Display paginated results while observing changes in Realm Swift - swift

Question

I have a typical messaging app where messages are stored as Realm objects. I want to display messages of a conversation in a collection/table view in a safe way observing the results
let results = realm.objects(Message.self).filter(predicate)
// Observe Results Notifications
notificationToken = results.observe { [weak self] (changes: RealmCollectionChange) in
guard let tableView = self?.tableView else { return }
switch changes {
case .initial:
// Results are now populated and can be accessed without blocking the UI
tableView.reloadData()
case .update(_, let deletions, let insertions, let modifications):
// Query results have changed, so apply them to the UITableView
tableView.beginUpdates()
tableView.insertRows(at: insertions.map({ IndexPath(row: $0, section: 0) }),
with: .automatic)
tableView.deleteRows(at: deletions.map({ IndexPath(row: $0, section: 0)}),
with: .automatic)
tableView.reloadRows(at: modifications.map({ IndexPath(row: $0, section: 0) }),
with: .automatic)
tableView.endUpdates()
case .error(let error):
// An error occurred while opening the Realm file on the background worker thread
fatalError("\(error)")
}
}
Now, this would work assuming that I display all messages. Since they can be a lot I need to load paginated.
How can I track changes then?
I'm searching for a method to get a sort of id of the changed message, but I couldn't find anything.

Answer 1

Realm objects are lazily loaded so they don't 'take up space' until they are accessed. In our case we have have 1000 objects in results but only display 10 at a time. Those are the 10 that 'take up space'. So it may not be an issue to have a large results dataset.
When you populate a results object from Realm, each object has an index. Think of a results as an array. The first object is index 0, the second object in index 1 etc.
When an object is modified in realm, that information is passed to your observer to which you can then update your UI.
Say we have a PersonClass Realm object that has a persons name and email
PersonClass: Object {
#objc dynamic var name = ""
#objc dynamic var email = ""
}
and we want to display a list of people, and if an email address changes we want to update the UI with that change.
When the app starts, we load all of the people into a Results class var.
override func viewDidLoad() {
super.viewDidLoad()
self.personResults = realm.objects(PersonClass.self)
Then we add an observer to those results so the app is notified of changes. .initial will run when the results have been loaded so it's a good place to populate your dataSource and refresh your tableView.
func doObserve() {
self.notificationToken = self.personResults!.observe { (changes: RealmCollectionChange) in
switch changes {
case .initial: // initial object load complete
if let results = self.personResults {
for p in results {
print(p.name, p.email) //populate datasource, refresh
}
}
break
case .update(_, let deletions, let insertions, let modifications):
for index in deletions {
print(" deleted object at index: \(index)")
}
for index in insertions {
print(" inserted object at index: \(index)")
}
for index in modifications {
print(" modified object at index: \(index)")
let person = self.personResults![index]
print("modified item: \(person.name) \(person.email)")
}
break
case .error(let error):
fatalError("\(error)")
break
}
}
}
In this example, when an person stored at index #2 changes his email, the observer responds to that and prints the name and the new email to console.
But...
Realm is live updating and if you refresh your tableView or even just that row so the cell re-loads from the object, the UI will be updated. I don't know what 'How can I track changes then?' means in your use case, but you could actually remove all of the for loops and just have a tableView.reloadData in the .update section and the UI will be updated as data changes. Or, you could use the index and just update that row.
Keep in mind Realm objects in Results are live and always stay fresh as data changes.
Another note is that many Realm objects will have a unique identifier for the object, defined like this
class PersonClass: Object {
#objc dynamic var person_id = UUID().uuidString
override static func primaryKey() -> String? {
return "person_id"
}
}
which can be used to access that specific object within Realm, but not directly related to your question.