Iterating For In Loop In Swift Using Parse Server - ios

Question

I have a chat filled with users and of course a username array. I want to get the profile picture associated with the username in order for each user in the username array. Parse, however, can only sort by ascending/descending order that I am aware of.
Therefore, I need to figure out how to sort the data once received.
I am ultimately appending a url to be used as the pic.
func getPics(_ completionHandler: #escaping () -> Void) {
let query = PFQuery(className: "_User")
var dictionary: [String : Int] = [:]
var unit = 0
for username in usernameArray {
unit += 1
dictionary[username] = unit
}
query.findObjectsInBackground(block: { (objects: [PFObject]?, error: Error?) in
if let objects = objects {
for object in objects {
if error == nil {
for user in self.usernameArray {
let pfuser = object["username"] as! String
if pfuser == user {
let imageFile = object["profilePic"] as? PFFileObject
let imageFileString = imageFile?.url as! String
if let url = URL(string: imageFileString) {
let replacedImageUrlString = imageFileString.replacingOccurrences(of: "[removed for privacy]", with: "removed for privacy")
let url = replacedImageUrlString as NSString
self.urlArray.append(url)
}
}
}
}
}
completionHandler()
}
})
}

Answer 1

I am not aware of Parse server, So I dont really know if there exists provision to get the response in specific order, if it exists that should be the optimal solution. But there is a generic issue with your solution, its the time complexity.
You have two nested for loops which makes its worst case complexity to be O(n^2), I guess the least you can do is to reduce its complexity to O(n)
func getPics(_ completionHandler: #escaping () -> Void) {
let query = PFQuery(className: "_User")
var dictionary: [String : Int] = [:]
var unit = 0
for username in usernameArray {
unit += 1
dictionary[username] = unit
}
query.findObjectsInBackground(block: { (objects: [PFObject]?, error: Error?) in
if let objects = objects, error == nil {
let objectsDict = Dictionary(grouping: objects, by: { $0["username"] as! String /* typically you should be accessing $0.username, but again am not aware of PFObject */})
for user in self.usernameArray {
if let pfuser = objectsDict[user]?[safe: 0] as? PFObject {
let imageFile = pfuser["profilePic"] as? PFFileObject
let imageFileString = imageFile?.url as! String
if let url = URL(string: imageFileString) {
let replacedImageUrlString = imageFileString.replacingOccurrences(of: "[removed for privacy]", with: "removed for privacy")
let url = replacedImageUrlString as NSString
self.urlArray.append(url)
}
}
}
completionHandler()
}
})
}
Once you get the array of PFObject, you can create a dictionary with username as the key and PFObject as value, once you have the dictionary you can get the PFObject for specific username in O(1), so you can run a single for loop which reduces your code's complexity to O(n)
P.S If you are wondering what [safe: 0] is you can add this handy extension to safely access object at specific index in an array
link: Safe (bounds-checked) array lookup in Swift, through optional bindings?
extension Collection {
subscript (safe index: Index) -> Element? {
return indices.contains(index) ? self[index] : nil
}
}
P.P.S: My answer is completely ignoring the complexity of Dictionary(grouping: API itself, I tried to look for the info, but couldnt find. But I think its O(n) not really sure though, whatever it is if its not O(n^2) you will still be benefited