fade_indexed_stack.dart

import 'package:flutter/material.dart';

class FadeIndexedStack extends StatefulWidget {
  final int index;
  final List<Widget> children;
  final Duration duration;

  const FadeIndexedStack({
    Key key,
    this.index,
    this.children,
    this.duration = const Duration(
      milliseconds: 800,
    ),
  }) : super(key: key);

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

class _FadeIndexedStackState extends State<FadeIndexedStack>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;

  @override
  void didUpdateWidget(FadeIndexedStack oldWidget) {
    if (widget.index != oldWidget.index) {
      _controller.forward(from: 0.0);
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  void initState() {
    _controller = AnimationController(vsync: this, duration: widget.duration);
    _controller.forward();
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: _controller,
      child: IndexedStack(
        index: widget.index,
        children: widget.children,
      ),
    );
  }
}

If you need the previous widget to fade out rather than disappear abruptly, you need to do so 😁

import 'package:flutter/material.dart';

class FadeIndexedStack extends StatefulWidget {
  const FadeIndexedStack({
    super.key,
    required this.index,
    required this.children,
    this.duration = const Duration(milliseconds: 200),
  });

  final int index;
  final List<Widget> children;
  final Duration duration;

  @override
  State<FadeIndexedStack> createState() => _FadeIndexedStackState();
}

class _FadeIndexedStackState extends State<FadeIndexedStack> with TickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<double> _animation;

  int _currentIndex = -1;
  int? _previousIndex;

  @override
  void didUpdateWidget(FadeIndexedStack oldWidget) {
    if (widget.index != oldWidget.index) {
      setState(() {
        _previousIndex = _currentIndex;
        _currentIndex = widget.index;
        _controller.forward(from: 0.0);
      });
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  void initState() {
    super.initState();
    _currentIndex = widget.index;
    _controller = AnimationController(vsync: this, duration: widget.duration);
    _animation = CurvedAnimation(parent: _controller, curve: Curves.easeInOut);
    _controller.forward();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        if (_previousIndex != null)
          FadeTransition(
            opacity:
            Tween<double>(begin: 1.0, end: 0.0).animate(_animation),
            child: IndexedStack(
              index: _previousIndex,
              children: widget.children,
            ),
          ),
        FadeTransition(
          opacity: Tween<double>(begin: 0.0, end: 1.0).animate(_animation),
          child: IndexedStack(
            index: _currentIndex,
            children: widget.children,
          ),
        ),
      ],
    );
  }
}

Nice one, bro