fetchRecordCompletionBlock and semaphores - help understanding execution order - swift

Question

Context:
App with all data in CloudKit
ViewController calls a query to load the data for a tableview
tableview crashes because the array of data for the tableview hasn't
come back from CK
I've researched semaphores and have it nearly
working But can't seem to figure out where to place the
semaphore.signal() to get the exact right behaviour
within viewDidLoad, I call the function:
Week.fetchWeeks(for: challenge!.weeks!) { weeks in
self.weeks = weeks
}
and the function:
static func fetchWeeks(for references: [CKRecord.Reference],
_ completion: #escaping ([Week]) -> Void) {
let recordIDs = references.map { $0.recordID }
let operation = CKFetchRecordsOperation(recordIDs: recordIDs)
operation.qualityOfService = .utility
let semaphore = DispatchSemaphore(value: 0)
operation.fetchRecordsCompletionBlock = { records, error in
let weeks = records?.values.map(Week.init) ?? []
DispatchQueue.main.async {
completion(weeks)
//Option 1: putting semaphore.signal() here means it never completes
// beyond initialization of the week records
}
//Option 2: putting semaphore.signal() here means it completes after the
// initialization of the Week items, but before completion(weeks) is done
// so the array isn't initialized in the view controller in time. so the
// VC tries to use weeks and unwraps a nil.
semaphore.signal()
}
Model.currentModel.publicDB.add(operation)
semaphore.wait() // blocking the thread until .signal is called
}
Note: I have tested that the weeks array within the view controller is properly set eventually - so it does seem to be purely a timing issue :)
I've tested placement of .signal() and if I put it within the 'DispatchQueue.main.async' block, it never gets triggered - probably because that block itself is waiting for the signal.
However if I put it anywhere else, then the viewcontroller picks up at that point and the completion(weeks) doesn't get called in time.
Maybe it is obvious - but as my first time working with semaphores - I'm struggling to figure it out!
Update 1: It works with DispatchQueue(label: "background")
I was able to get it working once I twigged that the semaphore.wait() was never going to get called with semaphore.signal() on the main thread.
So I changed it from:
DispatchQueue.main.async
to
DispatchQueue(label: "background").async and popped the semaphore.signal() inside and it did the trick
Comments/critiques welcome!
static func fetchWeeks(for references: [CKRecord.Reference],
_ completion: #escaping ([Week]) -> Void) {
NSLog("inside fetchWeeks in Week ")
let recordIDs = references.map { $0.recordID }
let operation = CKFetchRecordsOperation(recordIDs: recordIDs)
operation.qualityOfService = .utility
let semaphore = DispatchSemaphore(value: 0)
operation.fetchRecordsCompletionBlock = { records, error in
if error != nil {
print(error?.localizedDescription)
}
let weeks = records?.values.map(Week.init) ?? []
DispatchQueue(label: "background").async {
completion(weeks)
semaphore.signal()
}
}
Model.currentModel.publicDB.add(operation)
semaphore.wait() // blocking the thread until .signal is called
}
}
Update 2: Trying to avoid use of semaphores
Per comment thread - we shouldn't need to use semaphores with CloudKit - so it is likely that I'm doing something stupid :)
moving fetchWeeks() to the viewController to try to isolate the issue...but it still blows up as fetchWeeks() has't completed before the code tries to execute the line after and use the weeks array
my viewController:
class ChallengeDetailViewController: UIViewController {
#IBOutlet weak var rideTableView: UITableView!
//set by the inbound segue
var challenge: Challenge?
// set in fetchWeeks based on the challenge
var weeks: [Week]?
override func viewDidLoad() {
super.viewDidLoad()
rideTableView.dataSource = self
rideTableView.register(UINib(nibName: K.cellNibName, bundle: nil), forCellReuseIdentifier: K.cellIdentifier)
rideTableView.delegate = self
fetchWeeks(for: challenge!.weeks!) { weeks in
self.weeks = weeks
}
//This is where it blows up as weeks is nil
weeks = weeks!.sorted(by: { $0.weekSequence < $1.weekSequence })
}
//moved this to the view controller
func fetchWeeks(for references: [CKRecord.Reference],
_ completion: #escaping ([Week]) -> Void) {
let recordIDs = references.map { $0.recordID }
let operation = CKFetchRecordsOperation(recordIDs: recordIDs)
operation.qualityOfService = .utility
operation.fetchRecordsCompletionBlock = { records, error in
if error != nil {
print(error?.localizedDescription)
}
let weeks = records?.values.map(Week.init) ?? []
DispatchQueue.main.sync {
completion(weeks)
}
}
Model.currentModel.publicDB.add(operation)
}

Answer 1

Once again: Never use semaphores with the CloudKit API.
First of all declare data source arrays always as non-optional empty arrays to get rid of unnecessary unwrapping the optional
var weeks = [Week]()
The mistake is that you don't use the fetched data at the right place.
As the closure is asynchronous you have to proceed inside the closure
fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
self?.weeks = weeks
self?.weeks = weeks.sorted(by: { $0.weekSequence < $1.weekSequence })
}
or simpler
fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
self?.weeks = weeks.sorted{ $0.weekSequence < $1.weekSequence }
}
And if you need to reload the table view do it also inside the closure
fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
self?.weeks = weeks.sorted{ $0.weekSequence < $1.weekSequence }
self?.rideTableView.reloadData()
}
To do so you have to call completion on the main thread
DispatchQueue.main.async {
completion(weeks)
}
And finally delete the ugly semaphore!
let semaphore = DispatchSemaphore(value: 0)
...
semaphore.signal()
...
semaphore.wait()