Combine中的RxSwift `ActivityIndicator`功能

extension ObservableType {
    func trackActivity(_ activityIndicator: ActivityIndicator) -> Observable<Element> {
        return activityIndicator.trackActivityOfObservable(self)
    }
}

class ActivityIndicator: SharedSequenceConvertibleType {
    typealias Element = Bool
    typealias SharingStrategy = DriverSharingStrategy

    private let _lock = NSRecursiveLock()
    private let _relay = BehaviorRelay(value: false)
    private let _loading: SharedSequence<SharingStrategy, Bool>

    init() {
        _loading = _relay.asDriver()
            .distinctUntilChanged()
    }

    fileprivate func trackActivityOfObservable<O: ObservableConvertibleType>(_ source: O) -> Observable<O.Element> {
        return source.asObservable()
            .do(onNext: { _ in
                self._lock.lock()
                self._relay.accept(true)
                self._lock.unlock()
            }, onError: { _ in
                self._lock.lock()
                self._relay.accept(false)
                self._lock.unlock()
            }, onCompleted: {
                self._lock.lock()
                self._relay.accept(false)
                self._lock.unlock()
            }, onSubscribe: {
                self._lock.lock()
                self._relay.accept(true)
                self._lock.unlock()
            }, onDispose: {
                self._lock.lock()
                self._relay.accept(false)
                self._lock.unlock()
            })
    }

    func asSharedSequence() -> SharedSequence<SharingStrategy, Element> {
        return _loading
    }
}