iOS: `OfflineManager.listOfflineRegions()` crashes when called immediately after `OfflineManager.createInstance()` on app start

[BartoszStasiurka]

Platforms

iOS

Version of flutter-maplibre

0.3.2

Bug Description

On iOS, calling OfflineManager.listOfflineRegions() immediately after OfflineManager.createInstance() during a fresh app start causes a runtime crash.

The problem appears to be timing-related:
if there is a short delay between createInstance() and listOfflineRegions(), the issue disappears and the method works correctly.

This suggests that the iOS OfflineManager is not fully initialized when listOfflineRegions() is called right after instance creation.

Steps to Reproduce

1. Go to the example app.
2. Replace offline_page.dart with the code sample listed below.
3. Go to the offline page (iOS).

Expected Results

listOfflineRegions() should be safe to call immediately after createInstance() without throwing an exception.

Actual Results

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: Null check operator used on a null value
#0      OfflineManagerIos.listOfflineRegions (package:maplibre/src/platform/ios/offline_manager.dart:113:33)
offline_manager.dart:113
#1      _OfflinePageState.initState.<anonymous closure> (package:maplibre_example/offline_page.dart:24:22)
offline_page.dart:24
#2      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:948:45)
future_impl.dart:948
#3      Future._propagateToListeners (dart:async/future_impl.dart:977:13)
future_impl.dart:977
#4      Future._addListener.<anonymous closure> (dart:async/future_impl.dart:543:9)
future_impl.dart:543
#5      _microtaskLoop (dart:async/schedule_microtask.dart:40:35)
schedule_microtask.dart:40
#6      _startMicrotaskLoop (dart:async/schedule_microtask.dart:49:5)
schedule_microtask.dart:49

Code Sample

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

@immutable
class OfflinePage extends StatefulWidget {
  const OfflinePage({super.key});

  static const location = '/offline';

  @override
  State<OfflinePage> createState() => _OfflinePageState();
}

class _OfflinePageState extends State<OfflinePage> {
  late OfflineManager offlineManager;
  List<OfflineRegion> offlineRegions = [];

  @override
  void initState() {
    OfflineManager.createInstance().then((m) {
      offlineManager = m;
      offlineManager.listOfflineRegions().then((value) {
        print('value $value');
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Offline Map')),
      body: Center(child: Text('Regions length: ${offlineRegions.length}')),
    );
  }
}

[josxha]

That's an interesting bug. From the maplibre native documentation:

/// This property is set to `nil`, indicating that the receiver does not yet know
/// the existing packs, for an undefined amount of time starting from the moment
/// the shared offline storage object is initialized until the packs are fetched
/// from the database. After that point, this property is always non-nil, but it
/// may be empty to indicate that no packs are present.
/// 
/// To detect when the shared offline storage object has finished loading its
/// `packs` property, observe KVO change notifications on the `packs` key path.
/// The initial load results in an `NSKeyValueChangeSetting` change.

At the moment, dealing with swift native observers in dart is still cumbersome. I resolved this issue in #481 for now by polling every 50ms until the data is available.