Swift DateFormatter - print week of year and year - ios

Question

I'm trying to print a given date in the format of "[week in a year] [year]" combination using a DateFormatter. I have given it a pattern of "w yyyy" but it returns an empty string.
let date = DateComponents(calendar: Calendar.current, year: 2018, month: 9, day: 28).date
let formatter = DateFormatter()
formatter.locale = Locale.current
formatter.setLocalizedDateFormatFromTemplate("w yyyy")
formatter.string(from: date!) // this returns ""
formatter.setLocalizedDateFormatFromTemplate("w")
formatter.string(from: date!) // this returns the correct number, like "36"
According to the Date Formatting Guide, iOS 5 uses version tr35-19. I assume that it hasn't changed since then.
In the unicode.org documentation, there is no special mention of how week of year is behaving in conjunction with a year format. What am I missing here?
EDIT I understand that I can use DateComponents to get the numbers and formatting them that way, but this question is more about why the format "w yyyy" is special.

Answer 1

Don't use setLocalizedDateFormatFromTemplate(). I'm not familiar with that method, but it sounds like it expects a pre-defined date string template, and your format string must not match any known templates. If you change your line
formatter.setLocalizedDateFormatFromTemplate("w yyyy")
to
formatter.dateFormat = "w yyyy"
It works as expected.
Edit:
It seems you should use:
formatter.dateFormat = "w Y"
y and yyyy will not give the correct results with certain dates.

Answer 2

Try this :
let date = Date()
let calendar = Calendar.current
let year = calendar.component(.year, from: date)
let month = calendar.component(.month, from: date)
let day = calendar.component(.day, from: date)
let weekOfYear = calendar.component(.weekOfYear, from: Date.init(timeIntervalSinceNow: 0))
OR
let date = Date()
let calendar = Calendar.current
let year = calendar.component(.year, from: date)
let month = calendar.component(.month, from: date)
let day = calendar.component(.day, from: date)
let weekOfYear = calendar.component(.weekOfYear, from: Date())