Autocomplete-CitiesCache.swift

actor CitiesCache {

    let source: CitiesSource

    init(source: CitiesSource) {
        self.source = source
    }

    var cities: [String] {
        if let cities = cachedCities {
            return cities
        }

        let cities = source.loadCities()
        cachedCities = cities

        return cities
    }

    private var cachedCities: [String]?
}