A function always returns nil, due to misplacement of statements assigning values - swift

Question

In such a function I get a nil value but I do not understand why. The code in the middle returns an image (and I'm sure about it, I've also checked with some print statements). I do not know how is it possible that it always returns nil. It is like it ignores all the code running in the middle and considers just the first and last statement.
func getImagesDownloaded(reference: StorageReference) -> UIImage {
var imagePassedIn : UIImage?
reference.getData(maxSize: 10*1024*1024) { (data, error) in
guard let imageObject = UIImage(data: data!) else {print("Error has occurred: \(String(describing: error?.localizedDescription))"); return}
imagePassedIn = imageObject
}
if imagePassedIn == nil {
print("Error, getImagesDownloaded is not working")
}
return imagePassedIn!
}

Answer 1

The issue is that StorageReference.getData is an asynchronous function, but you're trying to synchronously return a value. You need to use a completion handler to return the asyncronously retrieved value.
func getImagesDownloaded(reference: StorageReference, completion: (UIImage?,Error?)->()) {
reference.getData(maxSize: 10*1024*1024) { (data, error) in
guard error == nil, let data = data else {
completion(nil,error)
return
}
guard let image = UIImage(data: data) else {
completion(nil, FirebaseErrors.expectedImage)
return
}
completion(image,nil)
}
}
enum FirebaseErrors: Error {
case expectedImage
}
Then you need to use it like this:
getImagesDownloaded(reference: yourStorageReference, completion: { image, error in
guard let image = image, error == nil else {
print(error)
return
}
// Assign your image to a UIImageView or do anything else with it inside the closure (before the ending `}`)
yourImageView.image = image
})

Answer 2

You are using a closure and the imageObject is probably not returned when you are doing the nil check.
Swift is executed line by line and when you have async code it executes the next line and doesn't wait for the result.
you should move the imagePassedIn check in the closure.

Answer 3

You're setting imagePassedIn inside a completion block. This means that when you are ready to set it, you've already retuned it.
In a few words, when using completion blocks, the code bellow won't be waiting for it to finish in order to execute.
Update your functions to this:
func getImagesDownloaded(reference: StorageReference, _ completion: (UIImage) -> ()) {
reference.getData(maxSize: 10*1024*1024) { (data, error) in
guard let data = data, let imageObject = UIImage(data: data) else {print("Error has occurred: \(String(describing: error?.localizedDescription))"); return}
completion(imageObject)
} else {
print("Error, getImagesDownloaded is not working")
}
}