Synchronize nested async network requests inside a while loop by using Semaphores - swift

Question

I have a func that gets a list of Players. When i fetch the players i need only to show those who belongs to the current Team so i am showing only a subset of the original list by filtering them. I don't know in advance, before making the request, how much players belong to the Team selected by the User, so i may need to do additional requests until i can display on the TableView at least 10 rows of Players. The User by pulling up from the bottom of the TableView can request more players to display. To do this i am calling a first async func request which in turn calls, inside a while, another nested async func request. Here a code to give you an idea of what i am trying to do:
let semaphore = DispatchSemaphore(value: 0)
func getTeamPlayersRequest() {
service.getTeamPlayers(...)
{
(result) in
switch result
{
case .success(let playersModel):
if let validCurrentPage = currentPageTmp ,
let validTotalPages = totalPagesTmp ,
let validNextPage = self.getTeamPlayersListNextPage()
{
while self.playersToShowTemp.count < 10 && self.currentPage < validTotalPages
{
self.currentPage = validNextPage //global var
self.fetchMorePlayers()
self.semaphore.wait() //global semaphore
}
}
case .failure(let error):
//some code...
}
})
}
private func fetchMorePlayers(){
// Completion handler of the following function is never called..
service.getTeamPlayers(requestedPage: currentPage, completion: {
(result) in
switch result
{
case .success(let playersModel):
if let validPlayerList = playersList,
let validPlayerListData = validPlayerList.data,
let validTeamModel = self.teamPlayerModel,
let validNextPage = self.getTeamPlayersListNextPage()
{
for player in validPlayerListData
{
if ( validTeamModel.id == player.team?.id)
{
self.playersToShowTemp.append(player)
}
}
}
self.currentPage = validNextPage
self.semaphore.signal() //global semaphore
case .failure(let error):
//some code...
}
}
}
I have tried both with DispatchGroup and Semaphore but i don't get it what i am doing wrong. I debugged the code and saw that the first async call get executed in a different queue (not the main queue) and a different thread. The nested async call getexecuted on a different thread but i don't know if it's the same concurrent queue of the first async call.
The completion handler of thenested call it's never called. Does anyone know why? is the self.semaphore.wait(), even if it get executed after the fetchMorePlayers() return, blocking/preventing the nested async completion handler to be called?
I am noticing through the Debugger that the completion() in the Xcode vars window has the note "swift partial apply forwarder for closure #1"

Answer 1

If we inline the function call in your loop, it looks something like this:
while self.playersToShowTemp.count < 10 && self.currentPage < validTotalPages
{
self.currentPage = validNextPage //global var
nbaService.getTeamPlayers(requestedPage: currentPage, completion: { ... })
self.semaphore.wait() //global semaphore
}
So nbaService.getTeamPlayers schedules a request, probably on the main DispatchQueue and immediately returns. Then you call wait on your semaphore, which blocks, probably before GCD even tries to run the task scheduled by nbaService.getTeamPlayers.
That's a problem on DispatchQueue.main, which is a serial queue. It has to be a serial queue for UI updates to work. What normally happens is on some iteration of the run loop you make a request, and return.. that bubbles back up to the run loop, which checks for more events and queued tasks. In this case, when your completion handler in getTeamPlayersRequest is waiting to be run, the run loop (via GCD) executes it for that iteration. Then you block the main thread, so the run loop can't continue. If you do need to block always do it on a different DispatchQueue, preferably a .concurrent one.
There is sometimes confusion about what .async does. It only means "run this later and right now return control back to the caller". That's all. It does not guarantee that your closure will run concurrently. It merely schedules it to be run later (possibly soon) on whatever DispatchQueue you called it on. If that queue is a serial queue, then it will be queued to run in its turn in that dispatch queue's run loop. If it's a concurrent queue (ie one you specifically set the attributes to include .concurrent). Then it will run, possibly at the same time as other tasks on that same DispatchQueue.
To avoid that instead of using a loop you can use async-chaining.
private func fetchMorePlayers(while condition: #autoclosure #escaping () -> Bool){
guard condition() else { return }
nbaService.getTeamPlayers(requestedPage: currentPage, completion: {
(result) in
switch result
{
case .success(let playersModel):
if let validPlayerList = playersList,
let validPlayerListData = validPlayerList.data,
let validTeamModel = self.teamPlayerModel,
let validNextPage = self.getTeamPlayersListNextPage()
{
for player in validPlayerListData
{
if ( validTeamModel.id == player.team?.id)
{
self.playersToShowTemp.append(player)
}
}
}
self.currentPage = validNextPage
// Chain to next call
self.fetchMorePlayers(while: condition))
case .failure(let error):
//some code...
}
}
}
Then in getTeamPlayersRequest you can do this:
func getTeamPlayersRequest() {
service.getTeamPlayers(...)
{
(result) in
switch result
{
case .success(let playersModel):
if let validCurrentPage = currentPageTmp ,
let validTotalPages = totalPagesTmp ,
let validNextPage = self.getTeamPlayersListNextPage()
{
self.currentPage = validNextPage //global var
self.fetchMorePlayers(while: self.playersToShowTemp.count < 10 && self.currentPage < validTotalPages)
}
case .failure(let error):
//some code...
}
})
}
This avoids the need to block on a semaphore, because each subsequent request happens in the completion handler of the previously completed one. The only issue is if you need for the completion handler in getTeamPlayersRequest to block while the fetchMorePlayers requests are being fetched, because now it won't you can re-introduce the semaphore. In that case the guard statement in fetchMorePlayers becomes:
guard condition() else
{
self.semaphore.signal()
return
}
That way it only signals on the last completion handler in the chain. You may need to block in a different DispatchQueue though. I think if you need to block, you probably have something about your design that needs to be reconsidered.

Answer 2

If you find yourself reaching for semaphores, it is almost always a mistake. Semaphores are inefficient at best, and introduce deadlock risks if misused. Semaphores should generally be avoided. (Don't get me wrong: Semaphores can be useful in some very narrow use cases, but this is not one of them.)
Use asynchronous patterns. One simple approach might be to recursively call the routine, calling the completion handler when done:
func startFetching(#escaping completion: () -> Void) {
fetchPlayers(page: 0, completion: completion)
}
private func fetchPlayers(page: Int, #escaping completion: () -> Void) {
// prepare request
// now perform request
performRequest(...) { ...
if let error = error {
completion()
return
}
...
if doesNeedMorePlayers {
fetchPlayers(page: page + 1, completion: completion)
} else {
completion()
}
}
}
Personally, I might probably add another closure to emit the players retrieved as we go along, e.g. like, if not actually, a Combine Publisher. Or if you want to update the UI all at once at the very end, just pass the players retrieved thus far as additional parameter in this recursive routine and pass the whole array back in the completion handler. But avoid globals or other state properties.
But the broader idea is to scrupulously avoid semaphores and instead embrace asynchronous patterns.