Handling async callbacks in a for-in loop with DispatchGroup only works when all loops succeed
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The code below is a rough sketch of the task. A database is queried, it returns a collection of results, that collection is looped in search of a specific property, if that property is found, a file storage is immediately queried and its async completion handler returns the file in the loop. Because I am handling async callbacks inside a for-in loop, I use a DispatchGroup to manage that. This setup works only if all of the documents in the collection have the someIdentifier property. If one document in the collection does not have the property, the dispatch group never calls notify() and we are stuck in limbo.
someDatabaseQuery.retrieveSomeData { (data, error) in
guard let data = data, error == nil else {
return
}
// database has retrieved data, create dispatch group
let dispatchGroup = DispatchGroup()
for document in data { // loop through collection
// check if document has some identifier
guard let someIdentifier = document["someIdentifier"] as? String else {
return
}
dispatchGroup.enter() // identifier found, enter dispatch
// perform async operation inside loop
Filestorage.getSomeFile(forURL: someIdentifier) { (data, error) in
guard let file = data, error == nil else {
return
}
// download the file
dispatchGroup.leave() // leave dispatch
}
}
dispatchGroup.notify(queue: .main) {
// all data grabbed, load table
}
}
swift grand-central-dispatch
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
add a comment |
The code below is a rough sketch of the task. A database is queried, it returns a collection of results, that collection is looped in search of a specific property, if that property is found, a file storage is immediately queried and its async completion handler returns the file in the loop. Because I am handling async callbacks inside a for-in loop, I use a DispatchGroup to manage that. This setup works only if all of the documents in the collection have the someIdentifier property. If one document in the collection does not have the property, the dispatch group never calls notify() and we are stuck in limbo.
someDatabaseQuery.retrieveSomeData { (data, error) in
guard let data = data, error == nil else {
return
}
// database has retrieved data, create dispatch group
let dispatchGroup = DispatchGroup()
for document in data { // loop through collection
// check if document has some identifier
guard let someIdentifier = document["someIdentifier"] as? String else {
return
}
dispatchGroup.enter() // identifier found, enter dispatch
// perform async operation inside loop
Filestorage.getSomeFile(forURL: someIdentifier) { (data, error) in
guard let file = data, error == nil else {
return
}
// download the file
dispatchGroup.leave() // leave dispatch
}
}
dispatchGroup.notify(queue: .main) {
// all data grabbed, load table
}
}
swift grand-central-dispatch
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
add a comment |
The code below is a rough sketch of the task. A database is queried, it returns a collection of results, that collection is looped in search of a specific property, if that property is found, a file storage is immediately queried and its async completion handler returns the file in the loop. Because I am handling async callbacks inside a for-in loop, I use a DispatchGroup to manage that. This setup works only if all of the documents in the collection have the someIdentifier property. If one document in the collection does not have the property, the dispatch group never calls notify() and we are stuck in limbo.
someDatabaseQuery.retrieveSomeData { (data, error) in
guard let data = data, error == nil else {
return
}
// database has retrieved data, create dispatch group
let dispatchGroup = DispatchGroup()
for document in data { // loop through collection
// check if document has some identifier
guard let someIdentifier = document["someIdentifier"] as? String else {
return
}
dispatchGroup.enter() // identifier found, enter dispatch
// perform async operation inside loop
Filestorage.getSomeFile(forURL: someIdentifier) { (data, error) in
guard let file = data, error == nil else {
return
}
// download the file
dispatchGroup.leave() // leave dispatch
}
}
dispatchGroup.notify(queue: .main) {
// all data grabbed, load table
}
}
swift grand-central-dispatch
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
The code below is a rough sketch of the task. A database is queried, it returns a collection of results, that collection is looped in search of a specific property, if that property is found, a file storage is immediately queried and its async completion handler returns the file in the loop. Because I am handling async callbacks inside a for-in loop, I use a DispatchGroup to manage that. This setup works only if all of the documents in the collection have the someIdentifier property. If one document in the collection does not have the property, the dispatch group never calls notify() and we are stuck in limbo.
someDatabaseQuery.retrieveSomeData { (data, error) in
guard let data = data, error == nil else {
return
}
// database has retrieved data, create dispatch group
let dispatchGroup = DispatchGroup()
for document in data { // loop through collection
// check if document has some identifier
guard let someIdentifier = document["someIdentifier"] as? String else {
return
}
dispatchGroup.enter() // identifier found, enter dispatch
// perform async operation inside loop
Filestorage.getSomeFile(forURL: someIdentifier) { (data, error) in
guard let file = data, error == nil else {
return
}
// download the file
dispatchGroup.leave() // leave dispatch
}
}
dispatchGroup.notify(queue: .main) {
// all data grabbed, load table
}
}
swift grand-central-dispatch
swift grand-central-dispatch
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
edited Nov 22 '18 at 6:21
bsod
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
asked Nov 22 '18 at 2:55
bsodbsod
2,65721124
2,65721124
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
You must call leave if you call enter. But the guard inside the getSomeFile completion block can prevent the call to leave being made even though you called enter.
