ObservableUserDefaults

Original: https://stackoverflow.com/a/54889233/5227717

These extensions make UserDefaults observable and easy to use.

What's wrong?

See documentation of UserDefaults.didChangeNotification

Summary

Posted when user defaults are changed within the current process.

Declaration

public class let didChangeNotification: NSNotification.Name

Discussion

This notification is posted on the thread that changes the user defaults. The notification object is the UserDefaults object. The notification doesn't contain a userInfo dictionary. This notification isn't posted when changes are made outside the current process, or when ubiquitous defaults change. You can use key-value observing to register observers for specific keys of interest in order to be notified of all updates, regardless of whether changes are made within or outside the current process.

Example & Usage

Define Keys

// IMPORTANT: DON'T use DOT `.` in key.
// DOT `.` used to define `KeyPath` and this is what we don't need here.
extension UserDefaults.Key {
    static let AppleMomentumScrollSupported  : UserDefaults.Key  = "AppleMomentumScrollSupported"     // Bool
    static let enableNetworkDiscovery        : UserDefaults.Key  = "defaults:enableNetworkDiscovery"  // Bool
    static let usesDetailedToolTips          : UserDefaults.Key  = "defaults:usesDetailedToolTips"    // Bool
    static let screenshotSavingPath          : UserDefaults.Key  = "defaults:screenshotSavingPath"    // String
}

Register Initial Values

// From Source
var initialValues: [UserDefaults.Key: Any?] = [
    .enableNetworkDiscovery  : true,
    .usesDetailedToolTips    : false,
    .screenshotSavingPath    : FileManager.default
        .urls(for: .picturesDirectory, in: .userDomainMask)
        .first?.appendingPathComponent("JSTColorPicker").path,
    .pixelMatchAAColor       : NSColor.systemYellow,
    .pixelMatchDiffColor     : NSColor.systemRed,
]

// From Property List
(try?
    PropertyListSerialization.propertyList(
        from: Data(contentsOf: Bundle.main.url(forResource: "InitialValues", withExtension: "plist")!),
        options: [],
        format: nil
    )
    as? [String : Any?])?.forEach({ initialValues[UserDefaults.Key(rawValue: $0.key)] = $0.value })

UserDefaults.standard.register(defaults: initialValues)

Access Direct Values

let discoveryEnabled: Bool = UserDefaults.standard[.enableNetworkDiscovery]

// Access Optional Values
let savingPath: String? = UserDefaults.standard[.screenshotSavingPath]
guard let savingPath: String = UserDefaults.standard[.screenshotSavingPath] else { return }

Modify Direct Values

@IBAction func discoveryMenuItemTapped(_ sender: NSMenuItem) {
    let enabled = sender.state == .on
    sender.state = !enabled ? .on : .off
    UserDefaults.standard[.enableNetworkDiscovery] = !enabled
}

Observe Changes of Single Value

// You must keep a reference to this object.
let observable = UserDefaults.standard.observe(key: .AppleMomentumScrollSupported) { (defaults, defaultKey, defaultValue: Bool) in
    let momentumScrollSupported = defaultValue
    // do something
}

Observe Changes of Multiple Values

class MyClass {
    private var observables           : [Observable]?
    private var usesDetailedToolTips  : Bool = false

    private func setupHandlers() {
        observables = UserDefaults.standard.observe(
            keys: [.AppleMomentumScrollSupported, .usesDetailedToolTips], 
            callback: { [weak self] in self?.applyDefaults($0, $1, $2) /* You must use weak self here! */ }
        )
    }

    private func applyDefaults(_ defaults: UserDefaults, _ defaultKey: UserDefaults.Key, _ defaultValue: Any) {
        if defaultKey == .usesDetailedToolTips, let toValue = defaultValue as? Bool {
            if usesDetailedToolTips != toValue {
                usesDetailedToolTips = toValue
                // do something
            }
        }
    }
}