求各位大家指点这个函数的执行步骤

小的菜鸟，求大神解惑，感激涕零~~~~
func createAdder(numberToAdd : Int)-> (Int)->Int{
func adder(number : Int)->Int {
return number + numberToAdd
}
return adder
}
var addTwo = createAdder(2)
addTwo(2) // =4