如何为字符串扩展添加自定义初始化方法？extensionString{init(_amount:Double,decimalPlaces:UInt){self.init()letdecimalFormat="%0.\(String(decimalPlaces))f"letcurrencyAmount=String(format:decimalFormat,amount)letcurrencySign=NSLocalizedString("Defaults.CurrencySign",comment:"currencysign")letformattedString="\(currenc