Dispatch for loading UIImages producing unexpected results - ios

Question

I am building an app that has several images(Plans) that are loaded and setup on a scrollview with a page content. I am trying to load the first image on the main thread and then load the surrounding images from it's index on a background thread. All the UIImageViews are in an array (planImages) to reference when I need to set their images with the loaded data.
The problem is the images aren't always updating with the recently loaded images. This is my first time using Dispatch, so is there something I am doing wrong? Here is the section of code that is giving me the issue.
func setupPagesInScrollView(firstImage: Int)
{
let numberOfPages: CGFloat = CGFloat(selectedDetails!.numberOfPlans)
scrollView!.contentSize = CGSize(width: scrollView!.frame.width * numberOfPages, height: scrollView!.frame.height)
pageControl!.numberOfPages = selectedDetails!.numberOfPlans
if(planImages.count < selectedDetails!.numberOfPlans)
{
for index in planImages.count...selectedDetails!.numberOfPlans - 1
{
let iView = UIImageView()
iView.frame = CGRect(x: 0, y: 0, width: scrollView!.frame.width, height: scrollView!.frame.height)
planImages.append(iView)
scrollView!.addSubview(iView)
iView.translatesAutoresizingMaskIntoConstraints = false
iView.topAnchor.constraint(equalTo: scrollView!.topAnchor).isActive = true
iView.leftAnchor.constraint(equalTo: scrollView!.leftAnchor, constant: scrollView!.frame.width * CGFloat(index)).isActive = true
iView.heightAnchor.constraint(equalToConstant: scrollView!.contentSize.height).isActive = true
iView.widthAnchor.constraint(equalToConstant: scrollView!.frame.width).isActive = true
}
}
DispatchQueue.global(qos: .background).async
{
//Load first selected image
let path = self.selectedDetails!.dirPath + self.selectedDetails!.plansName
self.setupImageAtIndex(path: path, index: firstImage)
//Build Image data around first image on background threads
var mask = 1
let max = self.selectedDetails!.numberOfPlans
while firstImage + mask < max || firstImage - mask > 0
{
if(firstImage + mask < max)
{
self.setupImageAtIndex(path: path, index: firstImage + mask)
}
if(firstImage - mask >= 0)
{
self.setupImageAtIndex(path: path, index: firstImage - mask)
}
mask = mask + 1
}
//Remove extra images from memory if needed
if(self.planImages.count > self.selectedDetails!.numberOfPlans)
{
let dif = self.planImages.count - self.selectedDetails!.numberOfPlans
for _ in 1...dif
{
let index = self.planImages.count - 1
let plan = self.planImages[index]
self.planImages.remove(at: index)
plan.removeFromSuperview()
plan.image = nil
}
}
}
}
private func setupImageAtIndex(path: String, index: Int)
{
let imageName = index + 1 > 9 ? path + String(index + 1) : path + "0" + String(index + 1)
planImages[index].image = UIImage(contentsOfFile: imageName)
}

Answer 1

As per apple documentation You must update UI elements on the main queue.
Update your code like this
private func setupImageAtIndex(path: String, index: Int)
{
let imageName = index + 1 > 9 ? path + String(index + 1) : path + "0" + String(index + 1)
DispatchQueue.main.async {
planImages[index].image = UIImage(contentsOfFile: imageName)
}
}