Swift 项目开发中遇到将银行卡按照4位一组，中间空格的形式展示，扩展了String类，扩展方法如下:

    public func formateForBankCard(joined: String = " ") -> String {
        guard self.characters.count > 0 else {
            return self
        }
        let length: Int = self.characters.count
        let count: Int = length / 4
        var data: [String] = []
        for i in 0..<count {
            let start: Int = 4 * i
            let end: Int = 4 * (i + 1)
            data.append(self[start..<end])
        }
        if length % 4 > 0 {
            data.append(self[4 * count..<length])
        }
        let result = data.joined(separator: " ")
        return result
    }

测试代码:

        print("输出结果:\("123456789".formateForBankCard())")