Smooth Border

iOS-style continuous corner radius (squircle) for Flutter. Zero dependencies.

Unlike standard BorderRadius.circular() which uses circular arcs with an abrupt transition, SmoothRectangleBorder uses cubic bezier curves that create a smooth, gradual transition between the straight edge and the curve — matching the visual style of iOS and Figma rounded rectangles.

Features

• SmoothRectangleBorder — a ShapeBorder that works with any Flutter widget
• SmoothRectangleClipper — a CustomClipper<Path> for ClipPath
• Adjustable smoothing — from 0.0 (standard circular) to 1.0 (maximum smoothness)
• Zero dependencies — pure Dart/Flutter, no external packages
• All 4 corners symmetric — works correctly with any aspect ratio

Getting Started

dependencies:
  smooth_border: ^1.0.2

import 'package:smooth_border/smooth_border.dart';

Usage

Container / ShapeDecoration

Container(
  decoration: ShapeDecoration(
    shape: SmoothRectangleBorder(borderRadius: 16),
    color: Colors.white,
    shadows: [
      BoxShadow(color: Colors.black12, blurRadius: 8, offset: Offset(0, 2)),
    ],
  ),
  child: Text("iOS Card"),
)

With border

Container(
  decoration: ShapeDecoration(
    shape: SmoothRectangleBorder(
      borderRadius: 16,
      side: BorderSide(color: Colors.grey.shade300),
    ),
    color: Colors.white,
  ),
)

Card

Card(
  shape: SmoothRectangleBorder(borderRadius: 12),
  child: ListTile(title: Text("Smooth Card")),
)

ClipPath (images, containers)

ClipPath(
  clipper: SmoothRectangleClipper(borderRadius: 20),
  child: Image.network("https://..."),
)

ElevatedButton / OutlinedButton

ElevatedButton(
  style: ElevatedButton.styleFrom(
    shape: SmoothRectangleBorder(borderRadius: 12),
  ),
  onPressed: () {},
  child: Text("Smooth Button"),
)

Dialog

showDialog(
  context: context,
  builder: (_) => Dialog(
    shape: SmoothRectangleBorder(borderRadius: 20),
    child: Padding(
      padding: EdgeInsets.all(24),
      child: Text("Smooth Dialog"),
    ),
  ),
);

Smoothing levels

// Standard circular (no smoothing)
SmoothRectangleBorder(borderRadius: 16, smoothing: 0.0)

// iOS/Figma default (recommended)
SmoothRectangleBorder(borderRadius: 16, smoothing: 0.6)

// Maximum smoothness
SmoothRectangleBorder(borderRadius: 16, smoothing: 1.0)

Golden Tests

Visual reference for each feature, generated from golden tests.

Standard vs Smooth (radius 8)

Direct comparison showing the subtle difference at small radius. The smooth variant has a gradual shoulder transition instead of an abrupt circular arc.

Standard vs Smooth (radius 16)

At medium radius the difference becomes clearly visible — the smooth corner starts curving earlier and has a wider, softer shoulder.

Standard vs Smooth (radius 24)

At large radius the squircle shape is very pronounced compared to the standard circular arc.

Radius Range (0–32)

How the smooth corner shape changes as radius increases from 0 (sharp) to 32 (pill-like).

Smoothing Levels (0.0 → 1.0)

The smoothing parameter at a fixed radius of 20. At 0.0 it matches standard BorderRadius.circular(). At 0.6 (iOS default) the shoulder is visible. At 1.0 the curve is maximally smooth.

Border Styles

SmoothRectangleBorder with different BorderSide configurations: no border, thin, thick (3px), and colored.

Aspect Ratios

Verifies all 4 corners stay symmetric regardless of shape: square, wide rectangle, tall rectangle, and small box.

ClipPath

SmoothRectangleClipper clipping containers at different radius and smoothing values.

How it works

Each corner is drawn with 2 cubic bezier segments instead of a single circular arc. The control points are derived from cos(45deg) ratios, matching the algorithm used by Figma and iOS for continuous corner radius.

The smoothing parameter controls how far the curve extends beyond the circular arc region:

smoothing	Effect
0.0	Identical to RoundedRectangleBorder
0.6	iOS/Figma default — subtle but visible difference
1.0	Maximum smoothness — very pronounced shoulder

API Reference

SmoothRectangleBorder

Property	Type	Default	Description
borderRadius	double	0	Corner radius in logical pixels
smoothing	double	0.6	Smoothing intensity (0.0–1.0)
side	BorderSide	BorderSide.none	Border line style

Implements OutlinedBorder, so it works everywhere ShapeBorder is accepted.

SmoothRectangleClipper

Property	Type	Default	Description
borderRadius	double	0	Corner radius in logical pixels
smoothing	double	0.6	Smoothing intensity (0.0–1.0)

Implements CustomClipper<Path> for use with ClipPath.

Contributing

Contributions are welcome! Open an issue or submit a pull request on GitHub.

License

MIT License. See LICENSE for details.