How to work with async functions swift? Completion handlers [duplicate] - swift

Question

This question already has answers here:
Run code only after asynchronous function finishes executing
(2 answers)
Closed 5 years ago.
Im trying to wait for the function to process in order to show my image. I have try many things but none of this worked. I know this is an async function and basically i have to wait in order to get the right values but I dont know how to fix this function right here. I hope you can help me out. Thank you!
func createListProductsGood(Finished() -> void) {
refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in
let prod = snapshot.value as! NSDictionary
let active = snapshot.key
let rejected = prod["NotInterested"] as! String
let photoURL = prod["photoURL"] as! String
var findit = false
// print(rejected)
if (rejected != self.userUID){
//print(active)
if rejected.contains(","){
var pointsArr = rejected.components(separatedBy: ",")
for x in pointsArr{
if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
// print("dont show")
findit = true
return
}
}
if (findit == false){
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active
self.productPhoto.image = UIImage(data: data as Data)
}}
}
}else{
print(active)
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active
self.productPhoto.image = UIImage(data: data as Data)
}}
}
}
})
finished()
}
Edited:
This is how my viewDidLoad looks like:
override func viewDidLoad() {
super.viewDidLoad()
setAcceptedOrRejected()
createListProductsGood{_ in
}
}
func createListProductsGood(finished: #escaping (_ imageData: Data) -> Void) {
refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in
let prod = snapshot.value as! NSDictionary
let active = snapshot.key
let rejected = prod["NotInterested"] as! String
let photoURL = prod["photoURL"] as! String
var findit = false
// print(rejected)
if (rejected != self.userUID){
//print(active)
if rejected.contains(","){
var pointsArr = rejected.components(separatedBy: ",")
for x in pointsArr{
if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
// print("dont show")
findit = true
return
}
}
if (findit == false){
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active
DispatchQueue.main.async {
self.productPhoto.image = UIImage(data: data as Data)
}
}}
}
}else{
print(active)
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active
DispatchQueue.main.async {
self.productPhoto.image = UIImage(data: data as Data)
}
}}
}
}
})
}
This is my second method:
func setAcceptedOrRejected() {
refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in
let prod = snapshot.value as! NSDictionary
if self.ProductId == snapshot.key{
self.texto = prod["NotInterested"] as! String
self.refProducts.child("Products").child(self.ProductId).updateChildValues(["NotInterested": self.texto + ", " + self.userUID])
} })
}

Answer 1

You should change:
func createListProductsGood(Finished() -> void) {
to:
func createListProductsGood(finished: #escaping (_ something: SomeType) -> Void) {
or to be more specific:
func createListProductsGood(finished: #escaping (_ imageData: Data) -> Void) {
then wherever in your function you get the image, you call
finished(imageData)
so you can pass the imageData through a closure to where its needed.
then you call this function like this:
createListProductsGood{ imageData in
...
let image = UIImage(data: imageData)
// update UI from main Thread:
DispatchQueue.main.async {
self.productPhoto.image = image
}
}
Also:
it's not convention to use Finished(), you should use finished()
using void is wrong. You must use Void or ()
If you're having problems with closures and completionHandlers, I recommend you first try getting your hands dirty with a simple UIAlertController. See here. Try creating an action with a closure, e.g. see here
EDIT :
Thanks to Leo's comments:
func createListProductsGood(finished: #escaping(_ imageData: Data?, MyError?) -> Void) {
let value: Data?
let error = MyError.someError("The error message")
refProducts.child("Products").queryOrderedByKey().observe(.childAdded, with: { snapshot in
let prod = snapshot.value as! NSDictionary
let active = snapshot.key
let rejected = prod["NotInterested"] as! String
let photoURL = prod["photoURL"] as! String
var findit = false
// print(rejected)
if (rejected != self.userUID){
//print(active)
if rejected.contains(","){
var pointsArr = rejected.components(separatedBy: ",")
for x in pointsArr{
if x.trimmingCharacters(in: NSCharacterSet.whitespaces) == self.userUID {
// print("dont show")
findit = true
return
}
}
if (findit == false){
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active // REMOVE
self.productPhoto.image = UIImage(data: data as Data) // REMOVE
finished(data, nil) //ADD
}else{
finished(nil,error) //ADD
}
}
}
}else{
print(active)
if let url = NSURL(string: photoURL) {
if let data = NSData(contentsOf: url as URL) {
self.ProductId = active // REMOVE
self.productPhoto.image = UIImage(data: data as Data) // REMOVE
finished(data,nil) //ADD
}else{
finished(nil,error) //ADD
}
}
}
}
})
}
And then you call it like:
createListProductsGood { imageData, error in guard let value = imageData, error == nil else { // present an alert and pass the error message return }
...
let image = UIImage(data: imageData)
// update UI from main Thread:
DispatchQueue.main.async {
self.ProductId = active
self.productPhoto.image = image } }
Basically this way the createListProductsGood takes in 2 closures, one for if the image is present, another for if an error was returned.