位置指定と配置したダイアログ基準としたTransitionアニメーションのサンプル

aligneddialog.dart

import 'package:aligned_dialog/aligned_dialog.dart';
import 'package:flutter/material.dart';

void main() => runApp(const GeneralDialogApp());

class GeneralDialogApp extends StatelessWidget {
  const GeneralDialogApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      restorationScopeId: 'app',
      home: GeneralDialogExample(),
    );
  }
}

class GeneralDialogExample extends StatelessWidget {
  const GeneralDialogExample({super.key});

  @override
  Widget build(BuildContext context) {
    final key = GlobalKey<_MyDialogState>();
    return Scaffold(
      body: Center(
        child: Builder(builder: (context) {
          return OutlinedButton(
            onPressed: () {
              // これで位置指定でウィジェットを表示する。
              // GlobalKey経由でtransitionsBuilder内でanimationをState側に
              // 渡してアニメーション動作を実施する
              showAlignedDialog(
                context: context,
                builder: (context) => MyDialog(
                  key: key,
                  child: const SizedBox(
                      width: 100,
                      height: 100,
                      child: Center(child: Text('Alert!'))),
                ),
                transitionsBuilder:
                    (context, animation, secondaryAnimation, child) {
                  key.currentState?.animation = animation;
                  return child;
                },
                followerAnchor: Alignment.topCenter,
                targetAnchor: Alignment.bottomCenter,
              );
            },
            child: const Text('Open Dialog'),
          );
        }),
      ),
    );
  }
}

/// [ProxyAnimation]に[Animation]を渡せるように[StatefulWidget]にして
/// [child]をダイアログ化するウィジェット
class MyDialog extends StatefulWidget {
  const MyDialog({required this.child, super.key});

  final Widget child;

  @override
  State<MyDialog> createState() => _MyDialogState();
}

class _MyDialogState extends State<MyDialog> {
  final ProxyAnimation _animation = ProxyAnimation();

  set animation(Animation<double> value) => _animation.parent = value;

  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: _animation,
      child: ScaleTransition(
        scale: _animation,
        alignment: Alignment.topLeft,
        child: RotationTransition(
          turns: _animation,
          child: Material(
              shape: const RoundedRectangleBorder(
                  borderRadius: BorderRadius.all(Radius.circular(4.0))),
              child: widget.child),
        ),
      ),
    );
  }
}