Observer (Наблюдатель) — шаблон проектирования на примере Swift
https://medium.com/@uladzislau.komar/observer-%D0%BD%D0%B0%D0%B1%D0%BB%D1%8E%D0%B4%D0%B0%D1%82%D0%B5%D0%BB%D1%8C-%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD-%D0%BF%D1%80%D0%BE%D0%B5%D0%BA%D1%82%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D1%8F-%D0%BD%D0%B0-%D0%BF%D1%80%D0%B8%D0%BC%D0%B5%D1%80%D0%B5-swift-f05645e29b61