Cannot convert value of type 'NSStoryboardSegue.Identifier?' to expected argument type 'String'

Question

Im using the following code to show a view controller,im identifying the segue using the identifier property.This code works fine in swift3 but when updating to swift4 I get the following error

Cannot convert value of type 'NSStoryboardSegue.Identifier?' to expected argument type 'String'

override func prepare(for segue: NSStoryboardSegue, sender: Any?) {
    if (segue.identifier == "segue") {
        //get a reference to the destination view controller
        let destinationVC:ProgressView = segue.destinationController as! ProgressView

        //set properties on the destination view controller
        destinationVC.fileArray=fileArray
        destinationVC.croptype=croptype
        destinationVC.outdir=outdir
        destinationVC.fileformat=fileformat
        destinationVC.tflag=tflag
        if(resize==true)
        {
        destinationVC.resize=true
        destinationVC.rwidth=rwidth
        destinationVC.rheight=rheight
        destinationVC.preserve_aspect_ratio=preserve_aspect_ratio
        }

    }
}

Please advice

Answer 1

In Swift 4 the type of the segue identifier has been changed to NSStoryboardSegue.Identifier

Two solutions

1. Compare the rawValue – and safely unwrap the identifier

   if let identifier = segue.identifier, identifier.rawValue == "segue" { ...
   

2. (recommended) Create an extension

   extension NSStoryboardSegue.Identifier {
       static let segue = NSStoryboardSegue.Identifier("segue")
   }
   

   and compare

   if let identifier = segue.identifier, identifier == .segue { ...