Set variable asynchronously and use within view controller

Question

Let's say I have the following asynchronous query:

var kittens: [PFObject]!

self.tempView.addGestureRecognizer(UITapGestureRecognizer(target: self, action: "pushToView"))

var query = PFQuery(className: "Kittens")
query.findObjectsInBackgroundWithBlock({ (objects, error) in
    if let kittenObjects = objects as? [PFObject] {
        self.kittens = kittenObjects
    }
})

I have a method that presents a view controller:

func pushToView() {
    let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
    let viewController: KittensViewController = mainStoryboard.instantiateViewControllerWithIdentifier("kittensViewController") as! KittensViewController
    viewController.kittens = self.kittens
    self.presentViewController(viewController, animated: true, completion: nil)
}

Because I'm setting self.kittens inside of an asynchronous query block it obviously doesn't work when I try to access self.kittens inside the presented view controller (it's always null).

What's the best method to get something like this working?

Answer 1

There are a couple of options. The main thing is that you need to update KittensViewController when the query completes.

• Make KittensViewController a property in the view controller that holds the kittens array. When the query completes, you not only update self.kittens, but also self.kittensViewController.kittens = kittenObjects. This is not best design, but it should work.

• Better design would be to have the query in an own controller class that fires a NSNotification when done. Then, you could listen anywhere on your code when new kittens arrive and update your views.

The Query throws the notification

    var query = PFQuery(className: "Kittens")
    query.findObjectsInBackgroundWithBlock({ (objects, error) in
        if let kittenObjects = objects as? [PFObject] {
             dispatch_async(dispatch_get_main_queue()) {
                 NSNotificationCenter.defaultCenter().postNotificationName("KittensQueryDidFinish", object:kittenObjects, userInfo:dataDict))
        }
    })

The ViewController registers for the notification and updates his model

In your viewDidLoad add the following:

NSNotificationCenter.defaultCenter().addObserverForName("KittensQueryDidFinish", object: nil, queue: NSOperationQueue.mainQueue) { note in
     let kittens = note.object
     [weak self].kittens = kittens
}

With that method, you get updates everywhere you need. Now you have to solve the problem when the kittens already have been loaded. Therefore, I'd suggest you introduce a shared controller object (aka singleton) that does the query and stores the kittens. Then you can access kittens from everywhere.