Error acessing list of Realm's objects: Realm accessed from incorrect thread

Question

I have a list of points of interest. This points were loaded from a Realm database. Each point should present its distance to the user's position.

Each time I get a new location, I calculate the distance to all points. To avoid a frozen screen, I was doing the math in a background thread, after i display the list in a table in the main thread.

func updatedLocation(currentLocation: CLLocation) {

    let qualityOfServiceClass = QOS_CLASS_BACKGROUND
    let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
    dispatch_async(backgroundQueue, {
        for point in self.points{
            let stringDistance = self.distanceToPoint(currentLocation, destination: point.coordinate)
            point.stringDistance = stringDistance
        }

        dispatch_async(dispatch_get_main_queue(), { () -> Void in
            self.tableView?.reloadData()
        })
    })
}

However I get this error:

libc++abi.dylib: terminating with uncaught exception of type realm::IncorrectThreadException: Realm accessed from incorrect thread.

I know i am getting this error because I'm accessing the realm objects in a background thread, however, they are already loaded into an array and I never make a new query to the database. In addition, the var i'm updating his not saved into the database.

Any idea how to solve this? I wanted to avoid doing the math in the main thread.

thanks in advance

Answer 1

I assume you wrap Realm Results objects into Array like the following:

let results = realm.objects(Point)
self.points = Array(results)

However, that is not enough. Because each element in the array is still tied with Realm, that cannot be access another thread.

A recommended way is re-create Realm and re-fetch the Results each threads.

dispatch_async(backgroundQueue, {
    let realm = try! Realm()
    let points = realm.objects(...)

    try! realm.write {
        for point in points{
            let stringDistance = self.distanceToPoint(currentLocation, destination: point.coordinate)
            point.stringDistance = stringDistance
        }
    }

    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        ...
    })
})

Realm objects have live-update feature. When committed changed to Realm objects on sub-thread, those changes reflect to the objects in other thread immediately. So you do not need to re-fetch the query in the main thread. What you should do is just reload the table view.

If you'd like to wrap array and pass it to other thread directly, you should wrap all elements of resutls as follows:

let results = realm.objects(Point)
self.points = results.map { (point) -> Point in
    return Point(value: point)
}