Shape System - M3 Expressive

Overview

Material 3 Expressive introduces a dynamic shape system that moves beyond simple rounded rectangles. Shapes are used as containers, buttons, and state indicators, focusing on fluidity and hierarchy.

Design Principles

Expressive Shapes: Use non-standard shapes (pixel triangles, teardrops, squircles) to convey brand personality.
Shape Hierarchy: Larger, more complex shapes denote higher importance.
Fluid Morphing: Shapes should seamlessly transition between states (e.g., FAB to Extended FAB).
Containment: Use shapes to group related content clearly.

Corner Token System

M3 uses a tokenized corner radius system:

Token	Radius	Example Usage
`none`	0dp	Full-width elements, square buttons
`extra-small`	4dp	Tooltips, selection controls
`small`	8dp	Chips, snackbars
`medium`	12dp	Cards, small dialogs
`large`	16dp	Extended FABs, menus
`extra-large`	28dp	FABs, navigation rails/bars
`full`	100%	Circular buttons, pill-shaped tags

Implementation in Flutter

1. Rounded Rectangle (Standard M3)

ElevatedButton(
  style: ElevatedButton.styleFrom(
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(16), // Large token
    ),
  ),
  onPressed: () {},
  child: const Text('Standard M3'),
)

2. Squircle / Continuous Corners

For a more premium feel, use "continuous" corners (similar to iOS) instead of standard circular arcs.

// Use a package or custom clipper for true squircles
Container(
  decoration: ShapeDecoration(
    shape: SmoothRectangleBorder(
      borderRadius: BorderRadius.circular(24),
      smoothness: 1, // Full squircle
    ),
    color: Colors.blue,
  ),
  child: const Padding(
    padding: EdgeInsets.all(16),
    child: Text('Squircle Container'),
  ),
)

3. Teardrop Shape (Expressive)

Commonly used for active indicators or expressive buttons.

class TeardropShape extends ShapeBorder {
  @override
  EdgeInsetsGeometry get dimensions => EdgeInsets.zero;
 
  @override
  Path getInnerPath(Rect rect, {TextDirection? textDirection}) => getOuterPath(rect);
 
  @override
  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
    return Path()
      ..moveTo(rect.left + rect.width / 2, rect.top)
      ..arcToPoint(
        Offset(rect.right, rect.top + rect.height / 2),
        radius: Radius.circular(rect.width / 2),
      )
      ..lineTo(rect.right, rect.bottom)
      ..lineTo(rect.left + rect.width / 2, rect.bottom)
      ..arcToPoint(
        Offset(rect.left, rect.top + rect.height / 2),
        radius: Radius.circular(rect.width / 2),
      )
      ..close();
  }
 
  @override
  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {}
 
  @override
  ShapeBorder scale(double t) => this;
}

4. Shape Morphing Animation

class MorphingShapeButton extends StatefulWidget {
  const MorphingShapeButton({super.key});
 
  @override
  State<MorphingShapeButton> createState() => _MorphingShapeButtonState();
}
 
class _MorphingShapeButtonState extends State<MorphingShapeButton> {
  bool _isRound = true;
 
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () => setState(() => _isRound = !_isRound),
      child: AnimatedContainer(
        duration: const Duration(milliseconds: 500),
        curve: Curves.elasticOut,
        width: 100,
        height: 100,
        decoration: BoxDecoration(
          color: Colors.deepPurple,
          borderRadius: BorderRadius.circular(_isRound ? 50 : 8),
        ),
        child: const Center(
          child: Icon(Icons.refresh, color: Colors.white),
        ),
      ),
    );
  }
}

Best Practices

Consistency: Stick to a "shape family" across your app. If you use squircles, use them everywhere.
Accessible Targets: Ensure that even with expressive shapes, the touch target remains at least 48x48dp.
Contrast: Shapes should be distinguishible from their background; use subtle shadows or outlines.
Performance: Complex custom paths in CustomPainter should be optimized. Use const paths where possible.

Resources

Motion Guide for shape transitions
Color Schemes for container colors

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