working with RxSwift
RxSwift tutorial: https://medium.com/ios-os-x-development/learn-and-master-%EF%B8%8F-the-basics-of-rxswift-in-10-minutes-818ea6e0a05b
RxSwift visual presentation observables: http://rxmarbles.com/#map
Getting started with RxSwift: https://www.raywenderlich.com/900-getting-started-with-rxswift-and-rxcocoa