Firebase Async Callback in NSOperation not returning - swift

Question

I have a AsyncOperation class defined as such
import Foundation
class ASyncOperation: NSOperation {
enum State: String {
case Ready, Executing, Finished
private var keyPath: String {
return "is" + rawValue
}
}
var state = State.Ready {
willSet {
willChangeValueForKey(newValue.keyPath)
willChangeValueForKey(state.keyPath)
}
didSet {
didChangeValueForKey(oldValue.keyPath)
didChangeValueForKey(state.keyPath)
}
}
override var ready: Bool {
return super.ready && state == .Ready
}
override var executing: Bool {
return super.ready && state == .Executing
}
override var finished: Bool {
return super.ready && state == .Finished
}
override var asynchronous: Bool {
return true
}
override func start() {
if cancelled {
state = .Finished
return
}
main()
state = .Executing
}
override func cancel() {
state = .Finished
}
}
and a subclass of it ImageLoadOperation.
import Foundation
import UIKit
import Firebase
class ImageLoadOperation: ASyncOperation {
var imagePath: String?
var image: UIImage?
override func main(){
let storage = FIRStorage.storage()
let storageRef = storage.referenceForURL("gs://salisbury-zoo- 91751.appspot.com")
if let path = imagePath {
let imageReference = storageRef.child(path)
imageReference.dataWithMaxSize(3 * 1024 * 1024) { (data, error) -> Void in
if (error != nil) {
self.image = nil
} else {
self.image = UIImage(data: data!)
self.state = .Finished
}
}
}
}
}
So I go to call the Operation in a Queue
let queue = NSOperationQueue()
let imageLoad = ImageLoadOperation()
queue.addOperation(imageLoad)
let img:UIImage? = imageLoad.image
But it always returns nil. When I put a print statement in the callback of ImageLoadOperation the image is there and state is set to finished. When I add
queue.waitUntilAllOperationsAreFinished()
Inbetween queue.addOperation and let img:UIImage? = imageLoad.load then the entire application stalls as the main thread is blocked. Any other ideas on how I could get the image to be there outside the scope of the callback? I have also tried doing it without a NSOperationQueue and just as an NSOperation with no luck.

Answer 1

The queue.addOperation function adds the operation, and it starts executing in a background thread. It therefore returns well before the background thread is finished, which is why the image is nil.
And as the documentation states, waitUntilAllOperationsAreFinished will block the thread until the operations are finished. This is very undesirable on the main thread.
imageReference.dataWithMaxSize is an asynchronous operation that has a completion handler (where you are currently setting self.image). You need something in there to trigger code to run that will allow you to use imageLoad.image. How you do this will depend on the architecture of your app.
If your image is to be displayed in a UITableViewCell, for example, you will need to store the image in an array of images, possibly where the index matches the table row, and then reload at least that row of the tableView. This is because by the time the image has been received, the cell may no longer exist for that row. Obviously you would not want this code sitting inside your ImageLoadOperation class. Instead it should be passed into main() as a completion handler.