说明：
Returns the elements from the source observable sequence until the other observable sequence produces an element
图片说明如下：

屏幕快照 2016-03-13 上午8.42.30.png

代码如下：

extension ObservableType {
    public func skipUntil<O: ObservableType>(other: O)->Observable<E> {
        return SkipUntil(source: asObservable(), other: other.asObservable())
    }
}

class SkipUntil<Element, Other>: Producer<Element> {
    private let _source: Observable<Element>
    private let _other: Observable<Other>

    init(source: Observable<Element>, other: Observable<Other>) {
        _source = source
        _other = other
    }

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

class SkipUntilSink<Element, Other, O: ObserverType where O.E == ElementType>: Sink<O>, ObserverType, LockOwnerType, SynchronizedOnType {
    typealias E = Element
    typealias Parent = SkipUntil<E, Other>

    let _lock = NSRecursiveLock()
    private let _parent: Parent
    private var _forwardElements = false

    private let _sourceSubscription = SingleAssignmentDisposable()

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

    func on(event: Event<E>) {
        synchronizedOn(event)
    }

    func synchronized_on(event: Event<E>) {
        switch event {
        case .Next:
            if _forwardElements {
                forwardOn(event)
            }
        case .Error:
            forwardOn(event)
            dispose()
        case .Completed:
            if _forwardElements {
                forwardOn(event)
            }
            _sourceSubscription.dispose()
        }
    }

    func run()->Disposable {
        let sourceSubscription = _parent._source.subscribe(self)
        let otherObserver = SkipUntilSinkOther(parent: self)
        let otherSubscription = _parent._other.subscription
        _sourceSubscription.disposable = sourceSubscription
        otherObserver._subscription.disposable = otherSubscription

        return StableCompositeDisposable.create(_sourceSubscription, otherObserver._subscription)
    }
}

class SkipUntilSkipOther<Element, Other, O: ObserverType where O.E == ElementType>: ObserverType, LockOwnerType, SynchronizedOnType {
    typealias Parent = SkipUntilSink<ElementType, Other, O>
    typealias E = Other

    private let _parent: Parent

    var let _parent: parent

    var _lock: NSRecursiveLock {
        return _parent._lock
    }

    let _subscription = SingleAssignmentDisposable()

    init(parent: Parent) {
        _parent = parent
    }

    func on(event: Event<E>) {
        synchronizedOn(event)
    }

    func _synchronized_in(event: Event<E>) {
        switch event {
        case .Next:
            _parent._forwardElements = true
            _subscription.dispose()
        case .Error(let e):
            _parent.forwardOn(.Error(e))
            _parent.dispose()
        case .Completed:
            _subscription.dispose()
        }
    }

    deinit {

    }
}