Let's say I got Int array and array for some class instances
var IDs = [Int]() var items = [Item]() For every item in IDs array I want to run async function to request item from server
for id in IDs { let item = requestItem(id) items.append(item) } As requestItem works in async way - how can execute code only after all items are loaded?
func requestItem(id: Int, completion: @escaping (Item) -> ()) { DispatchQueue.global(qos: .background).async { let item = Item() completion(item) } } var IDs = [Int]() var items = [Item]() for id in IDs { requestItem(id: id) { (derivedItem) in items.append(derivedItem) } } 
requestItem() to help clarify. Basically, you will call your completion inside the async queue in requestItem, once you have got the value of itemYou have to use completion block for your function which makes server request and need to apply some more logic to show user about something is happening.
var IDs = [Int]() var items = [Item]() for id in IDs { requestItem(itemId : id, finished : { (objItem : Item) -> Void in items.append(objItem) // Below condition is to check weather all async service call response are came then hide loader or show items to user, etc. whatever you want to do after get all calls responses if IDs.count == items.count { // Show item to user or perform your further operation } }) } Function which has completion block
func requestItem(itemId : Int, finished: (objItem : Item) -> Void) { print("Item details fetched!") // Store your response in your Item class object and pass it like below let objItem = Item() // You constructor or any other logic to store value in objItem // Call finished when you have data initialized in your objItem object finished(objItem) } 