Replacing NSDate value if two NSDate objects aren't equal - xcode

Question

basically I'm writing an app and I want some of its values to update weekly - so, using NSDate and NSUserDefaults I currently have this scenario set out:
let userDefaults = NSUserDefaults.standardUserDefaults()
var startOfWeek: NSDate?
The above is global^
var referenceDate = NSDate.timeIntervalSinceReferenceDate()
while referenceDate > 604800 {
if referenceDate > 604800 {
referenceDate -= 604800
}
This basically takes the amount of seconds that have elapsed since the 1st of Jan 2001 and subtracts 604800, which is the amount of seconds in a week, until you have an amount of seconds that is less than a week. This amount of seconds is basically the amount of seconds into the week, you're in.
After this I then create a variable which is just the negative of how many seconds into the week we are, a variable which is the current time and use them to create a new date object which is the start point for the week:
var weekTimer: NSTimeInterval = (-referenceDate)
var startOfWeek = userDefaults.objectForKey("startOfWeekKey") as? NSDate
var currentTime = NSDate(timeIntervalSinceNow: 0)
if startOfWeek == nil {
startOfWeek = NSDate(timeInterval: weekTimer, sinceDate: currentTime)
userDefaults.objectForKey("startOfWeekKey")
}
else {
}
So basically it creates a value for the start of the week and saves it to NSUserDefaults under the key "startOfWeekKey" if the value doesn't already exist. If the value does already exist, because it's already been saved, it's happy with this value and nothing happens.
Now, we create a new value to compare to our startOfWeek value so we can test if we enter a new week.
var startOfWeekCheck = NSDate(timeInterval: weekTimer sinceDate: currentTime)
Now we compare them:
if startOfWeek! == startOfWeekCheck {
}
else {
println("New week begins") //update other, irrelevant stuff here
startOfWeek = startOfWeekCheck
}
No matter what, new week begins is always printed, I added the following line to check what was happening:
println("\(startOfWeek!) \(startOfWeekCheck)")
And it prints the same value to the logs
I'm pretty unsure what to do at this point. Did I mess up? Can you not compare the two date objects like that? Is this a ridicuously inefficient manner to create a way to reset/change something once a week?
Note, this is not the exact code, I typed this all by hand because the code is on my macbook and I'm on my desktop, so if there are any typos / w/e don't worry about it, no compile errors on my mac.
Thanks!

Answer 1

This looks way too complicated. There is no need to do all the date calculations yourself. Especially because you do them wrong. Not every week has 604800 seconds. In many timezones you have one week per year which has 601200 seconds, and one week which has 608400 seconds. Also known as Daylight saving time. There are even countries that skipped a whole day in one year.
You have to use NSCalendar and NSDateComponents for correct date calculations.
But I would do it completely different:
Save the next refresh date in NSUserDefaults (default to distantPast)
If today >= next refresh date: refresh & write new refresh date.
Something like this:
let calendar = NSCalendar.currentCalendar()
// If nextRefreshDate was never written by us (i.e. first start of app) it will containt distantPast, which is in year 1 or so
NSUserDefaults.standardUserDefaults().registerDefaults(["nextRefreshDate" : NSDate.distantPast()])
// since we registered a default value we will always receive a valid NSDate object, so we can force unwrap
let nextRefreshDate = NSUserDefaults.standardUserDefaults().objectForKey("nextRefreshDate") as NSDate
// equivalent to: (nextRefreshDate < today) || (nextRefreshDate == today)
if nextRefreshDate.compare(NSDate()) != NSComparisonResult.OrderedDescending {
println("Refresh")
let refreshSuccess = true // get from your internet download code
if refreshSuccess {
// don't save the next refresh date until the refresh was actually sucessful. This makes sure that we refresh on next launch if this refresh fails for some reason
var startOfCurrentWeek: NSDate?
if calendar.rangeOfUnit(.CalendarUnitWeekOfMonth, startDate: &startOfCurrentWeek, interval: nil, forDate: NSDate()) {
if let startOfCurrentWeek = startOfCurrentWeek {
if let startOfNextWeek = calendar.dateByAddingUnit(.CalendarUnitWeekOfMonth, value: 1, toDate: startOfCurrentWeek, options: nil) {
NSUserDefaults.standardUserDefaults().setObject(startOfNextWeek, forKey: "nextRefreshDate")
println("Next refresh scheduled for \(startOfNextWeek)")
}
}
}
}
}
else {
println("No refresh necessary. Next refresh scheduled for \(nextRefreshDate)")
}