Custom Curve in Flutter

A clean way to do this is using a ShapeBorder, so you can use it on any container. Similar to the ClipPath widget, you can define a path to use. The ShapeBorder gets you the path from a bounding Rect of this Container. In the code below, I draw the top arc above this rect, so the top border aligns (and you can put other Widgets right above this container too).

import 'package:flutter/material.dart';

class WavyBorder extends ShapeBorder {
  const WavyBorder({this.radius = 32});

  final double radius;

  @override
  EdgeInsetsGeometry get dimensions => EdgeInsets.zero;

  @override
  Path getInnerPath(Rect rect, {TextDirection? textDirection}) {
    throw UnimplementedError();
  }

  @override
  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
    return Path()
      // The top curve is above this container's Rect, keep this in mind with your other Widgets
      ..moveTo(rect.topLeft.dx, rect.topLeft.dy - radius)
      // The 90 degree arc at the top
      ..relativeArcToPoint(
        Offset(radius, radius),
        radius: Radius.circular(radius),
        clockwise: false,
      )
      // The horizontal line
      ..lineTo(rect.topRight.dx - radius, rect.topRight.dy)
      // The 90 degree arc at the bottom
      ..relativeArcToPoint(
        Offset(radius, radius),
        radius: Radius.circular(radius),
      )
      // Then just visit the bottom corners normally
      ..lineTo(rect.bottomRight.dx, rect.bottomRight.dy)
      ..lineTo(rect.bottomLeft.dx, rect.bottomLeft.dy)
      ..close();
  }

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {}

  @override
  ShapeBorder scale(double t) {
    throw UnimplementedError();
  }
}

Then, wherever you want to use it, you can add it in the ShapeDecoration of your Container:

Container(
  decoration: ShapeDecoration(
    shape: WavyBorder(radius: 32),
    ...
  ),
  child: ...,
),

Which looks like this:

Complete code for usage (main.dart):

import 'package:flutter/material.dart';
import 'wavy_container.dart';

void main() {
  runApp(const MaterialApp(
    home: HomeScreen(),
    debugShowCheckedModeBanner: false,
  ));
}

class HomeScreen extends StatefulWidget {
  const HomeScreen({super.key});

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  final TextEditingController controller = TextEditingController();
  final FocusNode focusNode = FocusNode();

  @override
  void initState() {
    super.initState();
    controller.text = 'Hallo';
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Flex(
        direction: Axis.vertical,
        mainAxisSize: MainAxisSize.max,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Expanded(
            child: Text(
              'This text fits neatly above the container. Watch out with the left margin right above it though!',
              style: Theme.of(context).textTheme.headlineMedium,
              textAlign: TextAlign.center,
            ),
          ),
          Expanded(
            flex: 2,
            child: Container(
              padding: const EdgeInsets.all(8),
              decoration: ShapeDecoration(
                shape: const WavyBorder(radius: 32),
                color: Theme.of(context).colorScheme.primary,
              ),
              child: Text(
                'Look at that pretty wavy border above here!',
                style: Theme.of(context)
                    .textTheme
                    .headlineLarge
                    ?.copyWith(color: Colors.white),
                textAlign: TextAlign.center,
              ),
            ),
          )
        ],
      ),
    );
  }
}