One solution is to use defer inside the completion block. Call leave inside the defer to ensure it is called no matter how you leave the block.
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
add a comment |
guard let someIdentifier = document["someIdentifier"] as? String else {
continue // this is the proper control flow statement
}
The problem was simply choosing the wrong control flow statement. When the guard failed inside the loop, return prevented the loop from finishing and never gave dispatch group a chance to notify. The else-clause in the guard should have been continue, which keeps control inside the loop (by letting it finish) and thus gives the dispatch group a chance to notify.
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
You must call leave if you call enter. But the guard inside the getSomeFile completion block can prevent the call to leave being made even though you called enter.
One solution is to use defer inside the completion block. Call leave inside the defer to ensure it is called no matter how you leave the block.
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
add a comment |
You must call leave if you call enter. But the guard inside the getSomeFile completion block can prevent the call to leave being made even though you called enter.
One solution is to use defer inside the completion block. Call leave inside the defer to ensure it is called no matter how you leave the block.
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
add a comment |
You must call leave if you call enter. But the guard inside the getSomeFile completion block can prevent the call to leave being made even though you called enter.
One solution is to use defer inside the completion block. Call leave inside the defer to ensure it is called no matter how you leave the block.
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
You must call leave if you call enter. But the guard inside the getSomeFile completion block can prevent the call to leave being made even though you called enter.
One solution is to use defer inside the completion block. Call leave inside the defer to ensure it is called no matter how you leave the block.
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
answered Nov 22 '18 at 3:46
rmaddyrmaddy
246k27328391
246k27328391
add a comment |
add a comment |
guard let someIdentifier = document["someIdentifier"] as? String else {
continue // this is the proper control flow statement
}
The problem was simply choosing the wrong control flow statement. When the guard failed inside the loop, return prevented the loop from finishing and never gave dispatch group a chance to notify. The else-clause in the guard should have been continue, which keeps control inside the loop (by letting it finish) and thus gives the dispatch group a chance to notify.
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
add a comment |
guard let someIdentifier = document["someIdentifier"] as? String else {
continue // this is the proper control flow statement
}
The problem was simply choosing the wrong control flow statement. When the guard failed inside the loop, return prevented the loop from finishing and never gave dispatch group a chance to notify. The else-clause in the guard should have been continue, which keeps control inside the loop (by letting it finish) and thus gives the dispatch group a chance to notify.
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
add a comment |
guard let someIdentifier = document["someIdentifier"] as? String else {
continue // this is the proper control flow statement
}
The problem was simply choosing the wrong control flow statement. When the guard failed inside the loop, return prevented the loop from finishing and never gave dispatch group a chance to notify. The else-clause in the guard should have been continue, which keeps control inside the loop (by letting it finish) and thus gives the dispatch group a chance to notify.
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
guard let someIdentifier = document["someIdentifier"] as? String else {
continue // this is the proper control flow statement
}
The problem was simply choosing the wrong control flow statement. When the guard failed inside the loop, return prevented the loop from finishing and never gave dispatch group a chance to notify. The else-clause in the guard should have been continue, which keeps control inside the loop (by letting it finish) and thus gives the dispatch group a chance to notify.
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
edited Nov 25 '18 at 3:59
Samuel Liew♦
45.5k33118157
45.5k33118157
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
answered Nov 22 '18 at 6:18
bsodbsod
2,65721124
2,65721124
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
add a comment |
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
Good that you found the root cause. Btw, wouldn't you have to leave the DispatchGroup for the second guard as well? If not, when there is an error in retrieving the file, the group won't be notified. <!-- language: swift --> guard let file = data, error == nil else { dispatchGroup.leave() // this has to be added? return }
– Ann Catherine Jose
Nov 25 '18 at 4:38
1
1
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
You are right and in the final version I implemented the defer method as mentioned by someone else to catch returns for any reason, including that guard. But somebody edited that part of my answer out.
– bsod
Nov 25 '18 at 12:33
add a comment |
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