RealmのIDを自動採番（オートインクリメント）する方法 ref: http://qiita.com/syou007/items/267612c402fdd28de7a8

file0.swift

class RObject: Object {
    // ID
    dynamic var id = 0

    // データを保存。
    func save() {
        let realm = try! Realm()
        if realm.isInWriteTransaction {
            if self.id == 0 { self.id = self.createNewId() }
            realm.add(self, update: true)
        } else {
            try! realm.write {
                if self.id == 0 { self.id = self.createNewId() }
                realm.add(self, update: true)
            }
        }
    }

    // 新しいIDを採番します。
    private func createNewId() -> Int {
        let realm = try! Realm()
        return (realm.objects(type(of: self).self).sorted(byKeyPath: "id", ascending: false).first?.id ?? 0) + 1
    }

    // プライマリーキーの設定
    override static func primaryKey() -> String? {
        return "id"
    }
}

file1.swift

class User: RObject {
    // 名前
    dynamic var name = ""
}

file2.swift

let user = User()
user.name = "hoge"
user.save()

file3.swift

let realm = try! Realm()
let user = realm.objects(User.self).first ?? User()
try! realm.write {
    user.name = "hoge"
    user.save()
}