A few actions...

CurrencyPickerResponsibles.swift

// Behavior 1: Save to preferences.
class CurrencyPickerSettingsResponsible { }

extension CurrencyPickerSettingsResponsible : CurrencyPickerProtocol {
 	func selected(currency: String, from: CurrencyPickerTableViewController) {
    let preferences = BTCPreferences.sharedPreferences()
		preferences.setObject(key, forKey: kBTCSelectedCurrencyKey)
		preferences.synchronize()

		NSNotificationCenter.defaultCenter().postNotificationName(kBTCCurrencyDidChangeNotificationName, object: key)
		from.navigationController?.popViewControllerAnimated(true)
    }
}

// Behavior 2: No action.
class CurrencyPickerNoAction { }

extension CurrencyPickerSettingsResponsible : CurrencyPickerProtocol {
 	func selected(currency: String, from: CurrencyPickerTableViewController) { }
}