NSSharingService doesn't call didFailToShareItems when user cancels

Hi,

I'm able to send emails with NSSharingService from my Swift-code app but if a user discards the email that has been condtructed and passed to Mail instead of clicking Send the didFailToShareItems method isn't called in the delegate. To set up the serivce I use:

if let sharingService: NSSharingService = NSSharingService.init(named: NSSharingServiceNameComposeEmail) {
     let emailMessage: String = "An email message. The content is here...."
     sharingService.recipients = [clientEmail]
     sharingService.subject = "An email from myApp"
     sharingService.delegate = self
     if sharingService.canPerform(withItems: [emailMessage, invoiceURL]) {
                sharingService.perform(withItems: [emailMessage, attachmentURL])
     } else {
                //notify user
     }
}

The delegate's methods are:

func sharingService(_ sharingService: NSSharingService, didFailToShareItems items: [Any], error: Error) {
        Swift.Print("Oops, failed to send.") //or something more constructive
    }

    func sharingService(_ sharingService: NSSharingService, didShareItems items: [Any]) {
        Swift.Print("Sent.") //or something more constructive
    }

Is there any way for my app to be informed of a Cancel action in Mail?

Thanks,

Todd.