如何在Swift 5中获得所选周的第一天

Question

I have an UIPickerView with 53 weeks and I try to get the first day (Monday) of a selected week. But every time when I select a week I get the wrong year. For example if I select a week lower than the current week (which is week 31 at this time), then I will get the year 2020 instead of 2019. If I select a Week greater than the current Week (for example week 32 of 2019) then I will get 4 August 2019 23:00 instead of 5 August 2019.

Here is the list with the correct week numbers of 2019: https://www.epochconverter.com/weeks/2019

Here is my small demo:

https://github.com/tygruletz/FirstDayOfSelectedWeek

Here is my code:

class ViewController: UIViewController {

    // Interface Links
    @IBOutlet weak var pickerView: UIPickerView!
    @IBOutlet weak var firstMondayLabel: UILabel!

    // Properties
    var weeksArray = [String]()

    //-- Get the number of weeks for current year.
    var numberOfWeeksInYear: Int {
        var calendar = Calendar(identifier: .iso8601)
        calendar.firstWeekday = 2
        let weekRange = calendar.range(of: .weekOfYear,
                                       in: .year,
                                       for: Date())
        return weekRange!.count
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        weeksArray = (1...numberOfWeeksInYear).map { "\($0)" }

        print("Number of weeks in current year: \(numberOfWeeksInYear)")
    }
}

// Functionality for Next Trailer Inspection Cell
extension ViewController: UIPickerViewDelegate, UIPickerViewDataSource{

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return weeksArray.count
    }

    func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
        return weeksArray[row]
    }

    func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {

        let mondayOfSelectedWeek = Date().dateCorrespondingTo(weekNumber: Int(weeksArray[row]) ?? 0)

        print("1st monday of week \(Int(weeksArray[row]) ?? 0) is: \(mondayOfSelectedWeek ?? Date())")

        firstMondayLabel.text = "\(mondayOfSelectedWeek!)"
    }
}

extension Date {

    func dateCorrespondingTo(weekNumber: Int) -> Date? {
        let thisCalendar = Calendar(identifier: .iso8601)
        return thisCalendar.date(bySetting: .weekOfYear, value: weekNumber, of: self)
    }
}

Answer 1

In your dateCorrespondingTo(weekNumber:) function, you are calleing date(bySetting:...) which is implementation dependent, and currently, when you call it, you set the date to now, and when you select week of year that is in the past, it won't go in the past but will calculate date in the future.

The fact that you are getting August 4, 23:00 is because the date is calculated in the universal time zone, so since your local date is one hour ahead, it recalculates the UTC time, back to your time zone. You really got August 5 00:00:00 but in UTC.

Here's a piece of code that will calculate the Monday of the week of the year (notice that iso8601 calendar has the first day of the week set to Sunday so you should use number 2 for the weekday parameter in DateComponents)

here's the example based on your code:

extension Date {

    func dateCorrespondingTo(weekNumber: Int) -> Date? {
        let thisCalendar = Calendar(identifier: .iso8601)

        let year = thisCalendar.component(.year, from: self)

        let dateComponents = DateComponents(calendar: thisCalendar, timeZone: TimeZone(abbreviation: "UTC"), hour: 0, minute: 0, second: 0, nanosecond: 0, weekday: 2, weekOfYear: weekNumber, yearForWeekOfYear: year)
        return thisCalendar.date(from: dateComponents)
    }
}