class User: Object {

    dynamic var name: String?
    dynamic var age: Int = 0
    dynamic var gender: String?
    dynamic var groups: Set<String> = []
    dynamic var items: [String] = []
    dynamic var url: URL?
    dynamic var birth: Date?
    dynamic var thumbnail: File?
    dynamic var followers: Relation<User> = []
}

// Group
class Group: Object {
    dynamic var name: String?
    dynamic var users: Set<String> = []
}

let group: Group = Group()
group.name = "iOS Development Team"
group.save { (error, ref) in

    do {
        let user: User = User()
        user.name = "john appleseed"
        user.gender = "man"
        user.age = 22
        user.items = ["Book", "Pen"]
        user.groups.insert(ref.key)
        user.save({ (error, ref) in
            group.users.insert(ref.key) // It is updated automatically
        })
    }

    do {
        let user: User = User()
        user.name = "Marilyn Monroe"
        user.gender = "woman"
        user.age = 34
        user.items = ["Rip"]
        user.groups.insert(ref.key)
        user.save({ (error, ref) in
            group.users.insert(ref.key) // It is updated automatically
        })
    }

}

User.observeSingle(FIRDataEventType.Value) { (users) in
    users.forEach({ (user) in
        // do samething
        if let groupId: String = user.groups.first {
            Group.observeSingle(groupId, eventType: .Value, block: { (group) in
                // do samething
            })
        }
    })
}

if let groupId: String = user.groups.first {
    Group.observeSingle(groupId, eventType: .Value, block: { (group) in
        group.remove()
    })
}

class User: Salada.Object {

    override class var _version: String {
        return "v1"
    }

    dynamic var name: String?
    dynamic var age: Int = 0
    dynamic var gender: String?
    dynamic var groups: Set<String> = []
    dynamic var items: [String] = []
    dynamic var location: CLLocation?
    dynamic var url: URL?
    dynamic var birth: Date?
    dynamic var thumbnail: File?
    dynamic var cover: File?
    dynamic var type: UserType = .first

    var tempName: String?

    override var ignore: [String] {
        return ["tempName"]
    }

    override func encode(_ key: String, value: Any?) -> Any? {
        if key == "location" {
            if let location = self.location {
                return ["latitude": location.coordinate.latitude, "longitude": location.coordinate.longitude]
            }
        } else if key == "type" {
            return self.type.rawValue as AnyObject?
        }
        return nil
    }

    override func decode(_ key: String, value: Any?) -> Any? {
        if key == "location" {
            if let location: [String: Double] = value as? [String: Double] {
                self.location = CLLocation(latitude: location["latitude"]!, longitude: location["longitude"]!)
                return self.location
            }
        } else if key == "type" {
            if let type: Int = value as? Int {
                self.type = UserType(rawValue: type)!
                return self.type
            }
        }
        return nil
    }
}

let user: User = User()
let image: UIImage = UIImage(named: "Salada")!
let data: NSData = UIImagePNGRepresentation(image)!
let thumbnail: File = File(data: data, mimeType: .jpeg)
user.thumbnail = thumbnail
user.save({ (error, ref) in
    // do something
})

let image: UIImage = #imageLiteral(resourceName: "salada")
let data: Data = UIImageJPEGRepresentation(image, 1)!
let file: File = File(data: data, mimeType: .jpeg)
item.file = file
let task: FIRStorageUploadTask = item.file?.save(completion: { (metadata, error) in
    if let error = error {
        print(error)
        return
    }
})

guard let user: User = self.datasource?.objectAtIndex(indexPath.item) else { return }
user.thumbnail?.dataWithMaxSize(1 * 2000 * 2000, completion: { (data, error) in
    if let error: NSError = error {
        print(error)
        return
    }
    cell.imageView?.image = UIImage(data: data!)
    cell.setNeedsLayout()
})

User.observeSingle(friend, eventType: .value, block: { (user) in
    if let user: User = user as? User {
        if let ref: FIRStorageReference = user.thumbnail?.ref {
            cell.imageView.sd_setImage(with: ref, placeholderImage: #imageLiteral(resourceName: "account_placeholder"))
        }
    }
 })

class Follower: Relation<User> {
    override class var _name: String {
        return "follower"
    }
}

class User: Object {
    let followers: Follower = []
}

// ViewController Sample

var dataSource: DataSource<User>?

override func viewDidLoad() {
    super.viewDidLoad()
    
    let options: Options = Options()
    options.limit = 10
    options.sortDescirptors = [NSSortDescriptor(key: "age", ascending: false)]
    self.dataSource = DataSource(reference: User.databaseRef, options: options, block: { [weak self](changes) in
        guard let tableView: UITableView = self?.tableView else { return }
        
        switch changes {
        case .initial:
            tableView.reloadData()
        case .update(let deletions, let insertions, let modifications):
            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):
            print(error)
        }
    })
}

// TableViewDatasource
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return self.dataSource?.count ?? 0
}

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell: TableViewCell = tableView.dequeueReusableCell(withIdentifier: "TableViewCell", for: indexPath) as! TableViewCell
    configure(cell, atIndexPath: indexPath)
    return cell
}

func configure(_ cell: TableViewCell, atIndexPath indexPath: IndexPath) {
    cell.disposer = self.dataSource?.observeObject(at: indexPath.item, block: { (user) in
        cell.imageView?.contentMode = .scaleAspectFill
        cell.textLabel?.text = user?.name
    })
}

private func tableView(_ tableView: UITableView, didEndDisplaying cell: TableViewCell, forRowAt indexPath: IndexPath) {
    cell.disposer?.dispose()
}

func tableView(_ tableView: UITableView, canPerformAction action: Selector, forRowAt indexPath: IndexPath, withSender sender: Any?) -> Bool {
    return true
}

func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {
    if editingStyle == .delete {
        self.dataSource?.removeObject(at: indexPath.item, cascade: true, block: { (key, error) in
            if let error: Error = error {
                print(error)
            }
        })
    }
}

class ViewController: UIViewController {
    private var disposer: Disposer<User>?

    override func viewDidLoad() {
        super.viewDidLoad()
        disposer = User.observe(userID, eventType: .value) { user in
             //...
        }
    }

    deinit {
        // ... auto remove observe internal disposer when it deinitialized.
        // or manually and clearly dispose
        // disposer?.dispose()
    }
}