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

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])> :)