Is my code Declarative (RxSwift) ? Please help me to explain why not?
In order for code to be declarative, it must express an invariant. Something that is true throughout the entire life of the variables involved. In other words, a direct chain from input to output. Think Excel. In Excel, every cell in the spreadsheet is either an input variable or a transformation of one or more cells which recursively go back to input variables and that's what makes it declarative. 

Your code is not declarative because it doesn't specify what user action would cause processPhotoApiService to get called.

https://medium.com/@danielt1263/imperative-vs-declarative-programming-a74f6cceff0e