of运算

/**
This method creates a new observable instance
with a variable number of elements.

see also:http://reactivex.io/documentation/operators/from.html

- parameter elements: Elements to generate
- parameter scheduler: Scheduler to send elements on. 
    If 'nil', elements are sent immediately on subscription
- returns: The observable sequence whose elements are pulled from the given arguments.
*/
public static func of(elements: E ..., scheduler: ImmediateSchedulerType? = nil)->Observable {
    return Sequence(elements: elements, scheduler: scheduler)
}

class Sequence: Producer {
    private let _elements: [E]
    private let _scheduler: ImmediateSchedulerType?

    init(elements: [E], scheduler: ImmediateSchedulerType?) {
        _elements = elements
        _scheduler = scheduler
    }

    override func subscribe(observer: O)->Disposable {
        // optimized version without scheduler
        guard _scheduler != nil else {
            for element in _elements {
                observer.on(.Next(element))
            }
            observer.on(.Completed)
            return NoDisposable.instance
        }

        let sink = SequenceSink(parent: self, observer: observer)
        sink.disposable = sink.run()
        return sink
    }
}

class SequenceSink: Sink {
    typealias Parent = Sequence

    private let _parent: Parent

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

    func run()->Disposable {
        return _parent._scheduler!.scheduleRecursive( (0, _parent._elements)) { (state, recurse) in
            if state.0 < state.1.count {
                self.forwardOn(.Next(state.1[state.0]))
                recurse((state.0 + 1, state.1))
            }else {
                self.forwardOn(.Completed)
            }
        }
    }
}