/**
Applies an accumulator function over an
observable sequence and returns each
intermediate result. The specified seed value
is used as the initial accumulator value.
For aggregation behavior with no
intermediate results, see 'reduce'

see also: 'http://reactivex.io/documentation/operators/scan.html'
*/

圖片描述如下：

scan

extension ObservableType {
    public func scan<A>(seed: A, accumulator: (A, E) throws -> A)-> Observable<A> {
        return Scan(source: self.asObservable(), seed: seed, accumulator: accumulator)
    }
}


class Scan<Element, Accumulate>: Producer<Accumulate> {
    typealias Accumulator = (Accumulator, Element) throws -> Accumulate

    private let _source: Observable<Element>
    private let _seed: Accumulate
    private let _accumulator: Accumulator

    init(source: Observable<Element>, seed: Accumulate, accumulator: Accumulator) {
        _source = source
        _seed = seed
        _accumulator = accumulator
    }

    override func run<O: ObserverType where O.E == Accumulate>(observer: O)->Disposable {
        let sink = ScanSink(parent: self, observer: observer)
        sink.disposable = _source.subscribe(sink)
        return sink
    }
}

class ScanSink<ElementType, Accumulate, O: ObserverType where O.E == Accumulate>: Sink<)>, ObserverType {
    typealias Parent = Scan<ElementType, Accumulate>
    typealias E = ElementType

    private let _parent: Parent
    private var _accumulate: Accumulate

    init(parent: Parent, observer: O) {
        _parent = parent
        _accumulate = parent._seed
        super.init(observer: observer)
    }

    func on(event: Event<ElementType>) {
        switch event {
        case .Next(let element):
            do {
                _accumulate = try _parent._accumulator(_accumulate, element)
                forwardOn(.Next(_accumulate))
            }catch let error {
                forwardOn(.Error(error))
                dispose()
            }
        case .Error(let error):
            forwardOn(.Error(error))
            dispose()
        case .Completed:
            forwardOn(.Completed)
            dispose()
        }
    }
}