Perform closure once for a given key

Extensions.swift

// MARK: - NSUserDefaults

extension NSUserDefaults {
    
    class func performOnceForKey(key: String, perform: () -> Void, elsePerform: (() -> Void)? = nil) {
        let once = self.standardUserDefaults().objectForKey(key)
        self.standardUserDefaults().setBool(true, forKey: key)
        self.standardUserDefaults().synchronize()
        
        if once == nil { perform() }
        else { elsePerform?() }
    }
}