flutter-go/lib/views/fourth_page/page_reveal.dart

import 'dart:math';

import 'package:flutter/material.dart';

class PageReveal extends StatelessWidget {

  final double revealPercent;
  final Widget child;

  PageReveal({
    this.revealPercent,
    this.child
  });

  @override
  Widget build(BuildContext context) {
    return ClipOval(
      clipper: new CircleRevealClipper(revealPercent),
      child: child,
    );
  }
}

class CircleRevealClipper extends CustomClipper<Rect>{

  final double revealPercent;


  CircleRevealClipper(
    this.revealPercent
  );

  @override
  Rect getClip(Size size) {

    final epicenter = new Offset(size.width / 2, size.height * 0.9);

    double theta = atan(epicenter.dy / epicenter.dx);
    final distanceToCorner = epicenter.dy / sin(theta);

    final radius = distanceToCorner * revealPercent;
    final diameter = 2 * radius;

    return new Rect.fromLTWH(epicenter.dx - radius, epicenter.dy - radius, diameter, diameter);
  }

  @override
  bool shouldReclip(CustomClipper<Rect> oldClipper) {
    return true;
  }

}