Swift计时器未触发

我有以下启动和停止计时器的代码.

private var timer: Timer? = nil

private func startTimer() {
    if (timer != nil) {
        return
    }
    logger.error("Starting timer on \(Thread.current)")
    timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true, block: { timer in
        self.logger.error("Timer fired on \(Thread.current)")
    })
}

private func stopTimer() {
    if (timer == nil) {
        return
    }
    logger.error("Stopping timer on \(Thread.current)")
    timer?.invalidate()
    timer = nil
}

计时器将多次启动和停止.我遇到了一个问题，它第一次按预期开始和停止，但第二次启动计时器时，它永远不会触发.下面是我从应用程序的两次连续运行中看到的日志(log).您可以看到，在第二次运行计时器后，从未调用过"Timer fired".

运行1:

Starting timer on <_NSMainThread: 0x2829e4140>{number = 1, name = main}
Timer fired on <_NSMainThread: 0x2829e4140>{number = 1, name = main}
Timer fired on <_NSMainThread: 0x2829e4140>{number = 1, name = main}
Timer fired on <_NSMainThread: 0x2829e4140>{number = 1, name = main}
Stopping timer on <_NSMainThread: 0x2829e4140>{number = 1, name = main}
Starting timer on <NSThread: 0x2829ecf00>{number = 10, name = (null)}

运行2:

Starting timer on <_NSMainThread: 0x281cb8400>{number = 1, name = main}
Timer fired on <_NSMainThread: 0x281cb8400>{number = 1, name = main}
Timer fired on <_NSMainThread: 0x281cb8400>{number = 1, name = main}
Stopping timer on <_NSMainThread: 0x281cb8400>{number = 1, name = main}
Starting timer on <NSThread: 0x281cbc140>{number = 8, name = (null)}

为什么计时器不会在第二次计时器响起？目前我能想到的唯一潜在的领先优势就是线索.现在，计时器的停止和随后的重新启动可能不会在主线程上发生，并且Invalify()的文档说明应该在启动计时器的同一线程上调用它.因此，另一个问题是，我如何确保在同一线程上调用了Start和Stop(不必是主线程)？谢谢