How to make a button with rounded edges in flutter?

Making a rounded corner button is so simple, but I want to make a button that its edges are also rounded like this:

maybe I should use CustomPaint?

SquircleBorder might help you:

Container(
            width: 56.0,
            height: 56.0,
            child: Material(
              color: Colors.blueGrey[400],
              shape: SquircleBorder(
                side: BorderSide(color: Colors.grey, width: 3.0),
              ),
              child: Icon(Icons.settings),
            ),
          ),

class SquircleBorder extends ShapeBorder {
  final BorderSide side;
  final double superRadius;

  const SquircleBorder({
    this.side: BorderSide.none,
    this.superRadius: 5.0,
  })
    : assert(side != null),
      assert(superRadius != null);

  @override
  EdgeInsetsGeometry get dimensions => EdgeInsets.all(side.width);

  @override
  ShapeBorder scale(double t) {
    return new SquircleBorder(
      side: side.scale(t),
      superRadius: superRadius * t,
    );
  }

  @override
  Path getInnerPath(Rect rect, {TextDirection textDirection}) {
    return _squirclePath(rect.deflate(side.width), superRadius);
  }

  @override
  Path getOuterPath(Rect rect, {TextDirection textDirection}) {
    return _squirclePath(rect, superRadius);
  }

  static Path _squirclePath(Rect rect, double superRadius) {
    final c = rect.center;
    final dx = c.dx * (1.0 / superRadius);
    final dy = c.dy * (1.0 / superRadius);
    return new Path()
      ..moveTo(c.dx, 0.0)
      ..relativeCubicTo(c.dx - dx, 0.0, c.dx, dy, c.dx, c.dy)
      ..relativeCubicTo(0.0, c.dy - dy, -dx, c.dy, -c.dx, c.dy)
      ..relativeCubicTo(-(c.dx - dx), 0.0, -c.dx, -dy, -c.dx, -c.dy)
      ..relativeCubicTo(0.0, -(c.dy - dy), dx, -c.dy, c.dx, -c.dy)
      ..close();
  }

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection textDirection}) {
    switch (side.style) {
      case BorderStyle.none:
        break;
      case BorderStyle.solid:
        var path = getOuterPath(rect.deflate(side.width / 2.0), textDirection: textDirection);
        canvas.drawPath(path, side.toPaint());
    }
  }
}