fadeInOnScroll.dart

import 'dart:developer';
import 'dart:math';

import 'package:flutter/material.dart';

class FadeInOnScroll extends StatefulWidget {
  final ScrollController controller;
  final double offset;
  final Widget child;
  final int duration;

  const FadeInOnScroll(
      {Key key,
      @required this.child,
      @required this.offset,
      @required this.duration,
      @required this.controller})
      : super(key: key);

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

class _FadeInOnScrollState extends State<FadeInOnScroll> {
  _FadeInOnScrollState();
  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: widget.controller,
      child: widget.child,
      builder: (BuildContext context, Widget child) {
        double _opacityLevel =
            (widget.controller.offset > widget.offset) ? 1.0 : 0.0;
        return AnimatedOpacity(
          duration: Duration(
              milliseconds: (widget.duration != null) ? widget.duration : 200),
          opacity: _opacityLevel,
          child: child,
        );
      },
    );
  }
}