Having issue with asynchronous processing using Parse

Question

I have a Parse database which has records consisting (among other items) of PFFile items of thumbnail images. The database is read-only and was created successfully using another program. I confirmed that all thumbnails are in Parse. When I try to retrieve the thumbnails using the function shown below, I get the notification before all the images are processed resulting on occasional failures based on the timing of the post-notification processing. How can I ensure that all records are processed?

    func convertPFilesToImages () {
    println("convertPFilesToImages")
    let notification = NSNotification(name: "imagesLoaded", object: self)
    for i in 0 ..< records.count {
        let userImageFile = records[i].icon
        records[i].image = UIImage()
        println("name: \(self.records[i].name) image: \(self.records[i].image)")
        userImageFile.getDataInBackgroundWithBlock {
            (imageData: NSData?, error: NSError?) -> Void in
            if error == nil {
                //println("no error")
                if let imageData = imageData {
                    let image = UIImage(data:imageData)!

                    self.records[i].image = image
                    // println below shows that not all images are converted
                    println("name: \(self.records[i].name) image: \(self.records[i].image)")
                }
            } else {
                println("ParseProcessing-convertToImage: error = \(error)")
            }
            if (i == self.records.count-1) {
                println("All images processed")
                // loop is done before all images are processed
                NSNotificationCenter.defaultCenter().postNotification(notification)
            }

        }
    }

}

Shai · Accepted Answer · 2015-06-14 11:33:57Z

2

Your for() loop executes async code, your problem is with getDataInBackground.

when i == self.records.count-1 it doesn't guarantee all calls have finished up.

getDataInBackround should be as converted to getData (or however Parse named the SYNCHRONOUS call), and your whole method should be called in a background thread.

You could also use a NSOperationQueue to load everything simultaneously, and then call [queue waitUntilFinished].

answered Jun 14, 2015 at 11:33

Shai

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1 Comment

Syed Tariq Over a year ago

Thanks for the prompt response. I incorrectly assumed that getDataInBackgroundWithBlock helps me. I will try an approach using the synchronous calls inside a background thread.

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