如何配置 DateFormatter 以捕获微秒

Question

iOS Date() returns 精度至少为微秒的日期。
我通过调用 Date().timeIntervalSince1970 检查了这条语句，结果是 1490891661.074981

然后我需要将日期转换为精度为微秒的字符串。
我按以下方式使用 DateFormatter：

let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSSZZZZZ"
print(formatter.string(from: date))

这导致
"2017-03-30T16:34:21.075000Z"

现在如果我们比较两个结果：
1490891661.074981 和 "2017-03-30T16:34:21.075000Z"
我们可以注意到 DateFormatter 将日期舍入到毫秒精度，同时仍然以微秒为单位显示零。

有人知道如何配置 DateFormatter 以便我可以保持微秒并获得正确的结果："2017-03-30T16:34:21.074981Z"?

Answer 1

(NS)DateFormatter的分辨率限制在毫秒，比较 NSDateFormatter milliseconds bug。一个可能的解决方案是检索所有日期组件（最多纳秒）作为数字并进行自定义字符串格式设置。日期格式化程序仍可用于时区字符串。

示例：

let date = Date(timeIntervalSince1970: 1490891661.074981)

let formatter = DateFormatter()
formatter.dateFormat = "ZZZZZ"
let tzString = formatter.string(from: date)

let cal = Calendar.current
let comps = cal.dateComponents([.year, .month, .day, .hour, .minute, .second, .nanosecond],
                               from: date)
let microSeconds = lrint(Double(comps.nanosecond!)/1000) // Divide by 1000 and round

let formatted = String(format: "%04ld-%02ld-%02ldT%02ld:%02ld:%02ld.%06ld",
                       comps.year!, comps.month!, comps.day!,
                       comps.hour!, comps.minute!, comps.second!,
                       microSeconds) + tzString

print(formatted) // 2017-03-30T18:34:21.074981+02:00

Answer 2

感谢@MartinR 解决了我的问题的前半部分，感谢@ForestKunecke 为我提供了解决问题后半部分的技巧。

在他们的帮助下，我创建了即用型解决方案，该解决方案以微秒精度从字符串转换日期，反之亦然：

public final class MicrosecondPrecisionDateFormatter: DateFormatter {

    private let microsecondsPrefix = "."
    
    override public init() {
        super.init()
        locale = Locale(identifier: "en_US_POSIX")
        timeZone = TimeZone(secondsFromGMT: 0)
    }
    
    required public init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override public func string(from date: Date) -> String {
        dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
        let components = calendar.dateComponents(Set([Calendar.Component.nanosecond]), from: date)

        let nanosecondsInMicrosecond = Double(1000)
        let microseconds = lrint(Double(components.nanosecond!) / nanosecondsInMicrosecond)
        
        // Subtract nanoseconds from date to ensure string(from: Date) doesn't attempt faulty rounding.
        let updatedDate = calendar.date(byAdding: .nanosecond, value: -(components.nanosecond!), to: date)!
        let dateTimeString = super.string(from: updatedDate)
        
        let string = String(format: "%@.%06ldZ",
                            dateTimeString,
                            microseconds)

        return string
    }
    
    override public func date(from string: String) -> Date? {
        dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
        
        guard let microsecondsPrefixRange = string.range(of: microsecondsPrefix) else { return nil }
        let microsecondsWithTimeZoneString = String(string.suffix(from: microsecondsPrefixRange.upperBound))
        
        let nonDigitsCharacterSet = CharacterSet.decimalDigits.inverted
        guard let timeZoneRangePrefixRange = microsecondsWithTimeZoneString.rangeOfCharacter(from: nonDigitsCharacterSet) else { return nil }
        
        let microsecondsString = String(microsecondsWithTimeZoneString.prefix(upTo: timeZoneRangePrefixRange.lowerBound))
        guard let microsecondsCount = Double(microsecondsString) else { return nil }
        
        let dateStringExludingMicroseconds = string
            .replacingOccurrences(of: microsecondsString, with: "")
            .replacingOccurrences(of: microsecondsPrefix, with: "")
        
        guard let date = super.date(from: dateStringExludingMicroseconds) else { return nil }
        let microsecondsInSecond = Double(1000000)
        let dateWithMicroseconds = date + microsecondsCount / microsecondsInSecond
        
        return dateWithMicroseconds
    }
}

用法：

let formatter = MicrosecondPrecisionDateFormatter()
let date = Date(timeIntervalSince1970: 1490891661.074981)
let formattedString = formatter.string(from: date) // 2017-03-30T16:34:21.074981Z

Answer 3

@Vlad Papko 的解决方案有一些问题：

对于如下日期：

2019-02-01T00:01:54.3684Z

它可以使字符串带有额外的零：

2019-02-01T00:01:54.03684Z

这是固定的解决方案，它很丑陋，但没有问题：

override public func string(from date: Date) -> String {
        dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
        let components = calendar.dateComponents(Set([Calendar.Component.nanosecond]), from: date)

        let nanosecondsInMicrosecond = Double(1000)
        let microseconds = lrint(Double(components.nanosecond!) / nanosecondsInMicrosecond)

        // Subtract nanoseconds from date to ensure string(from: Date) doesn't attempt faulty rounding.
        let updatedDate = calendar.date(byAdding: .nanosecond, value: -(components.nanosecond!), to: date)!
        let dateTimeString = super.string(from: updatedDate)

        let stingWithMicroseconds = "\(date.timeIntervalSinceReferenceDate)"
        let dotIndex = stingWithMicroseconds.lastIndex(of: ".")!
        let hasZero = stingWithMicroseconds[stingWithMicroseconds.index(after: dotIndex)] == "0"
        let format = hasZero ? "%@.%06ldZ" : "%@.%6ldZ"

        let string = String(format: format,
                            dateTimeString,
                            microseconds)

        return string
    }

Answer 4

有点hack，但没那么复杂，100% Swift:

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

// Get the number of microseconds with a precision of 6 digits
let now = Date()
let dateParts = Calendar.current.dateComponents([.nanosecond], from: now)
let microSeconds = Int((Double(dateParts.nanosecond!) / 1000).rounded(.toNearestOrEven))
let microSecPart = String(microSeconds).padding(toLength: 6, withPad: "0", startingAt: 0)

// Format the date and add in the microseconds
var timestamp = dateFormatter.string(from: now)
timestamp = timestamp.replacingOccurrences(of: "MICROS", with: microSecPart)

如何配置 DateFormatter 以捕获微秒

How to configure DateFormatter to capture microseconds

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