RxSwift timer與Swift timer比較和原理探索

RxSwift中timer不受runloop的影響可以正常計時,以下探索timer實現原理。

Timer

  1. swift創建timer
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        timer = Timer.init(timeInterval: 1, target: self, selector: #selector(timerFire), userInfo: nil, repeats: true)
        RunLoop.current.add(timer, forMode: .default)
    }
    
    @objc func timerFire() {
        print(NSDate())
    }
/*
輸出結果:
2019-08-04 10:27:38 +0000
2019-08-04 10:27:39 +0000
2019-08-04 10:27:40 +0000
2019-08-04 10:27:41 +0000
2019-08-04 10:27:42 +0000
……
*/
  • 當頁面有滾動事件時定時器會停止計時,滾動事件結束定時器繼續計時。
  • 把定時器加到Runloop的.commonMode中,滾動事件不影響定時器計時,但是在commonMode裏執行會影響整個程序,且不容易處理
  1. Timer.scheduledTimer的寫法
timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(timerFire), userInfo: nil, repeats: true)
  • Timer.scheduledTimer的寫法在頁面有滾動事件時定時器會停止計時,說明此方法加進了runloop的.defaultMode裏

DispatchSourceTimer

  1. swift創建gcdTimer
gcdTimer = DispatchSource.makeTimerSource()
        gcdTimer?.schedule(deadline: DispatchTime.now(), repeating: DispatchTimeInterval.seconds(1))
        gcdTimer?.setEventHandler(handler: {
            print("gcdTimer:\(NSDate())")
        })
        gcdTimer?.resume()
/*
輸出結果:
gcdTimer:2019-08-04 11:18:13 +0000
gcdTimer:2019-08-04 11:18:14 +0000
gcdTimer:2019-08-04 11:18:15 +0000
gcdTimer:2019-08-04 11:18:16 +0000
gcdTimer:2019-08-04 11:18:17 +0000
……
*/
  • gcdTimer,滾動事件不影響定時器計時,這種方式計時更準確有效
  1. gcdTimer操作
        gcdTimer?.suspend()     //掛起,gcdTimer掛起後一定要取消置空
        gcdTimer?.cancel()     //取消,先取消再置空
        gcdTimer = nil     //置空

CADisplayLink

  1. swift創建cadTimer
        cadTimer = CADisplayLink(target: self, selector: #selector(timerFire))
        cadTimer?.preferredFramesPerSecond = 1
        cadTimer?.add(to: RunLoop.current, forMode: .default)
  • cadTimer也是加入到runloop的.defaultMode裏,滾動事件時定時器會停止計時

RxSwift timer

  1. RxSwift創建timer
 Observable<Int>.interval(1, scheduler: MainScheduler.instance).subscribe(onNext: { (num) in
            print(num)
        })
        .disposed(by: disposeBag)
  • RxSwift的timer計時不受滑動時間影響,底層是封裝了GCD執行計時
  1. interval的實現
public static func interval(_ period: RxTimeInterval, scheduler: SchedulerType)
        -> Observable<Element> {
        return Timer(
            dueTime: period,
            period: period,
            scheduler: scheduler
        )
    }
  1. Timer類的實現
    Timer類繼承於Producer,執行run方法,run方法裏創建TimerSink
/*繼承於Producer,執行run方法,Producer裏的run方法由子類來實現*/
final private class Timer<Element: RxAbstractInteger>: Producer<Element> {
    fileprivate let _scheduler: SchedulerType
    fileprivate let _dueTime: RxTimeInterval
    fileprivate let _period: RxTimeInterval?

    // 初始化
    init(dueTime: RxTimeInterval, period: RxTimeInterval?, scheduler: SchedulerType) {
        self._scheduler = scheduler     //調度環境
        self._dueTime = dueTime     //定時器發起事件
        self._period = period     //間隔時間
    }

    //執行run方法
    override func run<Observer: ObserverType>(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where Observer.Element == Element {
        if self._period != nil {
            //創建TimerSink,然後執行sink.run
            let sink = TimerSink(parent: self, observer: observer, cancel: cancel)
            let subscription = sink.run()
            return (sink: sink, subscription: subscription)
        }
        else {
            let sink = TimerOneOffSink(parent: self, observer: observer, cancel: cancel)
            let subscription = sink.run()
            return (sink: sink, subscription: subscription)
        }
    }
}
  • TimerSink,init初始化,實現run方法
final private class TimerSink<Observer: ObserverType> : Sink<Observer> where Observer.Element : RxAbstractInteger  {
    typealias Parent = Timer<Observer.Element>

    private let _parent: Parent
    private let _lock = RecursiveLock()

    init(parent: Parent, observer: Observer, cancel: Cancelable) {
        self._parent = parent
        super.init(observer: observer, cancel: cancel)
    }

    func run() -> Disposable {
        //執行schedulePeriodic方法
        return self._parent._scheduler.schedulePeriodic(0 as Observer.Element, startAfter: self._parent._dueTime, period: self._parent._period!) { state in
            self._lock.lock(); defer { self._lock.unlock() }
            self.forwardOn(.next(state))
            return state &+ 1
        }
    }
}
  • schedulePeriodic方法,創建了GCD計時器並處理定時器回調
func schedulePeriodic<StateType>(_ state: StateType, startAfter: RxTimeInterval, period: RxTimeInterval, action: @escaping (StateType) -> StateType) -> Disposable {
        let initial = DispatchTime.now() + startAfter

        var timerState = state

        // 創建GCD計時器
        let timer = DispatchSource.makeTimerSource(queue: self.queue)
        timer.schedule(deadline: initial, repeating: period, leeway: self.leeway)
        
        // TODO:
        // This looks horrible, and yes, it is.
        // It looks like Apple has made a conceputal change here, and I'm unsure why.
        // Need more info on this.
        // It looks like just setting timer to fire and not holding a reference to it
        // until deadline causes timer cancellation.
        var timerReference: DispatchSourceTimer? = timer
        let cancelTimer = Disposables.create {
            timerReference?.cancel()
            timerReference = nil
        }

        //定時器回調,保存的定時器事件
        timer.setEventHandler(handler: {
            //容錯條件
            if cancelTimer.isDisposed {
                return
            }
            //發送狀態
            timerState = action(timerState)
        })
        timer.resume()
        
        return cancelTimer
    }
  • action是閉包的傳值,往上找可以在TimerSink的run方法找到action的實現,action裏執行self.forwardOn(.next(state))發送消息,也就是執行subscribe(onNext:)方法返回到閉包裏執行打印
func run() -> Disposable {
        return self._parent._scheduler.schedulePeriodic(0 as Observer.Element, startAfter: self._parent._dueTime, period: self._parent._period!) { state in      //尾隨閉包action實現
            self._lock.lock(); defer { self._lock.unlock() }
            self.forwardOn(.next(state))
            return state &+ 1     //按位與操作,地址+1,改變狀態
        }
    }
  1. timer序列停止計時
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        //timer已經加入到disposeBag裏,這裏將disposeBag重新賦值即可銷燬timer停止計時
        disposeBag = DisposeBag()
    }
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