halkyon/packages/halkyon_ui/lib/src/widget/scroll_hiding_fab.dart

import 'package:flutter/rendering.dart';
import 'package:material_ui/material_ui.dart';

/// A floating action button that slides away while the person scrolls down,
/// reading onwards, and comes back as soon as they scroll up. [builder]
/// builds the screen around the button it is handed (as its scaffold's
/// floating action button); the scrolling in that screen moves it.
///
/// While hidden, the button can't be tapped through its fading ghost. A list
/// short enough not to scroll keeps it in view: it covers nothing there, and
/// so does [pinned].
class const ScrollHidingFab({
  /// The button; none when null, as in windows that put the action in the
  /// app bar instead.
  required final Widget? fab,

  /// The screen, given the button to place.
  required final Widget Function(BuildContext context, Widget? fab) builder,

  /// Whether the button stays whatever the scrolling, as while it is a field
  /// being written in, which mustn't slide away under the person's typing.
  final bool pinned = false,
  super.key,
}) extends StatefulWidget {
  @override
  State<ScrollHidingFab> createState() => _ScrollHidingFabState();
}

class _ScrollHidingFabState() extends State<ScrollHidingFab> {
  var _visible = true;

  bool _onScroll(UserScrollNotification notification) {
    // Only the screen's own vertical scrolling: a sideways swipe between
    // tabs moves nothing.
    if (notification.metrics.axis != Axis.vertical || widget.pinned) {
      return false;
    }
    final visible = switch (notification.direction) {
      // A list that doesn't scroll covers nothing behind the button, and an
      // overscroll bounce there mustn't hide it.
      ScrollDirection.reverse => notification.metrics.maxScrollExtent <= 0,
      ScrollDirection.forward => true,
      ScrollDirection.idle => _visible,
    };
    if (visible != _visible) {
      setState(() => _visible = visible);
    }
    return false;
  }

  @override
  Widget build(BuildContext context) {
    final fab = widget.fab;
    final visible = _visible || widget.pinned;
    return NotificationListener<UserScrollNotification>(
      onNotification: _onScroll,
      child: widget.builder(
        context,
        fab == null
            ? null
            : AnimatedSlide(
                duration: const Duration(milliseconds: 200),
                curve: Curves.easeOut,
                // Twice its own height clears the window's edge whatever
                // the button's size.
                offset: visible ? Offset.zero : const Offset(0, 2),
                child: AnimatedOpacity(
                  duration: const Duration(milliseconds: 200),
                  opacity: visible ? 1 : 0,
                  child: IgnorePointer(ignoring: !visible, child: fab),
                ),
              ),
      ),
    );
  }
}