Flutter Design

Flutter Design Patterns

Flutter/Dart implementation companion for mobile-app-design-mastery skill. Translates Refactoring UI principles into Flutter code.

Prerequisite: This skill provides Flutter-specific syntax. For mobile design theory and decision-making, reference mobile-app-design-mastery skill.

⚠️ CRITICAL: Project Theme First

ALWAYS check existing theme configuration before creating new styles.

If the project has ThemeData, AppColors, AppTextStyles, or similar—USE THEM:

// Check lib/core/theme/ or lib/config/
// Common patterns:
AppColors.primary
AppTextStyles.headline
AppSpacing.md
context.theme.colorScheme.primary

Priority order:

1. Project-defined theme (AppColors, AppTextStyles, custom ThemeExtension)
2. Theme.of(context) access
3. Hardcoded values as last resort only

ThemeData Setup (Material 3)

MaterialApp(
  theme: ThemeData(
    useMaterial3: true,
    colorScheme: ColorScheme.fromSeed(
      seedColor: Colors.blue, // Brand color
      brightness: Brightness.light,
    ),
    textTheme: _textTheme,
  ),
  darkTheme: ThemeData(
    useMaterial3: true,
    colorScheme: ColorScheme.fromSeed(
      seedColor: Colors.blue,
      brightness: Brightness.dark,
    ),
    textTheme: _textTheme,
  ),
);

Color Access Patterns

// ✅ CORRECT: Use ColorScheme
final colors = Theme.of(context).colorScheme;
Container(color: colors.primary)
Text('Hello', style: TextStyle(color: colors.onSurface))

// ✅ With extension
extension BuildContextX on BuildContext {
  ColorScheme get colors => Theme.of(this).colorScheme;
}
// Usage: context.colors.primary

// ❌ AVOID: Hardcoded colors
Container(color: Colors.blue) // Ignores theme

ColorScheme roles:

Typography (TextTheme)

Material 3 Type Scale:

// ✅ CORRECT: Use TextTheme
Text('Title', style: Theme.of(context).textTheme.headlineMedium)

// With extension
extension BuildContextX on BuildContext {
  TextTheme get textTheme => Theme.of(this).textTheme;
}
// Usage: context.textTheme.bodyMedium

Spacing System (4dp Grid)

Create reusable spacing constants:

abstract class AppSpacing {
  static const double xs = 4;
  static const double sm = 8;
  static const double md = 12;
  static const double base = 16;
  static const double lg = 24;
  static const double xl = 32;
  static const double xxl = 48;
  
  // SizedBox shortcuts
  static const SizedBox gapXs = SizedBox(height: xs, width: xs);
  static const SizedBox gapSm = SizedBox(height: sm, width: sm);
  static const SizedBox gapMd = SizedBox(height: md, width: md);
  static const SizedBox gapBase = SizedBox(height: base, width: base);
  static const SizedBox gapLg = SizedBox(height: lg, width: lg);
}

// Usage
Padding(padding: EdgeInsets.all(AppSpacing.base))
Column(children: [widget1, AppSpacing.gapMd, widget2])

BoxDecoration Patterns

Card (Flat)

Container(
  decoration: BoxDecoration(
    color: colors.surface,
    borderRadius: BorderRadius.circular(12),
    border: Border.all(color: colors.outline.withOpacity(0.2)),
  ),
)

Card (Elevated)

Container(
  decoration: BoxDecoration(
    color: colors.surface,
    borderRadius: BorderRadius.circular(12),
    boxShadow: [
      BoxShadow(
        color: Colors.black.withOpacity(0.08),
        blurRadius: 10,
        offset: const Offset(0, 2),
      ),
    ],
  ),
)

Input Field

Container(
  decoration: BoxDecoration(
    color: colors.surface,
    borderRadius: BorderRadius.circular(8),
    border: Border.all(color: colors.outline),
  ),
)

Shadow Scale

abstract class AppShadows {
  static List<BoxShadow> sm = [
    BoxShadow(
      color: Colors.black.withOpacity(0.05),
      blurRadius: 4,
      offset: const Offset(0, 1),
    ),
  ];
  
  static List<BoxShadow> md = [
    BoxShadow(
      color: Colors.black.withOpacity(0.08),
      blurRadius: 8,
      offset: const Offset(0, 2),
    ),
  ];
  
  static List<BoxShadow> lg = [
    BoxShadow(
      color: Colors.black.withOpacity(0.1),
      blurRadius: 16,
      offset: const Offset(0, 4),
    ),
  ];
  
  static List<BoxShadow> xl = [
    BoxShadow(
      color: Colors.black.withOpacity(0.15),
      blurRadius: 24,
      offset: const Offset(0, 8),
    ),
  ];
}

// Usage
Container(decoration: BoxDecoration(boxShadow: AppShadows.md))

Anti-Patterns

// ❌ NEVER
Colors.blue                    // Hardcoded, ignores theme
TextStyle(fontSize: 16)        // Not from TextTheme
SizedBox(height: 17)           // Off 4dp grid
EdgeInsets.only(top: 20, left: 8)  // Asymmetric without reason

// ✅ INSTEAD
context.colors.primary
context.textTheme.bodyLarge
SizedBox(height: 16)           // On grid
EdgeInsets.all(16)             // Symmetric

Reference Files