GCD with nested Parse Queries

Question

func getPosts(skip: Int){
    var query = PFQuery(className: self.parseClassName!)
    query.includeKey("posted_by")
    query.findObjectsInBackgroundWithBlock {
        (objects: [AnyObject]?, error: NSError?) -> Void in

        if error == nil && objects != nil {
            if let objects = objects as? [PFObject] {

                var requestGroup = dispatch_group_create()

                for post in objects
                {
                    dispatch_group_enter(requestGroup)
                    let queryKommentar1 = PFQuery(className:"Comment")
                    queryKommentar1.whereKey("posted_to", equalTo: post)
                    queryKommentar1.limit = 3
                    queryKommentar1.includeKey("written_by")
                    queryKommentar1.findObjectsInBackgroundWithBlock() {
                        (commentObjects: [AnyObject]?, error: NSError?) -> Void in

                      //Creating UITableViewCells from data and store in array 
                      dispatch_group_leave(requestGroup)

                    }   
                }

                println("Successfully retrieved \(objects.count) posts.")
                dispatch_group_notify(requestGroup, dispatch_get_main_queue()) {
                    println("All done")

                }  
            }
        }
    }
}

So, I'm not sure if I misunderstood dispatch groups, but my intention is to make two Parse Queries, targeting different classes, and create TableViewCells from the data provided. This works fine, but since I don't want the data to load when the user is scrolling the table, I want to preload the data and create the cells, and store them in an Array. Since I would like to remove any Activity Indication, and reload the table, by the time the fetch is complete I though (after som Googleing) that dispatch groups might be a good solution for this. However, "All done" is never printed in the console.

When I made a dispatch group around the outer query (entering the group just before the query, and leaving as the last line in the block) that worked fine. What am I doing wrong? Is it impossible to use this when nesting asynchronous calls?

FYI, I removed a lot of code, like creating the cells and using the data from Parse, since I would like to spare you from reading that mess.

Answer 1

This dispatch group pattern is basically right.

I would suggest logging some message at dispatch_group_enter and at dispatch_group_leave and see if it's getting called as you think it should and that every enter is offset by a leave .

If the number of occurrences of dispatch_group_leave are less than the number of calls to dispatch_group_enter , the dispatch_group_notify block will not get called.

Perhaps you have some path in that inner findObjectsInBackgroundWithBlock closure that is preventing it from hitting the dispatch_group_leave call.