首先需要導(dǎo)入第三方庫(kù)RxSwift

import Foundation
import RxSwift

extension Observable where Element: FloatingPoint {
    
    /**
     用 Rx 封裝的 Timer
     # 用法 #
     ```swift
     
     Observable<TimeInterval>.timer(duration: 5, interval: 1)
     .subscribe(
     onNext: { remain in
     print("剩余：", remain)
     },
     onCompleted: {
     print("計(jì)時(shí)結(jié)束！")
     }
     )
     .disposed(by: disposeBag)
     
     ```
     
     ## 結(jié)果 ##
     
     ```swift
     剩余： 5.0
     剩余： 4.0
     剩余： 3.0
     剩余： 2.0
     剩余： 1.0
     剩余： 0.0
     計(jì)時(shí)結(jié)束！
     ```
     - parameter duration: 總時(shí)長(zhǎng)
     - parameter interval: 時(shí)間間隔
     - parameter ascending: true 為順數(shù)計(jì)時(shí)，false 為倒數(shù)計(jì)時(shí)
     */
    
    public static func timer(duration: RxTimeInterval = RxTimeInterval.infinity, interval: RxTimeInterval = 1, ascending: Bool = false, scheduler: SchedulerType = MainScheduler.instance)
        -> Observable<TimeInterval> {
            let count = (duration == RxTimeInterval.infinity) ? .max : Int(duration / interval) + 1
            return Observable<Int>.timer(0, period: interval, scheduler: scheduler)
                .map { TimeInterval($0) * interval }
                .map { ascending ? $0 : (duration - $0) }
                .take(count)
    }
}