A Widget that automatically fades its child based on scroll position

fade_on_scroll.dart

import 'package:flutter/material.dart';

class FadeOnScroll extends StatefulWidget {
  final ScrollController scrollController;
  final double zeroOpacityOffset;
  final double fullOpacityOffset;
  final Widget child;

  FadeOnScroll(
      {Key key,
      @required this.scrollController,
      @required this.child,
      this.zeroOpacityOffset = 0,
      this.fullOpacityOffset = 0});

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

class _FadeOnScrollState extends State<FadeOnScroll> {
  double _offset;

  @override
  initState() {
    super.initState();
    _offset = widget.scrollController.offset;
    widget.scrollController.addListener(_setOffset);
  }

  @override
  dispose() {
    widget.scrollController.removeListener(_setOffset);
    super.dispose();
  }

  void _setOffset() {
    setState(() {
      _offset = widget.scrollController.offset;
    });
  }

  double _calculateOpacity() {
    if (widget.fullOpacityOffset == widget.zeroOpacityOffset)
      return 1;
    else if (widget.fullOpacityOffset > widget.zeroOpacityOffset) {
      // fading in
      if (_offset <= widget.zeroOpacityOffset)
        return 0;
      else if (_offset >= widget.fullOpacityOffset)
        return 1;
      else
        return (_offset - widget.zeroOpacityOffset) / (widget.fullOpacityOffset - widget.zeroOpacityOffset);
    } else {
      // fading out
      if (_offset <= widget.fullOpacityOffset)
        return 1;
      else if (_offset >= widget.zeroOpacityOffset)
        return 0;
      else
        return (_offset - widget.fullOpacityOffset) / (widget.zeroOpacityOffset - widget.fullOpacityOffset);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Opacity(
      opacity: _calculateOpacity(),
      child: widget.child,
    );
  }
}

fading_widget_example.dart

import 'package:flutter/material.dart';

import './fade_on_scroll.dart';

class FadingWidgetExample extends StatelessWidget {
  final ScrollController scrollController = ScrollController();

  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      controller: scrollController,
      slivers: <Widget>[
        SliverAppBar(
          pinned: true,
          title: FadeOnScroll(
            scrollController: scrollController,
            fullOpacityOffset: 180,
            child: Text("I'm going to fade"),
          ),
        ),
        SliverList(
          delegate:
              SliverChildBuilderDelegate((BuildContext context, int index) {
            return ListTile(
              title: Text('Row $index'),
            );
          }),
        )
      ],
    );
  }
}

Great gist and code. But fade out part does not work correctly, because we need to return (1 - calculated) for fade out logic. Because you're using an opacity widget, you need to complement that value to 1 so that it works as expected/needed.

else {
      // fading out
      if (_offset <= widget.fullOpacityOffset) {
        return 1;
      } else if (_offset >= widget.zeroOpacityOffset) {
        return 0;
      } else {
        return 1 -
            (_offset - widget.fullOpacityOffset) /
                (widget.zeroOpacityOffset - widget.fullOpacityOffset);
      }
    }