非同期関数を途中で止める例

FutureCancel.dart

import 'package:async/async.dart';
import 'dart:async';

Future<int> test(String name, {List<bool>? cancel}) async {
  for (int i = 0; i < 4; i++) {
    await Future.delayed(const Duration(seconds: 1));
    print("[$name] count = $i");
    if (cancel != null && cancel[0]) {
      print("[$name] canceled");
      break;
    }
  }
  print("[$name] test end");
  return 3;
}

class Test {
  Test(this.name);

  final String name;
  final _command = CancelableCompleter<int>();

  void start() {
    _command.complete(_test(name));
  }

  void cancel() {
    _command.operation.cancel();
  }

  bool get isCompleted => _command.isCompleted;

  Future<int> get result => _command.operation.value;

  Future<int> _test(String name) async {
    for (int i = 0; i < 4; i++) {
      await Future.delayed(const Duration(seconds: 1));
      print("[$name] count = $i");
      if (_command.isCanceled) {
        print("[$name] canceled");
        return 100;
      }
    }
    print("[$name] test end");
    return 3;
  }
}

void main(List<String> args) {
  {
    test("0");
  }
  {
    final List<bool> cancel = <bool>[false];
    test("1", cancel: cancel);
    Future.delayed(const Duration(seconds: 2), () {
      print("[1] cancel");
      cancel[0] = true;
    });
  }
  Future(() async {
    final test = Test("2");
    test.start();
    print("test[2] = ${await test.result}");
  });
  Future(() async {
    final test = Test("3");
    test.start();
    Future.delayed(const Duration(seconds: 2), () {
      print("[3] cancel");
      test.cancel();
    });
    print("test[3] = ${await test.result}");
  });
  print("main end");
}