What is the most effective way to iterate over a date range in Swift?

We can take a power of Strideable protocol:

extension Date: Strideable {
    public func distance(to other: Date) -> TimeInterval {
        return other.timeIntervalSinceReferenceDate - self.timeIntervalSinceReferenceDate
    }

    public func advanced(by n: TimeInterval) -> Date {
        return self + n
    }
}

and then in your func:

// Iterate by 1 day
// Feel free to change this variable to iterate by week, month etc.
let dayDurationInSeconds: TimeInterval = 60*60*24
for date in stride(from: startDate, to: endDate, by: dayDurationInSeconds) {
    print(date)
}

Output:

2018-07-19 12:18:07 +0000
2018-07-20 12:18:07 +0000
2018-07-21 12:18:07 +0000
2018-07-22 12:18:07 +0000
2018-07-23 12:18:07 +0000
....

The code is taken from this proposal

If I understand your question correctly, the user will check off some weekdays and provide a duration as a number of days.

Assuming you have the selected weekdays in an array and the duration, you can get the list of matching dates as follows:

// User selected weekdays (1 = Sunday, 7 = Saturday)
var selectedWeekdays = [2, 4, 6] // Example - Mon, Wed, Fri
var duration = 10 // Example - 10 days

let calendar = Calendar.current
var today = Date()
let dateEnding = calendar.date(byAdding: .day, value: duration, to: today)!

var matchingDates = [Date]()
// Finding matching dates at midnight - adjust as needed
let components = DateComponents(hour: 0, minute: 0, second: 0) // midnight
calendar.enumerateDates(startingAfter: today, matching: components, matchingPolicy: .nextTime) { (date, strict, stop) in
    if let date = date {
        if date <= dateEnding {
            let weekDay = calendar.component(.weekday, from: date)
            print(date, weekDay)
            if selectedWeekdays.contains(weekDay) {
                matchingDates.append(date)
            }
        } else {
            stop = true
        }
    }
}

print("Matching dates = \(matchingDates)")