问题
I can't seem to find any info on this flag, on StackOverflow or elsewhere on the web. Apple's own documentation only says:
If a formatter is set to be lenient, when parsing a string it uses heuristics to guess at the date which is intended. As with any guessing, it may get the result date wrong (that is, a date other than that which was intended).
Maybe I'm misunderstanding how this is supposed to work, but I can't get it to work at all. My guess was something like this (with a relatively easy date to parse):
> import Foundation
> let df = DateFormatter()
> df.isLenient = true
> df.date(from: "12:00 1/1/2001")
$R0: Date? = nil
No matter what I try, I get nil.
I also see there's a doesRelativeDateFormatting flag, which claims to support phrases such as “today” and “tomorrow”", but that doesn't seem to do anything, either:
> df.doesRelativeDateFormatting = true
> df.date(from: "today")
$R1: Date? = nil
Any ideas?
回答1:
Here is an example where the lenient option makes a difference:
let df = DateFormatter()
df.timeZone = TimeZone(secondsFromGMT: 0)
df.isLenient = true
df.dateFormat = "yyyy-MM-dd"
print(df.date(from: "2015-02-29")) // Optional(2015-03-01 00:00:00 +0000)
2015 is not a leap year, so there is no February 29. With
isLenient = true, the date is interpreted as March 1.
With isLenient = false it is rejected:
let df = DateFormatter()
df.timeZone = TimeZone(secondsFromGMT: 0)
df.isLenient = false
df.dateFormat = "yyyy-MM-dd"
print(df.date(from: "2015-02-29")) // nil
来源：https://stackoverflow.com/questions/39730317/how-does-one-use-nsdateformatters-islenient-option