Parse message “Warning: A long-running operation is being executed on the main thread” despite running in background thread

Question

I have a lengthy piece of code which downloads data from Parse. This is the shortened version:

func runQuery(query:PFQuery) {

    var backgroundQueue = NSOperationQueue()
    backgroundQueue.addOperationWithBlock(){

    query.findObjectsInBackgroundWithBlock {
        (objects: [AnyObject]?, error: NSError?) -> Void in
        if error == nil {

           // ... some stuff here...

           // the following line produces the parse warning. 
           // file is a PFFile object.

           if let imageData = file.getData() {
               // ...
           }

           // even more stuff here ...
     }
}

I know that PFFile.getData() blocks the running thread, but this is ok, because it runs in a NSOperationQueue. So from my thinking, I'm processing in the background. And it works, when I call runQuery(), it immediately returns and the UI works. But why on earth do I get this warning and how can I get rid of it? I don't want to change the PFFile.GetData() call to PFFile.getDataInBackgroundWithBlock(), because that would require significant code changes. And I see not need for that.

Answer 1

NSOperationQueue () will return the main operation queue, which is running on the main thread. Not on a background thread.

Answer 2

Meanwhile, I found the problem.

query.findObjectsInBackgroundWithBlock { ... }

puts the code inside the block back on the main queue even if you are executing it on a background thread! I don't know if that is intended.

Knowing that, it is easy to solve the problem: Bracket the time-consuming download code in a NSOperationQueue(), which runs on a background thread. Then you can use the synchronous parse methods, which makes the code a lot easier to write and read. All warnings are gone.