propertywrapper+combine.swift

 import Combine
 
 @propertyWrapper
 struct UserDefault<Value> {
     let key: String
     let defaultValue: Value
     var container: UserDefaults = .standard
     private let publisher = PassthroughSubject<Value, Never>()
     
     var wrappedValue: Value {
         get {
             return container.object(forKey: key) as? Value ?? defaultValue
         }
         set {
             // Check whether we're dealing with an optional and remove the object if the new value is nil.
             if let optional = newValue as? AnyOptional, optional.isNil {
                 container.removeObject(forKey: key)
             } else {
                 container.set(newValue, forKey: key)
             }
             publisher.send(newValue)
         }
     }

     var projectedValue: AnyPublisher<Value, Never> {
         return publisher.eraseToAnyPublisher()
     }
 } 

Usage

let subscription = UserDefaults.$username.sink { username in
     print("New username: \(username)")
 }
 UserDefaults.username = "Test"
 // Prints: New username: Test