Instance Struct variable of a mutating function in Swift - struct

Question

I'm having troubles using swift Structure when they have a mutating function. In this example, I have a MediaManager that fetches some data, stores them into its instance variable and calls a completion where I reload the UI.
public struct MediaManager {
//var store = MovieStore()
var movies = [String]()
mutating func fetchMovies(completion: (() -> Void) ) -> Void {
// fetch movies
NSOperationQueue.mainQueue().addOperationWithBlock() {
self.movies = ["Ant Man", "Batman"]
print("Movies in store \(self.movies)")
print("Movies in manager \(mediaManager.allMovies())")
completion()
}
}
public func allMovies() -> [String] {
return movies
}
}
The problem is that the allMovies() is empty.
The output is
Movies in store ["Ant Man", "Batman"]
Movies in manager []
I could use a class instead of a struct but my understanding is that struct should be preferred over classes in swift. What is a proper solution for this?
cheers,
Jan

Answer 1

You have a closure there so every variable used by the closure must be captured, including self. However, self is a value type (struct) so capturing means copying value. When the closure is being executed, movies are set on a local copy of the struct.
Use a class (reference type). Value types are in no way preferred over reference types. Both have their uses. You have found a use case where a reference type is needed.

Answer 2

there are a lot of very similar questions around. your fetchMovies function returns immediately. thats all .... You add some block for later execution but at which time it will be executed?
Yes, Sultan is right, event though you need some kind of synchronization. you can play (in accordance with Sultan recommendation with reference type)
import XCPlayground
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
import Foundation
public class MediaManager {
//var store = MovieStore()
var movies = [String]()
func fetchMovies(completion: (() -> Void) ) -> Void {
// fetch movies
NSOperationQueue.mainQueue().addOperationWithBlock() {
self.movies = ["Ant Man", "Batman"]
print("Movies in store \(self.movies)")
print("Movies in manager \(self.allMovies())")
completion()
}
}
public func allMovies() -> [String] {
return movies
}
}
var mm = MediaManager()
mm.fetchMovies { print("completed") }
print("after fetch:", mm.allMovies())
with this result
after fetch: []
Movies in store ["Ant Man", "Batman"]
Movies in manager ["Ant Man", "Batman"]
completed