Swift3 从自定义格式字符串中提取日期

Question

使用以下过于复杂的代码几乎可以从字符串中提取日期：

let dateStringArray:[String] = ["29-01-2017 10:41:18:825325012","29-01-2017 10:41:18:894631028"]
let formatString = "dd-MM-yyyy HH:mm:ss:nnnnnnnnn"
let dayRange = formatString.range(of: "dd")
let monthRange = formatString.range(of: "MM")
let yearRange = formatString.range(of: "yyyy")
let hourRange = formatString.range(of: "HH")
let minuteRange = formatString.range(of: "mm")
let secondRange = formatString.range(of: "ss")
let nanoRange = formatString.range(of:"nnnnnnnnn")

let dateString = dateStringArray[0]


let dateComponents = DateComponents( year:Int(dateString.substring(with: yearRange!)),
                                     month:Int(dateString.substring(with: monthRange!)),
                                     day:Int(dateString.substring(with: dayRange!)),
                                     hour:Int(dateString.substring(with: hourRange!)),
                                     minute:Int(dateString.substring(with: minuteRange!)),
                                     second:Int(dateString.substring(with: secondRange!)),
                                     nanosecond:Int(dateString.substring(with: nanoRange!)))
let nano = Int(dateString.substring(with: nanoRange!))
let currentCalendar = Calendar.current
let dateExtracted = currentCalendar.date(from: dateComponents)
print("\(dateExtracted!)")

最终打印的输出是：“2017-01-29 10:41:18 +0000\n”

这里有问题 (1) 必须有更简单的方法。 (2) & 更重要的是，为什么纳秒分量显然没有出现？

Answer 1

(1) there must be an easier way

使用日期格式化程序：

let dateString = "29-01-2017 10:41:18:825325012"

let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "en_US_POSIX")
dateFormatter.dateFormat = "dd-MM-yyyy HH:mm:ss:SSSSSSSS"

let dateExtracted = dateFormatter.date(from: dateString)!

(2) why is the nanosecond component apparently not appearing?

Date的description方法不显示小数秒，但是你可以用

再次提取它

currentCalendar.component(.nanosecond, from: dateExtracted!)

结果可能与原始纳秒值略有不同由于舍入错误在字符串中，因为 Date 存储在内部浮点数（自参考日期以来的秒数 2001 年 1 月 1 日）。如果您按照上面的建议使用日期格式化程序，那么分辨率限制为毫秒（比较NSDateFormatter milliseconds bug）。

Swift3 从自定义格式字符串中提取日期

Swift3 extracting a Date from a Custom Format String

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