Using User Defaults to store data passed from another View Controller - swift

Question

I'm storing an integer using User Defaults, but the number I'm trying to store was transferred via segue (ageFromSettings) and needs to have a default value.
The problem is that the data that was transferred via segue gets stored when presented, but when the app restarts it stores the default value.
Any ideas how to save an item that was passed via segue?
class ViewController: UIViewController {
#IBOutlet weak var youHaveLabel: UILabel!
#IBOutlet weak var daysLeftLabel: UILabel!
#IBOutlet weak var leftToLiveLabel: UILabel!
#IBAction func settingsButtonPressed(_ sender: Any) {
performSegue(withIdentifier: "toSettings", sender: nil)
}
var daysLeft = 0
var ageFromSettings = 36500
override func viewDidAppear(_ animated: Bool) {
UserDefaults.standard.set(ageFromSettings, forKey: "persistedAge")
if let age = UserDefaults.standard.object(forKey: "persistedAge") as? Int{
print("age: \(age)")
daysLeft = age
}
if daysLeft == 36500 {
daysLeftLabel.alpha = 0
youHaveLabel.alpha = 0
leftToLiveLabel.text = "Set your age in Settings"
} else if daysLeft == -1{
leftToLiveLabel.text = "Day On Borrowed Time"
} else if daysLeft == 0{
leftToLiveLabel.text = "Statistically, you died today"
} else if daysLeft == 1{
leftToLiveLabel.text = "Day Left to Live"
} else if daysLeft < 0 {
leftToLiveLabel.text = "Days on Borrowed Time"
}
if daysLeft > -1 {
daysLeftLabel.text = String(daysLeft)
} else {
daysLeftLabel.text = String(daysLeft * -1)
}
}
}

Answer 1

The problem is that you are saying:
UserDefaults.standard.set(ageFromSettings, forKey: "persistedAge")
if let age = UserDefaults.standard.object(forKey: "persistedAge") as? Int{
That's nutty. In the second line, we know that UserDefaults contains a persistedAge key, and we know what its value is — because you just put it there, in the first line. And you're doing that even if you have already set a different persistedAge value on a previous run of the app.
Delete the first line. If your goal is to supply an initial default value, use register(defaults:_). That's what it's for. It will be used only just in case you have not called set(_:forKey:) yet.

Answer 2

In your case, you are setting the value of the persistedAge key every single time in the viewDidAppear(_:) method of your view controller. Right afterwards, you are retrieving the data and trying to read it, which is somewhat redundant. In the latter case, you will also be resetting the value of the persistedAge key every time your view controller appears.