swift – shareReplay功能如何工作？

public func share(replay: Int = 0,scope: SubjectLifetimeScope = .whileConnected)
        -> Observable<E> {
        switch scope {
        case .forever:
            switch replay {
            case 0: return self.multicast(PublishSubject()).refCount()
            default: return self.multicast(ReplaySubject.create(bufferSize: replay)).refCount()
            }
        case .whileConnected:
            switch replay {
            case 0: return ShareWhileConnected(source: self.asObservable())
            case 1: return ShareReplay1WhileConnected(source: self.asObservable())
            default: return self.multicast(makeSubject: { ReplaySubject.create(bufferSize: replay) }).refCount()
            }
        }
    }

0,1和默认值,有什么区别？为什么将1与defalut分开？

override func subscribe<O : ObserverType>(_ observer: O) -> Disposable where O.E == E {
        _lock.lock()

        let connection = _synchronized_subscribe(observer)
        let count = connection._observers.count
        let disposable = connection._synchronized_subscribe(observer)

        _lock.unlock()

        if count == 0 {
            connection.connect()
        }

        return disposable
    }

如何锁定工作,最困难的是这个功能.封锁的obserables如何正确连接到他们的观察者.