Using @available with stored properties

Question

I have an app that uses local notifications and supports iOS 10. I am trying to add iOS 9 support which requires me to use the old location notification API. I am trying to use @available and #available on my iOS 10 code and I can't figure out how to get my center variable to only be for devices running iOS 10.

When I set my target from iOS 10 to 9 I get the error message for this variable:

UNUserNotificationCenter is only available on iOS 10.0 or newer. 

It suggests I add @available(iOS 10.0, *) to my entire class which I don't want to do since there is code in this class that will be used for iOS 9. I appreciate any suggestions on how to limit my center property to just iOS 10.

class ViewController: UIViewController, UITextFieldDelegate {      let center = UNUserNotificationCenter.current()   ... } 

Answer 1

Here is one potential solution (thanks to blog post). The idea is to use a stored property with a type of Any and then create a computed property that will cast the stored property (and instantiate it if necessary).

private var _selectionFeedbackGenerator: Any? = nil @available(iOS 10.0, *) fileprivate var selectionFeedbackGenerator: UISelectionFeedbackGenerator {     if _selectionFeedbackGenerator == nil {         _selectionFeedbackGenerator = UISelectionFeedbackGenerator()     }     return _selectionFeedbackGenerator as! UISelectionFeedbackGenerator } 

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Another option is to use lazy (however, this makes the variable read-write):

@available(iOS 10.0, *) private(set) lazy var center = UNUserNotificationCenter.current() 

Answer 2

I know this is an older question but I wanted to add an answer for people who come here via Google as I did.

As kgaidis and Cœur mentioned, you can use @available on computed properties. However, lazy variables are considered computed properties and so you can use @available on them too. This has the nice benefit of removing the boilerplate of the extra stored property and the forced casts - in fact, it leaves no evidence of the property in your pre-iOS 10 code.

You can simply declare it like this:

@available(iOS 10.0, *) private(set) lazy var center = UNUserNotificationCenter.current() 

Unfortunately there's no way to make it completely read-only but the private(set) at least makes it read-only outside of the class.