How to deal with thread and Realm ? (iOS)

Question

I use Realm to store my model objects. In my object I have a function which generate NSData from its own properties values. This generation can be long, So I would like to generate my NSData in a thread with handler block.

My problem is that Realm data access is only possible on the Realm creation entity (actually the main thread). So when I access to my RealmObject properties in a thread, application crash. According to Realm specs, it's normal. But what is the best solution to make my NSData generation in a thread depending to Realm limitation ?

Actually I have two ideas :

• make a Realm specific dispatch queue and make all my Realm access write in this queue
• get all properties in need in a temp struct(or a set of variables) and work with this struct/variables to generate my NSData in a thread.

I assume that a lot of Realm users need to deal with threads and Realm, so what did you do in this kind of case ?

Answer 1

Pass the object id to the code that is running in the separate thread. Within that thread create a Realm instance (let realm = try! Realm()) and retrieve your object. Then you can do your long generation and return the result with a callback.

let objectId = "something"
dispatch_async(queue) {
  let realm = try! Realm()
  let myObject = realm.objectForPrimaryKey(MyObject.self, key: objectId)
  let result = myObject.longOperation()

  // call back with results
}

or

let objectRef = ThreadSafeReference(to: myObject)
DispatchQueue(label: "background").async {

   autoreleasepool {
        let realm = try! Realm()
        guard let myObject = realm.resolve(objectRef) else {
            return // object was deleted
        }

        let result = myObject.longOperation()
        // call back with results
   }
}