Swift - How to retrieve multiple images at once (GCD)? - ios

Question

Let me give you some insight on my application itself.
To put it in short, I am creating a social-networking app. Each post consists of an image, profile picture, and caption. Each post exists in my MySQL database. I am using my own framework to retrieve each post. However, once I retrieve each post I still have to retrieve the profile picture and image using the URLs which I retrieved from the database. I would like to retrieve all images at once rather than running in sequential order.
As of now, there are about 5 posts in the database. Loading the necessary images for one post takes about 4 seconds. So right now I am loading the images for one post then retrieving the next in sequential order. So this whole process takes around 20 seconds. So say have 50 posts then it will take an extremely long time to load all the posts. I have some knowledge of GCD (grand-dispatch-queues) however I don't know how to implement it in my app.
Here is my code for retrieving my posts and images:
ConnectionManager.sharedInstance.retrievePosts(UserInformationInstance.SCHOOL) {
(result: AnyObject) in
if let posts = result as? [[String: AnyObject]] {
print("Retrieved \(posts.count) posts.")
for post in posts {
let postIDCurrent = post["id"] as? Int
var UPVOTES = 0;
var UPVOTED: Bool!
var query = ""
if let profilePictureCurrent = post["profile_picture"] {
// Loading profile picture image
let url = NSURL(string: profilePictureCurrent as! String)
let data = NSData(contentsOfURL: url!)
let image = UIImage(data: data!)
UserInformationInstance.postsProfilePictures.append(image!)
print("added profile pic")
} else {
print("error")
}
if let postPictureCurrent = post["image"] {
if (postPictureCurrent as! String != "") {
// Loading image associated with post
let url = NSURL(string: postPictureCurrent as! String)
let data = NSData(contentsOfURL: url!)
let image = UIImage(data: data!)
let imageArray: [AnyObject] = [postIDCurrent!, image!]
UserInformationInstance.postsImages.append(imageArray)
print("added image pic")
}
} else {
print("error")
}
UserInformationInstance.POSTS.append(post)
}
} else {
self.loadSearchUsers()
}
}
So my question is, how can I retrieve all the images at the same time instead of retrieving one after the other?
It would be great if someone could give an explanation as well as some code :)

Answer 1

I would recommend to revise your approach. If your server is fine - it's not busy and well reachable, so that resources downloading is limited by device network adapter bandwidth (X mbps), then it does not matter how you downloading images - concurrently or sequently.
Let me show this. Downloading time of 10 files with size Y mb simultaneously is equal to downloading time of one file, but in this case the downloading speed will be 10 times slower per file:
X/10 - downloading speed per one file
Time = Amount / Speed
T = Y / (X/10) = 10 * Y / X
Now if your are downloading sequently:
T = 10 * (Y / X) = 10 * Y / X
I would recommend to show posts immediately once you retrived them from the storage, then you need to start image downloading asynchronously and set image once that's downloaded. That's the best practice in the industry, consider Facebook, Twitter, Instagram apps.