RealmPlayground.swift

import Foundation
import RealmSwift

final class Task: Object {
    @Persisted var id: String = ""
    @Persisted var title: String = ""
    @Persisted var text: String = ""
    override static func primaryKey() -> String? { "id" }
}

final class Folder: Object {
    @Persisted var id: String = ""
    @Persisted var title: String = ""
    @Persisted var text: String = ""
    override static func primaryKey() -> String? { "id" }
}

/// make protocol with common properties
protocol EntityProperty: Object {
    var title: String { get }
    var text: String { get }
}

/// confirm protocol by similar realm objects
extension Task: EntityProperty {}
extension Folder: EntityProperty {}

final class EntityPropertyObserver {
    var tokens: [AnyObject] = []

    /// in-memory Realm to play in sandbox project
    init() {
        Realm.Configuration.defaultConfiguration = .init(inMemoryIdentifier: "org.mvp.RealmPlayground")
    }

    func observe<T: EntityProperty>(ofType: T.Type, forPrimaryKey: String, keyPath: PartialKeyPath<T>) {
        do {
            let realm = try Realm()
            let result = realm.object(ofType: ofType, forPrimaryKey: forPrimaryKey)!
            let token = result.observe(keyPaths: [keyPath], { changes in
                switch changes {
                case let .change(_, properties):
                    let keyPathName = _name(for: keyPath)
                    // TODO: combine or closure callback with changes
                    print(properties.first(where: { $0.name == keyPathName})?.newValue)
                default:
                    break
                }
            })
            tokens.append(token)
        } catch {
            fatalError("\(error)")
        }
    }
}

final class Playground {
    let observer = EntityPropertyObserver()

    init() {
        // make default objects
        let folder = Folder()
        folder.id = "folder_id"
        folder.title = "a folder title"
        folder.text = "a folder text"

        let task = Task()
        task.id = "task_id"
        task.title = "a task title"
        task.text = "a task text"

        do {
            let realm = try Realm()
            try realm.write {
                realm.add(folder)
                realm.add(task)
            }
        } catch {
            fatalError("\(error)")
        }

        observer.observe(ofType: type(of: folder), forPrimaryKey: folder.id, keyPath: \.title)
        observer.observe(ofType: type(of: task), forPrimaryKey: task.id, keyPath: \.text)

        do {
            let realm = try Realm()
            try realm.write {
                folder.title = "the title"
                folder.text = "new folder text"
                task.title = "yet another title"
                task.text = "new task text"
            }
        } catch {
            fatalError("\(error)")
        }
    }
}