Absolute positioning with CustomSingleChildLayout

radial_slider.dart

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


class RadialSlider extends StatefulWidget {
  double radius;

  RadialSlider({
    this.radius,
  });

  @override
  _RadialSliderState createState() => _RadialSliderState();
}

class _RadialSliderState extends State<RadialSlider> {
  double angle = 60.0 / 180 * pi;

  @override
  Widget build(BuildContext context) {
    return Stack(
      alignment: Alignment(0.0, 0.0),
      children: <Widget>[
        Container(
          width: widget.radius * 2 + 10,
          height: widget.radius * 2 + 10,
          decoration: BoxDecoration(
            border: Border.all(),
            shape: BoxShape.circle,
          ),
        ),
        CustomSingleChildLayout(
          delegate: _CicleLayoutDelegate(
            angle: angle,
            radius: widget.radius,
          ),
          child: RaisedButton(
            onPressed: () {},
            shape: CircleBorder(),
          ),
        ),
      ],
    );
  }
}

class _CicleLayoutDelegate extends SingleChildLayoutDelegate {
  double radius;
  double angle;

  _CicleLayoutDelegate({this.radius, this.angle});

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    var center = Offset(
      (size.width - childSize.width) / 2,
      (size.height - childSize.height) / 2,
    );

    return center.translate(radius * cos(angle), radius * sin(angle));
  }

  @override
  bool shouldRelayout(_CicleLayoutDelegate oldDelegate) {
    return oldDelegate.angle != angle
    || oldDelegate.radius != radius;
  }
}