Proper Placement of dispatchGroup to reloadData - ios

Question

I have a tableview function that is pulling data from a database to render cells. I want to accomplish the goal of not reloading my tableview so much. I learned that dispatch groups would be the way to go beause I don't want to return to the completion block that reloads the tableView until all the data has been pulled however when I use the dispatchGroup it never reaches the completion it just stops. The placement of my variables may be in the wrong place but i just can't really see where I should put it. I have been moving it to different places and still nothing.
import UIKit
import Firebase
class FriendsEventsView: UITableViewController{
var cellID = "cellID"
var friends = [Friend]()
var attendingEvents = [Event]()
//label that will be displayed if there are no events
var currentUserName: String?
var currentUserPic: String?
var currentEventKey: String?
override func viewDidLoad() {
super.viewDidLoad()
self.title = "Friends Events"
view.backgroundColor = .white
// Auto resizing the height of the cell
tableView.estimatedRowHeight = 44.0
tableView.rowHeight = UITableViewAutomaticDimension
self.navigationItem.rightBarButtonItem = UIBarButtonItem(image: #imageLiteral(resourceName: "close_black").withRenderingMode(.alwaysOriginal), style: .done, target: self, action: #selector(self.goBack))
tableView.register(EventDetailsCell.self, forCellReuseIdentifier: cellID)
self.tableView.tableFooterView = UIView(frame: CGRect.zero)
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
DispatchQueue.global(qos: .background).async {
print("This is run on the background queue")
self.fetchEventsFromServer { (error) in
if error != nil {
print(error)
return
} else {
DispatchQueue.main.async {
self.tableView.reloadData()
print("This is run on the main queue, after the previous code in outer block")
}
}
}
}
}
#objc func goBack(){
dismiss(animated: true)
}
override func numberOfSections(in tableView: UITableView) -> Int {
// print(friends.count)
return friends.count
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
// print(friends[section].events.count)
return friends[section].collapsed ? 0 : friends[section].events.count
}
func tableView(_ tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {
return UITableViewAutomaticDimension
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: cellID) as! EventDetailsCell? ?? EventDetailsCell(style: .default, reuseIdentifier: cellID)
// print(indexPath.row)
cell.details = friends[indexPath.section].events[indexPath.row]
return cell
}
override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
let header = tableView.dequeueReusableHeaderFooterView(withIdentifier: "header") as? CollapsibleTableViewHeader ?? CollapsibleTableViewHeader(reuseIdentifier: "header")
// print(section)
header.arrowLabel.text = ">"
header.setCollapsed(friends[section].collapsed)
print(friends[section].collapsed)
header.section = section
// header.delegate = self
header.friendDetails = friends[section]
return header
}
override func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
return 50
}
func fetchEventsFromServer(_ completion: #escaping (_ error: Error?) -> Void ){
//will grab the uid of the current user
guard let myUserId = Auth.auth().currentUser?.uid else {
return
}
let ref = Database.database().reference()
//checking database for users that the current user is following
ref.child("following").child(myUserId).observeSingleEvent(of: .value, with: { (followingSnapshot) in
//handling potentail nil or error cases
guard let following = followingSnapshot.children.allObjects as? [DataSnapshot]
else {return}
//validating if proper data was pulled
let group = DispatchGroup()
for followingId in following {
group.enter()
UserService.show(forUID: followingId.key, completion: { (user) in
PostService.showFollowingEvent(for: followingId.key, completion: { (event) in
self.attendingEvents = event
var friend = Friend(friendName: (user?.username)!, events: self.attendingEvents, imageUrl: (user?.profilePic)!)
self.friends.append(friend)
})
})
}
this loop should return to the completon block in viewWillAppear following the execution of this if statement
if self.friends.count == following.count{
group.leave()
let result = group.wait(timeout: .now() + 0.01)
//will return this when done
completion(nil)
}
}) { (err) in
completion(err)
print("Couldn't grab people that you are currently following: \(err)")
}
}
Any help is greatly appreciated

Answer 1

You want to place the group.leave() inside of the PostService.showFollowingEvent callback.
Now you call enter following.count-times, but you call leave only once. For the group to continue you have to leave the group as many times as you entered it:
for followingId in following {
group.enter()
UserService.show(forUID: followingId.key, completion: { (user) in
PostService.showFollowingEvent(for: followingId.key, completion: { (event) in
self.attendingEvents = event
var friend = Friend(friendName: (user?.username)!, events: self.attendingEvents, imageUrl: (user?.profilePic)!)
self.friends.append(friend)
// leave here
group.leave()
})
})
}
Moreover, I would not recommend using group.wait since you are facing a possible deadlock. If any of the callbacks that are supposed to call group.leave are happening on the same thread as group.wait was called, they will never get called and you will end up with the frozen thread. Instead, use group.notify:
group.notify(queue: DispatchQueue.main) {
if self.friends.count == following.count {
completion(nil)
}
}
This will allow the execution on the main thread, but once all the tasks are finished, it will execute the provided callback closure.