Swift UITableViewController `await` until all data is loaded before rendering, or re-render after data has been loaded - ios

Question

I am on Swift 4. The goal is to load all the data in an address book, before render the address book in view. In a different language such as js, I may use await in each item in the loop, before telling the view to render the rows. I am looking for the canonical way to solve this issue in Swift 4 with UITableViewController.
Right now the address book is stored in backend with Amplify and GraphQL. I have a User model of form
type User #Model {
id: ID!
name: String!
bio : String!
}
and Contact of form
type Contact #model {
ownerId: ID!
userId: ID!
lastOpened: String
}
In ContactController: UITableViewController.viewDidLoad I fetch all Contact in database where the ownerId is my user's id-token, I then create an object using this contact information. And then for each Contact object instance, I get its corresponding User in database when the object is initialized. Per this post: Wait until swift for loop with asynchronous network requests finishes executing, I am using Dispatch group, and then reload the UITableView after the loop completes and the Dispatch group has ended. But when I print to console, I see that the loop completes before the Contact object has loaded its User information.
Code snippets:
class ContactsController: UITableViewController, UISearchResultsUpdating {
var dataSource : [Contact] = []
override func viewDidLoad() {
super.viewDidLoad()
let fetchContactGrp = DispatchGroup()
fetchContactGrp.enter()
self.getMyContacts(){ contacts in
for contact in contacts {
let _contactData = Contact(
userId : contact.userId
, contactId : contact.id
, timeStamp : contact.timeStamp
, lastOpened : contact.lastOpened
, haveAccount: true
)
_contactData.loadData()
self.dataSource.append(_contactData)
}
}
fetchContactGrp.leave()
DispatchQueue.main.async{
self.tableView.reloadData()
}
}
}
The function self.getMyContacts is just a standard GraphQL query:
func getMyContacts( callBack: #escaping ([Contact]) -> Void ){
let my_token = AWSMobileClient.default().username
let contact = Contact.keys
let predicate = contact.ownerId == my_token!
_ = Amplify.API.query(from: Contact.self, where: predicate) { (event) in
switch event {
case .completed(let result):
switch result {
case .success(let cts):
/// #On success, output a user list
callBack(cts)
case .failure(let error):
break
}
case .failed(let error):
break
default:
break
}
}
}
And the Contact object loads the User data from database:
class Contact {
let userId: String!
let contactId: String!
var name : String
var bio : String
var website: String
let timeStamp: String
let lastOpened: String
init( userId: String, contactId: String, timeStamp: String, lastOpened: String, haveAccount: Bool){
self.userId = userId
self.contactId = contactId
self.timeStamp = timeStamp
self.lastOpened = lastOpened
self.haveAccount = haveAccount
self.name = ""
self.bio = ""
self.website = ""
}
func loadData(){
/// #use: fetch user data from db and populate field on initation
let _ = Amplify.API.query(from: User.self, byId: self.userId) { (event) in
switch event {
case .completed(let res):
switch res{
case .success (let musr):
if (musr != nil){
let userData = musr!
let em = genEmptyString()
self.name = (userData.name == em) ? "" : userData.name
self.bio = (userData.bio == em) ? "" : userData.bio
self.website = (userData.website == em) ? "" : userData.website
print(">> amplify.query: \(self.name)")
} else {
break
}
default:
break
}
default:
print("failed")
}
}
}
}

Answer 1

It's because the function getMyContacts() is performing an Async task and the control goes over that and execute the leave statement. You need to call the leave statement inside the getMyContacts() function outside the for loop.
Try the following code:
override func viewDidLoad() {
super.viewDidLoad()
let fetchContactGrp = DispatchGroup()
fetchContactGrp.enter()
self.getMyContacts(){ contacts in
for contact in contacts {
let _contactData = Contact(
userId : contact.userId
, contactId : contact.id
, timeStamp : contact.timeStamp
, lastOpened : contact.lastOpened
, haveAccount: true
)
_contactData.loadData()
self.dataSource.append(_contactData)
}
fetchContactGrp.leave()
}
fetchContactGrp.wait()
DispatchQueue.main.async{
self.tableView.reloadData()
}
}

Answer 2

I posted a more general version of this question here: Using `DispatchGroup` or some concurency construct to load data and populate cells in `UITableViewController` sequentially
And it has been resolved.