class CalculateService
 {
      public int Num1;
      public int Num2;
       AsyncInterface interface;
      public void Num1Updated(int num1, Action<int> callback)
      {

       var result = TimeConsumingFunction(Num1, Num2);
       callback(result;)

       }

    public void Num2Updated(int num2, Action<int> callback)
    {

      var result = TimeConsumingFunction(Num1, Num2);
      callback(result;)

    }

 }

class SomeOtherClass
    {

       CalculateService serviceObj;

       void func()
      {
         ExternalData.Field1.Changed(x=>{serviceObj.Num1Updated(x, (res) =>
         {UpdateUI(res)});

          ExternalData.Field2.Changed(x=>{serviceObj.Num2Updated(x, (res) =>
         {UpdateUI(res)})
       }
     }