Conditional downcasting

We have already seen how to safely unwrap an optional value:

if let id = Int("12364677") 
{ 
  print("Valid id: \(id)") 
} 

In this case, we know that the Intinit method that takes a String argument returns an optional Int. We only need to test whether it contains a value or nil, and if it contains a value, we know that we can unwrap the value to get an Int.

If, however, we don't know what type may be returned by a function, we can attempt a downcast with the as? keyword:

let jsonResponse: [String: Any] = ["user": "Elliot", 
                                   "id": "Elliot2016"] 

if let userString = jsonResponse["user"] as? String 
{ 
  print("User string contains \(userString.characters.count) 
  characters") 
} 

This code will print the required information, since the downcast succeeds.

Now try this code:

if let id = jsonResponse["id"] as? Int 
{ 
  print("Valid id: \(id)") 
}  
else 
{ 
  print("Invalid id") 
} 

This will print the else statement, since the id downcast has returned nil.