as and as? In Swift

Use the as operator if you are sure that the downcasting will always

succeed, because the as operator will trigger runtime error if the downcasting

fails. 

Use the as? operator when you are not sure if the downcasting will succeed

because it will return optional value, which means that you will receive nil if the

downcasting failed.

for view in subViews

{

     if view is UILabel // (Method 1)

    {

        let label = view as UILabel

        label.text = "bla bla bla"

    }

   if let button = view as? UIButton // (Method 2)

   {

      button.titleLabel?.text = "button"

   }

}

in method 1, we used the as operator because

we are sure that the view variable will be UILabel after we have checked its type

using the is operator. 

in method 2, we use the as? operator and the condition

will be true only if the casting succeeds and gives a non nil value.