pluralOpenValve.swift

  public func openValve<O: MotionObservableConvertible>(whenAllTrue observables: [O]) -> MotionObservable<T> where O.T == Bool {
    return MotionObservable<T> { observer in
      var upstreamSubscription: Subscription?

      var connectUpstream = {
        upstreamSubscription = self.asStream().subscribe(next: observer.next,
                                                         state: observer.state,
                                                         coreAnimation: observer.coreAnimation)
      }

      var states: [Bool] = observables.map { _ in false }
      var valveSubscriptions: [Subscription] = []
      for (index, observable) in observables.enumerated() {
        let subscription = observable.asStream().subscribe(next: { value in
          states[index] = value

          var anyFalse = false
          for state in states {
            if state == false {
              anyFalse = true
              break
            }
          }
          let shouldOpen = !anyFalse
          if shouldOpen && upstreamSubscription == nil {
            connectUpstream()
          }

          if !shouldOpen && upstreamSubscription != nil {
            upstreamSubscription?.unsubscribe()
            upstreamSubscription = nil
          }

        }, state: { _ in }, coreAnimation: { _ in })
        valveSubscriptions.append(subscription)
      }
      return {
        for subscription in valveSubscriptions {
          subscription.unsubscribe()
        }
        upstreamSubscription?.unsubscribe()
        valveSubscriptions = []
        upstreamSubscription = nil
      }
    }
  }