Retrieve multiple images from Parse with getdatainbackground (IOS - Swift) - ios

Question

I try to do a query with Parse containing a few Strings and Images that will be added to an array. The strings in the array are all in the right order but not the images. I think its probably because some images are smaller than the other ones and so they get appended to the array earlier than they are supposed to. Is there any way to "save" space in the array for the images to keep them in the right order? It's probably not that hard to solve that but I am a Newbie :( Thank you!
query.findObjectsInBackground (block: { (objects:[PFObject]?, error: Error?) -> Void in
for object in objects! {
DispatchQueue.global(qos: .userInteractive).async {
// Async background process
if let imageFile : PFFile = self.bild.append(object.value(forKey: "Bild") as! PFFile) {
imageFile.getDataInBackground(block: { (data, error) in
if error == nil {
DispatchQueue.main.async {
// Async main thread
let image = UIImage(data: data!)
image2.append(image!)
}
} else {
print(error!.localizedDescription)
}
})
}
}
}
})

Answer 1

Your analysis is correct that the requests will complete in non-deterministic order, partly or mostly influenced by the amount of data that must be returned.
Instead of an array to which you append the UIImage (or data), use a mutable dictionary that maps strings to UIImage. A reasonable choice for the string key is the PFFile name.
EDIT I'm not a Swift writer, but I tried to express the idea below (don't depend on it compiling, but I think the idea is sound)
class MyClass {
var objects: [PFObject] = []
var images: [String: UIImage] = [:] // we'll map names to images
fetchObjects() {
// form the query
query.findObjectsInBackground (block: { (objects:[PFObject]?, error: Error?) -> Void in
self.objects = objects
self.fetchImages()
})
}
fetchImages() {
for object in self.objects! {
if let imageFile : PFFile = object["Bild"] as PFFile {
self.fetchImage(imageFile);
}
}
}
fetchImage(imageFile: PFFile) {
imageFile.getDataInBackground(block: { (data, error) in
if error == nil {
self.images[imageFile.name] = UIImage(data: data!)
// we can do more here: update the UI that with image that has arrived
// determine if we're done by comparing the count of images to the count of objects
} else {
// handle error
}
}
}
}
This will get the images in the background and keep them associated with their filenames using a dictionary. The OP code didn't explain what self.bild is, but I assumed it was an instance array of retrieved PFFiles. I replaced this with the images instance var.
Image file order is maintained by the object collection: to get the Nth image, get the Nth object, get it's "Bild" property, that PFFile's name is the key into your images dictionary.
var n = // some index into objects
var object : PFObject = self.objects[n]
var file : PFFile = object["Bild"]
var name : String = file.name
var nthImage = self.images[name] // is nil before fetch is complete