SimpleCloudNotifier/flutter/lib/components/bottom_fab/anchored_overlay.dart

import 'package:flutter/material.dart';

class AnchoredOverlay extends StatelessWidget {
  final bool showOverlay;
  final Widget Function(BuildContext, Offset anchor) overlayBuilder;
  final Widget child;

  const AnchoredOverlay({
    super.key,
    required this.showOverlay,
    required this.overlayBuilder,
    required this.child,
  });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
      return OverlayBuilder(
        showOverlay: showOverlay,
        overlayBuilder: (BuildContext overlayContext) {
          RenderBox box = context.findRenderObject() as RenderBox;
          final center = box.size.center(box.localToGlobal(const Offset(0.0, 0.0)));

          return overlayBuilder(overlayContext, center);
        },
        child: child,
      );
    });
  }
}

class OverlayBuilder extends StatefulWidget {
  final bool showOverlay;
  final Widget Function(BuildContext) overlayBuilder;
  final Widget child;

  const OverlayBuilder({
    super.key,
    this.showOverlay = false,
    required this.overlayBuilder,
    required this.child,
  });

  @override
  State<OverlayBuilder> createState() => _OverlayBuilderState();
}

class _OverlayBuilderState extends State<OverlayBuilder> {
  OverlayEntry? overlayEntry;

  @override
  void initState() {
    super.initState();

    if (widget.showOverlay) {
      WidgetsBinding.instance.addPostFrameCallback((_) => showOverlay());
    }
  }

  @override
  void didUpdateWidget(OverlayBuilder oldWidget) {
    super.didUpdateWidget(oldWidget);
    WidgetsBinding.instance.addPostFrameCallback((_) => syncWidgetAndOverlay());
  }

  @override
  void reassemble() {
    super.reassemble();
    WidgetsBinding.instance.addPostFrameCallback((_) => syncWidgetAndOverlay());
  }

  @override
  void dispose() {
    if (isShowingOverlay()) {
      hideOverlay();
    }

    super.dispose();
  }

  bool isShowingOverlay() => overlayEntry != null;

  void showOverlay() {
    overlayEntry = OverlayEntry(
      builder: widget.overlayBuilder,
    );
    addToOverlay(overlayEntry!);
  }

  void addToOverlay(OverlayEntry entry) async {
    print('addToOverlay');
    Overlay.of(context).insert(entry);
  }

  void hideOverlay() {
    print('hideOverlay');
    if (overlayEntry != null) {
      overlayEntry!.remove();
      overlayEntry = null;
    }
  }

  void syncWidgetAndOverlay() {
    if (isShowingOverlay() && !widget.showOverlay) {
      hideOverlay();
    } else if (!isShowingOverlay() && widget.showOverlay) {
      showOverlay();
    }
  }

  @override
  Widget build(BuildContext context) {
    return widget.child;
  }
}