My caching system causes a memory leak - ios

Question

I have a data structure that I have created that caches items using a combination of a Dictionary and a Queue. The idea is that items are added to the "Cache" and once it reaches its limit, the oldest item in the cache is removed:
struct Cache<A: Hashable, B> {
fileprivate var theCache = [A:B]()
fileprivate var keyQueue = Queue<A>()
fileprivate var maxItems: Int!
init(maxItems: Int) {
self.maxItems = maxItems
}
subscript(key : A?) -> B? {
get {
if key != nil {
return theCache[key!]
}
return nil
}
set(newValue) {
// If value being added to cache, check that the max no. of items isn't exceeded.
// If it is then remove the first item of the cache queue and add value to queue.
if key != nil {
if (theCache.count + 1 > maxItems) {
removeFirstItem()
}
addValue(key!, value: newValue)
}
}
}
/**
Tells us how many items there are in the cache
- returns: The count of the cache
*/
var count: Int {
return theCache.count
}
mutating func clear() {
theCache.removeAll()
for _ in 0..<keyQueue.count {
_ = keyQueue.dequeue()
}
}
fileprivate mutating func addValue(_ key: A, value: B?) {
theCache[key] = value // Memory leak 2
keyQueue.enqueue(key)
}
fileprivate mutating func removeFirstItem() {
let firstItemKey = keyQueue.dequeue()
if firstItemKey != nil {
theCache.removeValue(forKey: firstItemKey!)
}
}
}
I'm using this to cache UIImages in a UICollectionView, and have a strong reference to this cache from my model class.
Inside my cellForItemAt method, I have the following code to check for cached images. If there is a cached image then it adds that to the UICollectionCell otherwise it downloads it, adds it to the cache, then adds it to the UICollectionCell:
if let cellImage = model.imageCache[urlString] {
cell.imageView.image = cellImage
}
else {
model.downloadImage(
urlString,
completion: { [weak self] (error, image) -> () in
if let strongSelf = self {
if error == nil {
// Store the image in to our cache
strongSelf.model.imageCache[urlString] = image
// Update the cell with our image
if let cellToUpdate = collectionView.cellForItem(at: indexPath) as? MyCollectionCell {
cellToUpdate.imageView.image = image
}
}
else {
print("Error downloading image: \(error!.localizedDescription)")
}
}
}
)
}
However, according to Xcode 8 this whole process is causing a memory leak for the UIImages. Can anyone help me out with where the memory leak is occurring and how I can prevent it?
EDIT
Here's my downloadImage function. I've also added "Memory leak 1" and "Memory leak 2" as comments next to my code to show the backtrace of the memory leak xcode shows me:
Alamofire.request(urlString, method: .get)
.authenticate(usingCredential: credentials!)
.validate()
.downloadProgress { bytesRead, totalBytesRead, totalBytesExpectedToBeRead in
DispatchQueue.main.async {
progress?(Float(totalBytesRead)/Float(totalBytesExpectedToBeRead))
}
}
.responseData { response in
// Convert the downloaded data in to a UIImage object
var image: UIImage?
if response.result.error == nil {
image = UIImage(data: response.data!) // Memory leak 1
}
completion(response.result.error, image)
}