Observableを順番に処理させるサンプル ref: http://qiita.com/syou007/items/44e5d4541b22f973d12b

file0.swift

        let a:Observable<Int> = Observable.create { observable in
            NSOperationQueue().addOperationWithBlock({ () -> Void in
                sleep(5)
                observable.onNext(1)
                observable.onCompleted()
            })
            return NopDisposable.instance
        }

        let b:Observable<Int> = Observable.create { observable in
            NSOperationQueue().addOperationWithBlock({ () -> Void in
                sleep(5)
                observable.onNext(2)
                observable.onCompleted()
            })
            return NopDisposable.instance
        }

        let c:Observable<Int> = Observable.create { observable in
            NSOperationQueue().addOperationWithBlock({ () -> Void in
                sleep(5)
                observable.onNext(3)
                observable.onCompleted()
            })
            return NopDisposable.instance
        }

        a.concat(b).concat(c).subscribeNext { (value) in
            print(value)
        }