[sqflite] doc asset db and versioning

sqflite/doc/opening_asset_db.md

@@ -101,6 +101,84 @@ You might want to have a versioning strategy (not yet part of this project) to o
 it changes in the build system or might also allow the user to modify the database (in this case you must copy it
 first).
 
+#### One simple solution
+
+Since issues like 'I updated my asset database but the app still see the old one', I propose a simple solution.
+
+One simple solution is to uses a versioning system using an incremental number. An asset version file stores this number. When the app starts,
+it checks the version file currently existing on the file system, compare it to the one in the assets and decide
+to copy the asset (the db and the version file) or not.
+
+Let's assume that you have a version file named `db_version_num.txt` in your assets folder.
+The content of the file is a single line with the version number.
+
+`db_version_num.txt`
+```txt
+1
+```
+
+along with the database file `my_asset_database.db`
+
+You could have the following code to copy the asset database only if the version number is different:
+```dart
+/// Copy the asset database if needed and open it.
+///
+/// It uses an external version file to keep track of the asset version.
+Future<Database> copyIfNeededAndOpenAssetDatabase(
+    {required String databasesPath,
+      required String versionNumFilename,
+      required String dbFilename}) async {
+  var dbPath = join(databasesPath, dbFilename);
+
+  // First check the currently installed version
+  var versionNumFile = File(join(databasesPath, versionNumFilename));
+  var existingVersionNum = 0;
+  if (versionNumFile.existsSync()) {
+    existingVersionNum = int.parse(await versionNumFile.readAsString());
+  }
+
+  // Read the asset version
+  var assetVersionNum = int.parse(
+      (await rootBundle.loadString(url.join('assets', versionNumFilename))).trim());
+
+  // Compare them.
+  print('existing/asset: $existingVersionNum/$assetVersionNum');
+
+  // If needed, copy the asset database
+  if (existingVersionNum < assetVersionNum) {
+    print('copying new version $assetVersionNum');
+    // Make sure the parent directory exists
+    try {
+      await Directory(databasesPath).create(recursive: true);
+    } catch (_) {}
+
+    // Copy from asset
+    var data = await rootBundle.load(url.join('assets', dbFilename));
+    var bytes = Uint8List.sublistView(data);
+
+    // Write and flush the database bytes written
+    await File(dbPath).writeAsBytes(bytes, flush: true);
+    // Write and flush the version file
+    await versionNumFile.writeAsString('$assetVersionNum', flush: true);
+  }
+
+  var db = await openDatabase(dbPath);
+  return db;
+}
+```
+
+You can then call this function to open the database:
+```dart
+var db = await copyIfNeededAndOpenAssetDatabase(
+  databasesPath: await getDatabasesPath(),
+  // The asset database filename.
+  dbFilename: 'my_asset_database.db',
+  // The version num.
+  versionNumFilename: 'db_version_num.txt');
+```
+
+When you want to force updating the asset database, you can simply increment the number in the version file (2, 3...).
+
 ### Web support
 
 Check [opening_asset_db_web.md](../../packages_web/sqflite_common_ffi_web/doc/opening_asset_db_web.md) for web support.