hud.dart

import 'package:flutter/material.dart';

class HUD {
  BuildContext hudContext;
  void show(BuildContext context) {
    showGeneralDialog(
      context: context,
      pageBuilder: (hudContext, _a, _sa) {
        this.hudContext = hudContext;
        return const Center(child: CircularProgressIndicator());
      },
      barrierDismissible: false,
      barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
      barrierColor: Colors.black26,
      transitionDuration: const Duration(milliseconds: 150),
      transitionBuilder: (_c, animation, _sa, child) => FadeTransition(
            opacity: CurvedAnimation(
              parent: animation,
              curve: Curves.easeOut,
            ),
            child: child,
          ),
    );
  }

  void dismiss() {
    if (hudContext == null) {
      assert(false, 'hudContext should not be null.');
      return;
    }
    Navigator.of(hudContext)?.pop();
  }
}

Usage

final hud = HUD()..show(context);
Future.delayed(Duration(seconds: 2)).then((_) {
  // HUDが閉じる
  hud.dismiss();
  Future.delayed(Duration(seconds: 2)).then((_) {
    // 何も起きない
    hud.dismiss();
  });
});