Observable Current and Previous Value

Question

I have a Variable which is an array of enum values. These values change over time.

enum Option {     case One     case Two     case Three }  let options = Variable<[Option]>([ .One, .Two, .Three ]) 

I then observe this variable for changes. The problem is, I need to know the diff between the newest value and the previous value. I'm currently doing this:

let previousOptions: [Option] = [ .One, .Two, .Three ]  ...  options     .asObservable()     .subscribeNext { [unowned self] opts in         // Do some work diff'ing previousOptions and opt         // ....         self.previousOptions = opts     } 

Is there something built in to RxSwift that would manage this better? Is there a way to always get the previous and current values from a signal?

Answer 1

Here is a handy generic extension, that should cover these "I want the previous and the current value" use cases:

extension ObservableType {      func withPrevious(startWith first: E) -> Observable<(E, E)> {         return scan((first, first)) { ($0.1, $1) }.skip(1)     } } 

Answer 2

there you go

options.asObservable()     .scan( [ [],[] ] ) { seed, newValue in         return [ seed[1], newValue ]     }     // optional, working with tuple of array is better than array of array     .map { array in (array[0], array[1])  }      //optional, in case you dont want empty array     .skipWhile { $0.count == 0 && $1.count == 0 } 

it will return Observable<([Options], [Options])> :)