How to write a callback for the asynchronous function below - ios

Question

I have a function I'm using below. I want the data from that asynchronous function but it comes up nil if I try to use the data outside the fullResolutionImageData function. So I have to write a callback. But I don't know how to write the callback. Can someone show me how to write a callback for the asynchronous function below so I can access the data elsewhere. Can you show me how to write a callback so I can access files outside fullResolutionImageData asynchronous function.
public func fullResolutionImageData(completion: #escaping (Data?) -> ()) {
SharedQueues.imageProcessingQueue.addOperation { [path] in
let data = try? Data(contentsOf: URL(fileURLWithPath: path))
DispatchQueue.main.async {
completion(data)
}
}
}
In my view controller I have an action that uses this function
class ViewController: UIViewController{
func buttonAction(sender:UIButton!){
var photos2: [ImageSource]?
var files : [Data] = []
let _ = self.photos2?.map ({completion: path in
path.fullResolutionImageData{ data in
files.append(data) --> (prints 2)
}
})
print("(files.count)") --> (prints 0 How do I write callback?)
}
}

Answer 1

All u need is Dispatch Group.
func buttonAction(sender:UIButton!){
var photos2: [ImageSource]?
var files : [Data] = []
let myGroup = DispatchGroup()
let _ = self.photos2?.map ({completion: path in
myGroup.enter()
path.fullResolutionImageData{ data in
files.append(data) --> (prints 2)
myGroup.leave()
}
})
myGroup.notify(queue: DispatchQueue.main) { // 2
print("(files.count)")
}
}
Little bit of Explanation :
Dispatch groups are used for monitoring the completion of multiple asynchronous tasks. Like in your case getting the data from the file path which u are performing on background thread.
Things to be aware of :
Every single tasks that enters the group should leave the group no matter whether it succeeds in its goal or not. If any of the task entered the group fail to exit the thread will be blocked forever. So make sure fullResolutionImageData always executes the completion block!
EDIT:
Finally As pointed out by David in comments it makes sense to use foreach loop then using map if u are discarding the results
hence use
func buttonAction(sender:UIButton!){
var photos2: [ImageSource]?
var files : [Data] = []
let myGroup = DispatchGroup()
photos2?.forEach{ path in
myGroup.enter()
path.fullResolutionImageData{ data in
files.append(data) --> (prints 2)
myGroup.leave()
}
}
myGroup.notify(queue: DispatchQueue.main) { // 2
}
}