使用定时器时，出现了一个警告，Variable 'timer' was written to, but never read，代码如下
var timer : Timer
timer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { (bool) in })
解决方法，没有用到“timer”变量，去掉timer的定义即可
Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { (bool) in })
