Swift Timer仅在一次操作中触发一次

Question

我在Swift中的Operation的子类中设置了此代码。但我无法弄清楚为什么计时器只调用选择器一次。

class DogOperationRun : DogOperation {

let thisDog:Dog?
let operationID:Int?
var runDistance:Double?
var dogSpeed:Double?

var runTimeLeft:Double = 0.0
var currentRunDistance:Double = 0.0
private var interval = 0.0
private var cycle = 0.0
private var dogTimer:Timer?

init(thisDog:Dog, operationID:Int, runDistance:Double, dogSpeed:Double) {

    self.thisDog = thisDog
    self.operationID = operationID
    self.runDistance = runDistance
    self.dogSpeed = dogSpeed

    super.init(self.operationID!)
}

override func main() {
    guard isCancelled == false else {
        operationIsFinished = true
        return
    }

    startRun()
}

func startRun() {

    operationIsExecuting = true

    if let distance = runDistance, let thisDogSpeed = dogSpeed {
        runTimeLeft = distance
        interval = distance/thisDogSpeed
        dogTimer = Timer.scheduledTimer(timeInterval: 0.5, target: self, selector: #selector(runDog), userInfo: nil, repeats: true)
    }

}

@objc func runDog() {
    runTimeLeft -= interval
    cycle += 1

    if runTimeLeft > 0 {
        runDistance = interval * cycle
        print("\(self.thisDog!.name) has run \(runDistance!) meters!")
    } else {
        dogTimer?.invalidate()
        operationIsFinished = true
    }

}

}

和超类

import Foundation

class DogOperation: Operation {

public var id:Int


//going to over ride the existing variable and return my private one
override var isExecuting: Bool {
    return operationIsExecuting
}

var operationIsExecuting = false {

    didSet {

        willChangeValue(forKey: "isExecuting")
        print("Operation \(id) will execute")

        didChangeValue(forKey: "isExecuting")
        print("Operation \(id) is executing")
    }
}

override var isFinished: Bool {
    return operationIsFinished
}

var operationIsFinished = false {

    didSet {

        willChangeValue(forKey: "isFinished")
        print("Operation \(id) will finish")

        didChangeValue(forKey: "isFinished")
        print("Operation \(id) is finished")
    }
}


init(_ id:Int) {
    self.id = id
}
}

这样称呼它：

let lodiDogRun = DogOperationRun(thisDog: aDog, operationID: 1, runDistance: 100.0, dogSpeed: 12.0)
    let pretzelDogRun = DogOperationRun(thisDog: nextDog, operationID: 2, runDistance: 100.0, dogSpeed: 14.0)

    myOperationQueue.addOperations([lodiDogRun, pretzelDogRun], waitUntilFinished: true)

编辑Matt：dogRun被调用的唯一时间是我用dogTimer.fire（）强制计时器。对于我捕获它的实例，它正在线程5上运行：