我有一个看起来像这样的单元测试:
func testManyYearsAgo() {
for year in 2...77 {
let earlierTime = calendar.date(byAdding: .year, value: 0 - year, to: now)
// print(year)
// print(dateDifference.itWasEstimate(baseDate: now, earlierDate: earlierTime!))
XCTAssertEqual(dateDifference.itWasEstimate(baseDate: now, earlierDate: earlierTime!), "\(year) years ago")
}
}
now
被定义为 Date()
calendar
是 Calendar.current
它正在测试一个看起来像这样的类:
class DateDifference {
func itWasEstimate(baseDate: Date, earlierDate: Date) -> String {
let calendar = Calendar.current
let requestedComponent: Set<Calendar.Component> = [ .year, .month, .day, .hour, .minute, .second]
let timeDifference = calendar.dateComponents(requestedComponent, from: baseDate, to: earlierDate)
if timeDifference.year! < 0 {
if timeDifference.year! == -1 {
return "Last year"
} else {
return "\(abs(timeDifference.year!)) years ago"
}
}
return ""
}
}
当我运行单元测试时,我通常(但不总是)得到如下错误:
XCTAssertEqual failed: ("30 years ago") is not equal to ("31 years ago")
这些错误通常在年份值超过 12 之后开始。
如果我取消注释打印语句,无论我运行代码多少次,它都可以正常工作。
这让我相信也许有一些奇怪的异步事情正在发生,但我肯定不能通过观察来判断。我对快速开发还比较陌生,所以可能缺少一些基本的东西。
最佳答案
这是一个独立的可重现示例,展示了 问题:
var calendar = Calendar(identifier: .gregorian)
calendar.locale = Locale(identifier: "en_US_POSIX")
calendar.timeZone = TimeZone(secondsFromGMT: 0)!
let formatter = DateFormatter()
formatter.calendar = calendar
formatter.dateFormat = "yyyy-MM-dd HH:mm:ss.SSS"
let d1 = DateComponents(calendar: calendar, year: 2017, month: 1, day: 1, hour: 0,
minute: 0, second: 0, nanosecond: 456 * Int(NSEC_PER_MSEC)).date!
print("d1:", formatter.string(from: d1))
let d2 = calendar.date(byAdding: .year, value: -20, to: d1)!
print("d2:", formatter.string(from: d2))
let comps: Set<Calendar.Component> = [ .year, .month, .day, .hour, .minute, .second, .nanosecond]
let diff = calendar.dateComponents(comps, from: d1, to: d2)
print(diff)
print("difference in years:", diff.year!)
输出
d1: 2017-01-01 01:00:00.456 d2: 1997-01-01 01:00:00.456 year: -19 month: -11 day: -30 hour: -23 minute: -59 second: -59 nanosecond: -999999756 isLeapMonth: false difference in years: -19
Due to rounding errors (Date
uses a binary floating point number as
internal representation), the difference is computed as a tiny bit
less than 20 years, and the years component of the difference comes out as -19 instead of the expected -20.
As a workaround, you can round the dates to full seconds, that seems to fix the issue:
let baseDate = Date(timeIntervalSinceReferenceDate: baseDate
.timeIntervalSinceReferenceDate.rounded())
let earlierDate = Date(timeIntervalSinceReferenceDate: earlierDate
.timeIntervalSinceReferenceDate.rounded())
您也可以考虑向 Apple 提交错误报告。
关于swift - 为什么我的简单日期计算在 Swift 3.1 中有时会失败?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/43797129/