Background thread - two network calls [duplicate] - ios

Question

This question already has answers here:
DispatchGroup logical workflow
(2 answers)
Closed 1 year ago.
I have two methods in the completeOnboarding method and both of them have network operation which should be done in the background thread as follows. However, I am wondering if I am doing why completion(true) gets called first, how could I able to handle that issue?
DispatchQueue.global(qos: .background).async {
self?.completeOnboarding( completion: { (success) in
DispatchQueue.main.async {
if success {
print("success")
} else {
print("failed")
}
}
})
func completeOnboarding(completion: #escaping(Bool) -> Void){
// has network post operation
classRegistration() {(success) in
if !success {
completion(false)
return
}
}
// has network post operation
classLocation() { (success) in
if !success {
completion(false)
return
}
}
completion(true)
}

Answer 1

The final completion(true) is not waiting for classLocation() and classRegistration() calls. If you have multiple network calls and you want to wait for all of them to finish you could (one approach) add them to a DispatchGroup and wait for that one to finish:
func dispatchAndWait(completion: #escaping () -> Void) {
func networkOne(completion: #escaping (_ success: Bool) -> Void) {
print("[DEBUG] Enter \(#function)")
DispatchQueue.main.asyncAfter(deadline: .now() + Double.random(in: 0...3)) {
completion(Bool.random())
}
print("[DEBUG] Return \(#function)")
}
func networkTwo(completion: #escaping (_ success: Bool) -> Void) {
print("[DEBUG] Enter \(#function)")
DispatchQueue.main.asyncAfter(deadline: .now() + Double.random(in: 0...3)) {
completion(Bool.random())
}
print("[DEBUG] Return \(#function)")
}
// Create a DispatchGroup and add both calls
let dispatchGroup = DispatchGroup()
// Enter first network call
dispatchGroup.enter()
networkOne { success in
print("[DEBUG] Complete networkOne with success: \(success)")
// Exit first network call
dispatchGroup.leave()
}
// Enter second network call
dispatchGroup.enter()
networkTwo { success in
print("[DEBUG] Complete networkTwo with success: \(success)")
// Exit second network call
dispatchGroup.leave()
}
// notify gets called when all tasks have exited
dispatchGroup.notify(queue: DispatchQueue.main) {
completion()
}
}
One more advise:
classRegistration() {(success) in
if !success {
completion(false)
return
}
}
will never complete in case of success==true, you should make sure that the completion is called on every path
classRegistration() {(success) in
// ... do whatever needs to be done here
completion(success)
}

Answer 2

Assuming classRegistration needs to succeed for classLocation to begin --
quick & dirty --
func completeOnboarding(completion: #escaping(Bool) -> Void){
// has network post operation
classRegistration() {(success) in
if success {
// has network post operation
classLocation() { (success) in
completion(success)
}
} else {
completion(false)
}
}
}
a proper way (others include - NSOperations with dependency, Dispatch group)
func completeOnboarding(completion: #escaping(Bool) -> Void){
let serialQueue = DispatchQueue(label: "classname.serial")
var proceedWithSuccess = true
serialQueue.async {
serialQueue.suspend() //run 1 operation at a time
classRegistration() {(success) in
proceedWithSuccess = success
serialQueue.resume() //let next operation run
}
}
serialQueue.async {
guard proceedWithSuccess else { return }
serialQueue.suspend()
classLocation() { (success) in
proceedWithSuccess = success
serialQueue.resume()
}
}
serialQueue.async {
completion(proceedWithSuccess)
}
}
If you want classLocation() to fire even if registration fails - just get rid of guard statement above.
If it were up to me I'd use a custom NSOperation subclass for Async operation & explicitly mention dependency between operations but it needs a ton of boilerplate code (perhaps something to look into later); serial queue (or dispatch group from the other answer) oughta be enough for you in this case though.