     }
     
     @IBAction func swipedInput(sender: AnyObject) {
+        // If a user swipes on the input label, remove on digit.
+        // The iOS native calculator app also has this hidden feature.
         converter.removeLastInput()
         updateInterface()
     }
     
     @IBAction func longPressedInput(sender: UIGestureRecognizer) {
+        // Copy input label text to clipboard after a long press.
         if sender.state == .Began {
             UIPasteboard.generalPasteboard().string = inputCurrencyLabel.text
             print("Copied input currency value to clipboard.")


extension MainViewController: ChangeCurrencyViewControllerDelegate {
    
    func didChangeCurrency(currencyCode: String, targetCurrency: String) {
        if targetCurrency == "input" {
            converter.setInputCurrency(currencyCode)
            updateInterface()
            prefs.setObject(currencyCode, forKey: "input")
            print("Input currency updated to: \(currencyCode)")
        }
        if targetCurrency == "output" {
            converter.setOutputCurrency(currencyCode)
            updateInterface()
            prefs.setObject(currencyCode, forKey: "output")
            print("Output currency updated to: \(currencyCode)")
        }

        view.backgroundColor = UIColor(red:0.97, green:0.97, blue:0.97, alpha:1)
        view.clipsToBounds = true
        
        // If we have the last input currency used saved on the preferences
        // file, let's use it.
        if let currencyCode = prefs.stringForKey("input") {
            converter.setInputCurrency(currencyCode)
            updateInterface()
            print("Used saved input currency: \(currencyCode)")
        }
        
        // If we have the last input currency used saved on the preferences
        // file, let's use it.
        if let currencyCode = prefs.stringForKey("output") {
            converter.setOutputCurrency(currencyCode)
            updateInterface()
