How to use different animations delay on appear than on going away #136

import 'dart:async';

import 'package:flutter/widgets.dart';

/// Widget that delays opacity change when widget
/// appears (in) or disappears (out)
class DelayedAnimatedOpacity extends StatefulWidget {
  const DelayedAnimatedOpacity({
    super.key,
    Duration? delayIn,
    Duration? delayOut,
    this.duration = const Duration(milliseconds: 500),
    required this.child,
    required this.visible,
  })  : delayIn = delayIn ?? const Duration(milliseconds: 500),
        delayOut = delayOut ?? Duration.zero;

  final Duration delayIn;
  final Duration delayOut;
  final Duration duration;
  final Widget child;
  final bool visible;

  @override
  State<DelayedAnimatedOpacity> createState() => _DelayedAnimatedOpacityState();
}

class _DelayedAnimatedOpacityState extends State<DelayedAnimatedOpacity> {
  bool visible = false;
  Timer? _timer;

  @override
  void initState() {
    super.initState();
    if (widget.visible) {
      _timer = Timer(widget.delayIn, () {
        if (mounted) {
          setState(() {
            visible = true;
          });
        }
      });
    }
  }

  @override
  void didUpdateWidget(covariant DelayedAnimatedOpacity oldWidget) {
    super.didUpdateWidget(oldWidget);

    if (widget.visible != visible) {
      if (widget.visible) {
        _timer?.cancel();
        _timer = Timer(widget.delayIn, () {
          if (mounted) {
            setState(() {
              visible = true;
            });
          }
        });
      } else {
        _timer?.cancel();
        _timer = Timer(widget.delayOut, () {
          if (mounted) {
            setState(() {
              visible = false;
            });
          }
        });
      }
    }
  }

  @override
  void dispose() {
    _timer?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedOpacity(
      duration: widget.duration,
      opacity: visible ? 1 : 0,
      child: widget.child,
    );
  }
}