Swift - Stop segue and show alert

I "google" and the following link is helpful:
http://jamesleist.com/ios-swift-tutorial-stop-segue-show-alert-text-box-empty/

Reference: https://developer.apple.com/library/mac/documentation/AppKit/Reference/NSSeguePerforming_Protocol/#//apple_ref/occ/intfm/NSSeguePerforming/shouldPerformSegueWithIdentifier:sender:

Override above function and return false if you want to stop segue.

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