStatus.active  => c.accent,
        OrderStatus.blocked => c.danger,
      };
}

Full chip widget rendering all three signals: examples/status_encoding.dart.

Anti-patterns

• Color(0xFF…), Colors.blueGrey, Curves.easeOut, Duration(milliseconds: 200), BorderRadius.circular(16), or fontSize: 14 in a widget outside lib/theme/ — the gate fails; the fix is a slot read or a new slot, never // ignore.
• A static AppColors/Dimens const class, or AppColors.of returning ?? AppColors.fallback() — slots ride a ThemeExtension, and a fallback ships a palette no test ever verified.
• ColorScheme.fromSeed(...) for a product with an identity, or adding dynamic_color — the overrides don't propagate and wallpaper-derived palettes are untestable at build time.
• A palette-only dark theme, or extensions attached to the light ThemeData only — dark silently renders defaults and theme switches snap.
• A lerp that returns this unlabeled, or that forgets a newly added field — the theme snaps or a slot goes un-interpolated forever.
• Color as the only status channel — recoloring is fine; removing the glyph/label/weight it accompanies is a bug that vanishes in grayscale.
• Choosing a shorter duration or softer curve under reduced motion — collapse to Duration.zero; the user asked for stop.
• A bare shared_preferences read for theme, corrected a frame after runApp — a flash of the wrong theme is a luminance jolt; restore before first paint.
• import 'package:google_fonts/…' or FontVariation('opsz'|'ital', …) — the first ships an HTTP path; the second no-ops silently on a font without the axis.
• A CustomPainter calling Theme.of(context) inside paint() — snapshot tokens into painter fields at the widget layer.
• A token pipeline (DTCG JSON, Style Dictionary, Figma sync codegen) for a small single-package app — it buys a node_modules build-failure mode to solve a designer/engineer handoff that, when it's the same person, does not exist.

Definition of done

• scripts/check_raw_values.sh and scripts/check_font_bundling.sh are clean over lib/ (raw values confined to lib/theme/**; no google_fonts).
• Every aesthetic value a widget renders traces to a named semantic slot on a ThemeExtension, read via an asserting of(context).
• Tokens are two-tier: primitives named by measured value, semantic slots named by role; no widget reads a primitive.
• ColorScheme is hand-authored for both brightnesses (never fromSeed for identity); M3 role names kept; every extension attached to both ThemeDatas.
• Both themes pass AA independently, verified as a computeLuminance() unit test over declared fg/bg pairs (references/contrast-and-redundant-encoding.md).
• copyWith implemented; lerp interpolates colors/doubles and snaps non-interpolables (or is a commented deliberate step).
• Every stateful meaning carries ≥3 grayscale-legible signals; a greyscale golden still answers "what state is this?".
• Every animation reads a motion token and collapses to zero under MediaQuery.disableAnimationsOf.
• The persisted theme is restored before first paint; unknown values fall back explicitly, never to null.
• Fonts are bundled and license-registered; per-script fontFamilyFallback declared; no runtime font fetch.
• Painters receive a token snapshot; no Theme.of(context) in paint().

Related skills

• See accessibility-as-code for reading a11y state from MediaQuery, the never-color-alone rule, target sizes, and never clamping textScaler.
• See custom-canvas-and-gestures for the painter that consumes the token snapshot.
• See flutter-performance for const subtrees, .select rebuild scoping, and RepaintBoundary.
• See widget-composition for building views from small const widgets that read these slots.
• See state-management-riverpod for the themeModeProvider and composition-root injection.
• See app-startup-and-bootstrap for the main() ordering that restores the theme before runApp.
• See i18n-rtl-l10n for per-script fonts, directional geometry, and RTL goldens.
• See lint-and-style-config for promoting the no-raw-values grep into a CI gate.
• See motion-and-haptics for what the motion tokens are spent on: the moment catalog, the haptic event map, and the degradation each moment declares.
• See design-review-workflow for the once-per-app screenshot sweep that judges the assembled system.

References

• Flutter API — ThemeExtension: https://api.flutter.dev/flutter/material/ThemeExtension-class.html
• Flutter API — ThemeData (extensions, component themes, colorScheme): https://api.flutter.dev/flutter/material/ThemeData-class.html
• Flutter API — ColorScheme (roles; fromSeed caveats): https://api.flutter.dev/flutter/material/ColorScheme-class.html
• Flutter cookbook — Use themes to share colors and font styles: https://docs.flutter.dev/cookbook/design/themes
• Flutter cookbook — Use a custom font (bundling, fontFamilyFallback): https://docs.flutter.dev/cookbook/design/fonts
• Flutter API — MediaQueryData.disableAnimations / textScaler: https://api.flutter.dev/flutter/widgets/MediaQueryData-class.html
• Flutter API — Color.computeLuminance (WCAG relative luminance): https://api.flutter.dev/flutter/dart-ui/Color/computeLuminance.html
• Flutter API — LicenseRegistry: https://api.flutter.dev/flutter/foundation/LicenseRegistry-class.html
• Material Design 3 — design tokens: https://m3.material.io/foundations/design-tokens/overview
• W3C WAI — WCAG 2.2 SC 1.4.3 Contrast (Minimum): https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html
• W3C WAI — WCAG 2.2 SC 1.4.1 Use of Color: https://www.w3.org/WAI/WCAG22/Understanding/use-of-color.html