FMTC Docs
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Initialisation

FMTC is licensed under GPL-v3.
If you're developing a proprietary (non open-source) application, this affects you and your application's legal right to distribution. For more information, please see (Proprietary) Licensing.
Before using FMTC, ensure you comply with the appropriate rules and ToS set by your tile server. Failure to do so may lead to a permenant ban, or any other punishment.
This library and/or the creator(s) are not responsible for any violations you make using this package.
OpenStreetMap's can be found here: specifically bulk downloading is discouraged, and forbidden after zoom level 13. Other servers may have different terms.
The main basis of this package is the FlutterMapTileCaching object, which exposes the majority of FMTC's APIs (through FMTC.instance), and also contains most of the state needed to connect and communicate with the underlying systems.
Therefore, it must be asynchronously initialised on every app startup, usually in the main method that runs directly after the Flutter environment starts.
FMTC is a shorthand type alias for FlutterMapTileCaching, and works in exactly the same way. Often, documentation will use the shortened version to save space, and you should do so in your code as well.
You must call initialise() before trying to use instance.Failure to do so will throw a StateError.
main.dart
import 'package:flutter/widgets.dart';
import 'package:flutter_map_tile_caching/flutter_map_tile_caching.dart';
Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
await FlutterMapTileCaching.initialise();
// FMTC.instance;
// Run your app and do all of that other stuff
}

Initialisation Safety

In summary, in the extremely unlikely event of database corruption, FMTC can recover automatically (in most circumstances), with minimal data loss, given enough fatal app crashes - corruption cannot be caught in any other way.
Whilst it is (almost) impossible for one of the underlying Isar databases to become corrupted during normal usage, or even due to a bug, it can happen if the database file is modified manually.
In this case, it is currently impossible to avoid a fatal crash during initialisation, as the database reader/parser crashes all Dart threads & isolates without warning. Please see this issue I opened:
However, FMTC has a workaround.
By using a basic temporary text file, the IDs/filenames of successfully opened databases can be recorded. Once all databases have been opened, the file is deleted, meaning that the safety system will not intervene on the next app launch. However, if a database open attempt crashes the app, the file will not be deleted, and it will contain the list of safe databases.
Because the order in which databases are opened is determinate (alphabetical), the app can open every database one by one, (metaphorically) crossing it off the list. When there are no more safe databases, but more database files, the next database is deleted instead of being opened. The initialisation then continues as normal, still using the file appropriatley.
Therefore, one fatal crash is enough to detect one faulty database, and delete it, allowing the app to initialise normally on the next launch.
If there are multiple corrupted databases (n), one fatal crash is needed per database, so the app will successfully open after n + 1.
All of this means that FMTC can usually recover from a database corruption with minimal data loss. However, if the database filenames/IDs are also changed, the behaviour is unspecified, especially if the initialisation file already exists.