Swift 3: Property observer for singleton

Question

I wonder if there's any way to observe singleton class for changes in ony of its property

In my case, with Realm I have something like this

class User: Object {
   dynamic var name:String = ""
   dynamic var email:String = ""
   dynamic var id:String = ""
   dynamic var picURL:String = ""
   dynamic var pic:Data = Data()

    static let currentUser = User()
    {
        didSet
        {
            try! realm.write {
                realm.add(currentUser, update: true)
            }
    }
  }
}

What I want to achieve is I want to have only one user object in my app, and anytime any of its properties gets modified, I would like to save it to Realm.

However, the example above shows error that

'let' declarations cannot be observing properties

How would you approach it? Do you think my idea with having Realm object singleton and updating it anytime some property changes is a good idea?

I wonder if it actually makes any sense to have a singleton object of class that is persisted by Realm.

Answer 1

It's not possible to have a blanket observer for a single Realm Object at the moment, however there's an issue tracking it in the Realm GitHub and I believe it's being prioritized at the moment.

Alcivanio is correct in that you'll need to manually trigger an update event to ensure the changes are persisted to Realm. That being said, because Realm controls the accessors in persisted objects, didSet isn't guaranteed to be called once an object has been saved to Realm. For simplicity reasons, I'd recommend using a closure to update the user object.

Updated Answer: If your singleton is a Realm Object, you can't modify its properties outside of a write transaction. Like I wrote below, the easiest thing to do would be to be to have an update closure that managed the Realm write transaction, and you simply update the properties of your Singleton object inside it.

As you've also seen, you can't use didSet with a property defined with let, so it might be best to drop back to Objective-C conventions and use an internal instance variable.

So all up, maybe consider doing this:

fileprivate var _current: User? = nil

class User: Object {
    dynamic var name:String = ""
    dynamic var email:String = ""
    dynamic var id:String = ""
    dynamic var picURL:String = ""
    dynamic var pic:Data = Data()

    static var current: User
    {
        if _current == nil {
            let realm = try! Realm()

            // Query for an existing user
            _current = realm.objects(User.self).first

            // Create a new user if it didn't exist in Realm
            if _current == nil {
                 _current = User()
                 try! realm.write { realm.add(_current) }
            }
        }
        return _current
    }

    public func update(_ block: (() -> Void)) {
        let realm = try! Realm()
        try! realm.write(block)
    }
}

let user = User.current
user.update {
    user.name = "DCDC"
}

Answer 2

Well, if you just have one user, you should put this observer in each variable, instead of in the let user. And there you can call a method for user update.

dynamic var name: String {
    didSet{
        User.updateOnRealm()//create this method
    }
}

To write the update method, you'll have to write something like this

class func updateOnRealm() {
    //update code here
}

Answer 3

Based on what you wrote :

What I want to achieve is I want to have only one user object in my app, and anytime any of its properties gets modified, I would like to save it to Realm.

I would suggest this solution:

1. Create a class User which does not use the Singleton pattern.

2. Create a class UserManager which does conform to the Singleton pattern

3. Create custom setter methods for A User. In these setters you can then change the users properties WITH or WITHOUT writing to the database. You can also check the use INPUT (set the name to lowercase etc.).

With this approach you can easily test your User class, extend it, subclass and reuse.

If you need code snippets comment below =).