ios - How handle different threads in recursive function in swift? -

i have recursive function calculate factorial , want make calculation in background thread, how handle it? i'm trying this:

func factorial(of number: double) -> double {     var result: double = 0     if number > 170 {         return double.infinity     }     if number != double(int(number)) || number < 1 {return 0}     if number == 1 {         return 1     }     dispatchqueue.global(qos: .background).async {         result = number * factorial(of: number - 1)     }     return result  } 

i understand wrong, don't understand how have do. thank's help!

my recommendation split synchronous , asynchronous functions:

func factorial(of number: double, completion: @escaping (double) -> ()) {      dispatchqueue.global(qos: .background).async {         let result = factorial(of: number)         dispatchqueue.main.sync { //i don't know thread you're intending use, picked main :)             completion(result)         }     } }  func factorial(of number: double) -> double {      var result: double = 0     if number > 170 {         return double.infinity     }     if number != double(int(number)) || number < 1 {return 0}     if number == 1 {         return 1     }      result = number * factorial(of: number - 1)      return result } 

this allows meat of work happen in whichever way want, without having worry potential threading issues.