Add multiple query results to array - ios

Question

I am trying to run 4 queries from my parse database, each pulling just 1 object. I am trying to add each of the objects to an array that can then be used in a collection view. The queries run successfully but the app crashes because it is pulling null values. Here is my code:
var query1desc = ""
var query2desc = ""
var descs = [String]()
fileprivate func fetchUsers() {
let query1 = PFQuery(className: "Messages")
query1.limit = 1
query1.findObjectsInBackground{
(objects, error) -> Void in
if error == nil {
if objects?.count == 0 {
} else {
let object = objects![0] as! PFObject
self.query1desc = object["message"] as! String
}
}
}
let query2 = PFQuery(className: "Messages")
query2.limit = 1
query2.findObjectsInBackground{
(objects, error) -> Void in
if error == nil {
if objects?.count == 0 {
} else {
let object = objects![0] as! PFObject
self.query2desc = object["message"] as! String
}
}
}
self.descs = [self.query1desc, self.query2desc]
self.collectionView.reloadData()
Does anyone know of a way to fix this so that self.descs does not just provide nil values? When I print within the query itself I know that objects are being pulled. Thanks in advance.

Answer 1

I think desc variable holds nil value because your program is executing an array assignment code part while completion handlers are still processing query requests. You need to wait until all your query requests finish and then can collect all the results.
let workGroup = DispatchGroup()
let queueForQuery1 = DispatchQueue(label: "firstQuery")
let query1 = PFQuery(className: "Messages")
query1.limit = 1
workGroup.enter()
queueForQuery1.async(group:workGroup) {
query1.findObjectsInBackground{ (objects, error) -> Void in
if error == nil {
if objects?.count == 0 {
} else {
let object = objects![0] as! PFObject
self.query1desc = object["message"] as! String
}
}
workGroup.leave()
}
}
let queueForQuery2 = DispatchQueue(label: "secondQuery")
let query2 = PFQuery(className: "Messages")
query2.limit = 1
workGroup.enter()
queueForQuery2.async(group:workGroup) {
query2.findObjectsInBackground{ (objects, error) -> Void in
if error == nil {
if objects?.count == 0 {
} else {
let object = objects![0] as! PFObject
self.query1desc = object["message"] as! String
}
}
workGroup.leave()
}
}
workGroup.notify(queue: DispatchQueue.main){
self.descs = [self.query1desc, self.query2desc]
self.collectionView.reloadData()
}