Does the swift language change every time Xcode updates?

Could you be more specific ?

- From which version to which version of XCode ?

- What error do you get ?

- What is myMessage : a ViewController most likely ?

you need to apply this to an instance, eg :

self.presentViewController(myMessage, animated: true, competion: nil)

Is the line, " presentViewController(myMessage, animated: true, competion: nil) " still valid?

NO.

The method `presentViewController(_:animated:completion:)` has renamed to `present(_:animated:completion:)` in Swift 3, still the same in Swift 4:

present(myMessage, animated: true, competion: nil)

(Generally type-name suffixes are removed when importing Objective-C methods into Swift 3+.)

If you try Swift migrator included in Xcode 8 (not Xcode 9), your code (valid in Swift 2.x) would be successfully converted to Swift 3 syntax.

>Does the swift language change every time Xcode updates?

I'd not be surprised if it changes more frequently.