iOS Keep url array order when saving images into an array - ios

Question

I'm trying to do the following.
I have an array that contains several URLs for different images. These images will be displayed into a scrollview and I want to keep this order to be able to delete specific images at a specific index.
Right now I'm using a for loop, first I load the array with NSNull objects and afterward I run the loop replacing all the objects with images. This doesn't seem to be really efficient, specially when the app loads from the background.
This is my code:
func downloadImagesfromUrlArray(){
for var i=0;i < graphsURLlist.count; i++ {
if let data = NSData(contentsOfURL: graphsURLlist[i]){
let image = UIImage(data: data)
pageImages.replaceObjectAtIndex(i, withObject: image!)
//self.scaleImagesandScrollView()
}else{
NSLog("%#", i)
var alert = myAlertController(title: nil, message: "Error trying to update the charts. Try again later \(i)",
preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.Default, handler: nil))
self.presentViewController(alert, animated: true, completion: nil)
}
//Enter queue for every image to download
}
dispatch_group_leave(self.group)
}
Is there a better approach to do this?
Thanks

Answer 1

You better want to download images asynchronously and cache them. Before the images are downloaded it's better to display spinner, and then replace it with an image.
Here are a few related methods from my latest app:
func downloadAndCacheImg(url: String?, cell: AppCell, tableView: UITableView, indexPath: NSIndexPath) {
if url == nil { return }
let charactersToRemove = NSCharacterSet.alphanumericCharacterSet().invertedSet
let cacheFilename = "".join(url!.componentsSeparatedByCharactersInSet(charactersToRemove))
if let fullFilename = prefixFilenameWithDirectory(cacheFilename), img = UIImage(named: fullFilename) {
cell.setAppImage(img)
return
}
if let nsUrl = NSURL(string: url!) {
NSURLConnection.sendAsynchronousRequest(NSURLRequest(URL: nsUrl), queue: NSOperationQueue.mainQueue(), completionHandler: { (response, data, error) -> Void in
if let data = data, img = UIImage(data: data) {
if let fullFilename = self.prefixFilenameWithDirectory(cacheFilename) {
data.writeToFile(fullFilename, atomically: true)
}
if let cell = tableView.cellForRowAtIndexPath(indexPath) as? AppCell {
cell.setAppImage(img)
}
}
})
}
}
func prefixFilenameWithDirectory(filename: String) -> String? {
var dir = NSFileManager.defaultManager().URLForDirectory(.CachesDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: true, error: nil)
dir = dir?.URLByAppendingPathComponent(filename)
return dir?.path
}