Loading TableViewDataSource after a method finish - ios

Question

I want to call TableViewData Sources method for Seeting up Ui after it has been fethced from parse . With this i am able to fetch
func loadImages() {
var query = PFQuery(className: "TestClass")
query.orderByDescending("objectId")
query.findObjectsInBackgroundWithBlock ({(objects:[AnyObject]!, error: NSError!) in
if(error == nil){
self.getImageData(objects as [PFObject])
}
else{
println("Error in retrieving \(error)")
}
})//findObjectsInBackgroundWithblock - end
}
func getImageData(objects: [PFObject]) {
for object in objects {
let thumbNail = object["image"] as PFFile
println(thumbNail)
thumbNail.getDataInBackgroundWithBlock({
(imageData: NSData!, error: NSError!) -> Void in
if (error == nil) {
var imageDic = NSMutableArray()
self.image1 = UIImage(data:imageData)
//image object implementation
self.imageResources.append(self.image1!)
println(self.image1)
println(self.imageResources.count)
}
}, progressBlock: {(percentDone: CInt )-> Void in
})//getDataInBackgroundWithBlock - end
}//for - end
self.tableView.reloadData()
But not able to populate these fetched data to tableview like this
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
println("in table view")
println(self.imageResources.count)
return imageResources.count+1;
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell:CustomTableViewCell = tableView.dequeueReusableCellWithIdentifier("customCell") as CustomTableViewCell
var (title, image) = items[indexPath.row]
cell.loadItem(title: title, image: image)
println("message : going upto this line")
println(self.imageResources.count)
var (image1) = imageResources[indexPath.row]
cell.loadItem1(image1: image1)
return cell
}
Then on loaditem i am trying to show up the images and i have writen my own array to populate to the image array but i am geeting a zero value when populating so not able to set it up
Any Help is much appreciated!

Answer 1

You have several problems, all related to concurrency - your load is occurring in the background and in parallel.
The first problem is the use of self.image1 as a temporary variable in the loading process - this variable may be accessed concurrently by multiple threads. You should use a local variable for this purpose.
Second, you are appending to self.imageResources from multiple threads, but Swift arrays are not thread safe.
Third, you need to call reload on your tableview after you have finished loading all of the data, which isn't happening now because you call it while the background operations are still taking place.
Finally, your getImageData function is executing on a background queue, and you must perform UI operations (such as reloading a table) on the main queue.
The simplest option is to change get thumbnail loading to synchronous calls - This means that your thumbnails will load sequentially and may take a bit longer that the multiple parallel tasks but it is easier to manage -
func getImageData(objects: [PFObject]) {
for object in objects {
let thumbNail = object["image"] as PFFile
println(thumbNail)
let imageData? = thumbNail.getData
if (imageData != nil) {
let image1 = UIImage(data:imageData!)
//image object implementation
self.imageResources.append(image1!)
println(self.imageResources.count)
}
}//for - end
dispatch_async(dispatch_get_main_queue(), {
self.tableView.reloadData()
})
}
A more sophisticated approach would be to use a dispatch group and keep the parallel image loading. In order to do this you would need to guard the access to the shared array