JustScheduled运算

/**
Returns an observable sequence that contains a single element.
*/
public static func just(element: E, scheduler: ImmediateSchedulerType)->Observable<E> {
    return JustScheduled(element: element, scheduler: scheduler)
}

class JustScheduled<Element>: Producer<Element> {
    private let _scheduler: ImmediateSchedulerType
    private let _element: Element

    init(element: Element, scheduler: ImmediateSchedulerType) {

    }

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

class JustScheduledSink<O: ObserverType> : Sink<O> {
    typealias Parent = JustScheduled<O.E>

    private let _parent: Parent

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

    func run()->Disposable {
        let scheduler = _parent._scheduler
        return scheduler.schedule(_parent._element) { element in
            self.forwardOn(.Next(element))
            return scheduler.schedule(()) { _ in
                self.forwardOn(.Completed)
                return NopDisposable.instance
            }
        }
    }
}