为什么我的Timer选择器触发两次（RunLoop）？

Question

我继承了一些用于检测结束语的代码。

...
recognitionRequest.shouldReportPartialResults = true 
recognitionTask = speechRecognizer.recognitionTask(with: recognitionRequest, resultHandler: { (result, error) in

    if result != nil {

        if self.eosTimer != nil {
            self.eosTimer!.invalidate()
            self.eosTimer = nil
        }
        self.eosTimer = Timer(timeInterval: 2.0, target: self, selector: #selector(self.timerHasCompleted(timer:)), userInfo: nil, repeats: false)
        RunLoop.current.add(self.eosTimer!, forMode: .commonModes)
    }
    ...

我认为这是因为计时器被添加到RunLoop。没有这样做，计时器根本不会发射。如何让eosTimer只触发一次？