How to use KVO for UserDefaults in Swift?

Question

I'm rewriting parts of an app, and found this code: ``` fileprivate let defaults = UserDefaults.standard func storeValue(_ value: AnyObject, forKey key:String) { defaults.set(value, forKey: key) defaults.synchronize() NotificationCenter.default.post(name: Notification.Name(rawValue: "persistanceServiceValueChangedNotification"), object: key) } func getValueForKey(_ key:String, defaultValue:AnyObject? = nil) -> AnyObject? { return defaults.object(forKey: key) as AnyObject? ?? defaultValue } ``` When CMD-clicking the line `defaults.synchronize()` I see that `synchronize` is planned deprecated. This is written in the code: ``` /*! -synchronize is deprecated and will be marked with the NS_DEPRECATED macro in a future release. -synchronize blocks the calling thread until all in-progress set operations have completed. This is no longer necessary. Replacements for previous uses of -synchronize depend on what the intent of calling synchronize was. If you synchronized... - ...before reading in…

Accepted Answer

As of iOS 11 + Swift 4, the recommended way (according to SwiftLint) is using the block-based KVO API.

Example:

Let's say I have an integer value stored in my user defaults and it's called `greetingsCount`.

First I need to extend `UserDefaults` with a `dynamic var` that has the same name as the user defaults key you want to observe:

```
extension UserDefaults {
    @objc dynamic var greetingsCount: Int {
        return integer(forKey: "greetingsCount")
    }
}
```

This allows us to later on define the key path for observing, like this:

```
var observer: NSKeyValueObservation?

init() {
    observer = UserDefaults.standard.observe(\.greetingsCount, options: [.initial, .new], changeHandler: { (defaults, change) in
        // your change logic here
    })
}
```

And never forget to clean up:

```
deinit {
    observer?.invalidate()
}
```

How to use KVO for UserDefaults in Swift?

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