A custom implementation of the showModalBottomSheet, with rounded corners

rounded_modal.dart

import 'dart:async';

import 'package:flutter/material.dart';

/// Below is the usage for this function, you'll only have to import this file
/// [radius] takes a double and will be the radius to the rounded corners of this modal
/// [color] will color the modal itself, the default being `Colors.white`
/// [builder] takes the content of the modal, if you're using [Column]
/// or a similar widget, remember to set `mainAxisSize: MainAxisSize.min`
/// so it will only take the needed space.
///
/// ```dart
/// showRoundedModalBottomSheet(
///    context: context,
///    radius: 10.0,  // This is the default
///    color: Colors.white,  // Also default
///    builder: (context) => ???,
/// );
/// ```
Future<T> showRoundedModalBottomSheet<T>({
  @required BuildContext context,
  @required WidgetBuilder builder,
  Color color = Colors.white,
  double radius = 10.0,
}) {
  assert(context != null);
  assert(builder != null);
  assert(radius != null && radius > 0.0);
  assert(color != null && color != Colors.transparent);
  return Navigator.push<T>(
      context,
      _RoundedCornerModalRoute<T>(
        builder: builder,
        color: color,
        radius: radius,
        barrierLabel:
            MaterialLocalizations.of(context).modalBarrierDismissLabel,
      ));
}

class _RoundedModalBottomSheetLayout extends SingleChildLayoutDelegate {
  _RoundedModalBottomSheetLayout(this.progress);

  final double progress;

  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    return new BoxConstraints(
        minWidth: constraints.maxWidth,
        maxWidth: constraints.maxWidth,
        minHeight: 0.0,
        maxHeight: constraints.maxHeight * 9.0 / 16.0);
  }

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    return new Offset(0.0, size.height - childSize.height * progress);
  }

  @override
  bool shouldRelayout(_RoundedModalBottomSheetLayout oldDelegate) {
    return progress != oldDelegate.progress;
  }
}

class _RoundedCornerModalRoute<T> extends PopupRoute<T> {
  _RoundedCornerModalRoute({
    this.builder,
    this.barrierLabel,
    this.color,
    this.radius,
    RouteSettings settings,
  }) : super(settings: settings);

  final WidgetBuilder builder;
  final double radius;
  final Color color;

  @override
  Color get barrierColor => Colors.black54;

  @override
  bool get barrierDismissible => true;

  @override
  String barrierLabel;

  AnimationController _animationController;

  @override
  AnimationController createAnimationController() {
    assert(_animationController == null);
    _animationController =
        BottomSheet.createAnimationController(navigator.overlay);
    return _animationController;
  }

  @override
  Widget buildPage(BuildContext context, Animation<double> animation,
      Animation<double> secondaryAnimation) {
    return MediaQuery.removePadding(
      context: context,
      removeTop: true,
      child: Theme(
        data: Theme.of(context).copyWith(canvasColor: Colors.transparent),
        child: AnimatedBuilder(
          animation: animation,
          builder: (context, child) => CustomSingleChildLayout(
                delegate: _RoundedModalBottomSheetLayout(animation.value),
                child: BottomSheet(
                  animationController: _animationController,
                  onClosing: () => Navigator.pop(context),
                  builder: (context) => Container(
                        decoration: BoxDecoration(
                          color: this.color,
                          borderRadius: BorderRadius.only(
                            topLeft: Radius.circular(this.radius),
                            topRight: Radius.circular(this.radius),
                          ),
                        ),
                        child: SafeArea(child: Builder(builder: this.builder)),
                      ),
                ),
              ),
        ),
      ),
    );
  }

  @override
  bool get maintainState => false;

  @override
  bool get opaque => false;

  @override
  Duration get transitionDuration => Duration(milliseconds: 200);
}

This is now in the pub package rounded_modal.