RealmService.swift

public final class RealmService {
    public var realm: Realm {
        return try! Realm()
    }
  
    // MARK: - Interface
    
    public func nextID<Model: Object>(for modelType: Model.Type) -> Int {
        let className = String(describing: modelType)
        //
        // Stuck here, can't get realm instance.
        //
        guard let schema = realm.objects(RealmSchema.self).filter("className = %@", className).first else {
            let newModel = RealmSchema(with: className)
            save(newModel)
            
            return 0
        }
        let currentID = schema.nextID
        updateAsync(schema, { $0.incrementID() })
        
        return currentID
    }
    // ...
}