How to fix the warning of type casting in 'if let' statement in Xcode 8.3?

Question

Consider the following code:

let nsdate: NSDate? = NSDate()
if let date = nsdate as? Date { // a warning occurs here
    print(date)
}

The compiler complains like this: Conditional downcast from 'NSDate?' to 'Date' is a bridging conversion; did you mean to use 'as'?

A cast from NSData to Data has the same problem. How to fix the it?

Answer 1

Try to cast to an optional Date:

if let date = nsdate as Date?

You're trying to optional cast of optional NSDate to NON optional Date. As long as NSDate is bridged from obj-c to Date, so this cast always success, so no optional cast required here, just basic cast as is enough. Then you need to cast optional value, so the resulting value has to be optional too, therefore Date? is appropriate here.

Answer 2

Swift 3.1 distinguishes

• An optional down cast as? Foo

  It casts a more unspecific to a more specific type for example

  let dict : [String:Any] = ["Foo" : 12]
  let number = dict["Foo"] as? Int
  

• A bridge cast of an optional type as Foo?

  It bridges a (Core)Foundation type to a toll free Swift type and vice versa.

  It's the optional equivalent of the usual non-optional syntax

  let string : NSString = "Foo"
  let swiftString = string as String
  

The difference is subtle for the developer but very useful for the compiler.

Basically don't use the NS... Foundation classes in Swift 3 if there is a native Swift counterpart.