Timer not running at some class

I have a simple class to interval run some code when call it start ;

// Test.swift
public class Test {
  var timer Timer?
  public func start(){
    //  
    print("2“） // print
     timer = Timer.scheduledTimer(5,true,{ 
       // code here
       print("3")  // not print
     }
    // RunLoop.current.add(timer,forMode:.commonMode) // uncomment this line also not work
  }
}

// Class2.swift

class Class2: ParentClass{
  private let test = Test()
 @objc public func check(flag:String){
     if flag == "Y" {
      print("1")
        test.start()
      }
   }
}

But the timer seems not work --- not print "3"

But I try to create a timer at some other class, it work, just at Test class not run ....

Anyone know why ?