async_task.dart

import 'dart:async';

import 'package:flutter/material.dart';

/// See [asyncTask].
typedef Task<T> = Future<T?> Function(bool Function() cancelled);

/// Runs the future returned by [task] and shows a modal waiting dialog after
/// an initial [delay] (default 100 ms) with an optional [label] until the future
/// completes. The [task] is passed a function that returns whether the task
/// has been cancelled by the user. It should periodically check this and return
/// early if it is true.
Future<T?> asyncTask<T>({
  required BuildContext context,
  required Task<T> task,
  Duration? delay,
  Widget? title,
  Widget? cancelLabel,
}) async {
  T? result;
  var state = 0;
  var isCancelled = false;

  return Future.any([
    // run the task and set isDone when it completes so we can avoid showing
    // the waiting dialog if it completes quickly enough; if it was already
    // shown, we close it
    task(() => isCancelled).then((value) => result = value).whenComplete(() {
      if (state == 1 && context.mounted) {
        Navigator.of(context).pop();
      }
      state = 2;
    }),
    // show the waiting dialog after the delay; if the user cancels, we set
    // isCancelled so the task can return early; also set hasDialog so we
    // can close it if the task eventually completes
    Future<void>.delayed(delay ?? const Duration(milliseconds: 100), () async {
      if (state != 0 || !context.mounted) return;
      state = 1;
      return showDialog<void>(
        context: context,
        barrierDismissible: false,
        builder: (_) => AlertDialog(
          title: title,
          content: const IntrinsicHeight(child: Center(child: CircularProgressIndicator())),
          actions: [
            TextButton(
              onPressed: () => isCancelled = true,
              child: cancelLabel ?? const Text('Cancel'),
            ),
          ],
        ),
      );
    }),
  ]).then((_) => result);
}

void main() {
  runApp(const App());
}

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

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Page(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
      child: ElevatedButton(
        onPressed: () async {
          final result = await asyncTask(
            context: context,
            task: (isCancelled) async {
              for (var i = 0; i < 40; i++) {
                await Future<void>.delayed(const Duration(milliseconds: 90));
                if (isCancelled()) return null;
              }
              return 42;
            },
            title: const Text('Waiting…'),
          );
          print('result: $result');
        },
        child: const Text('Run Task'),
      ),
    ));
  }
}