afterNextTransaction(onCommit:onRollback:)

This method helps synchronizing the database with other resources, such as files, or system services.

In the example below, a CLLocationManager starts monitoring a CLRegion if and only if it has successfully been stored in the database:

/// Inserts a region in the database, and start monitoring upon
/// successful insertion.
func startMonitoring(_ db: Database, region: CLRegion) throws {
    // Make sure database is inside a transaction
    try db.inSavepoint {

        // Save the region in the database
        try insert(...)

        // Start monitoring if and only if the insertion is
        // eventually committed to disk
        db.afterNextTransaction { _ in
            // locationManager prefers the main queue:
            DispatchQueue.main.async {
                locationManager.startMonitoring(for: region)
            }
        }

        return .commit
    }
}

The method above won’t trigger the location manager if the transaction is eventually rollbacked (explicitly, or because of an error).

The onCommit and onRollback closures are executed in the writer dispatch queue, serialized will all database updates.