I'm kind of new to programming in general, so I have this maybe simple question. Actually, writing helps me to identify the problem faster.
Anyway, I have an app with multiple asynchronous calls, they are nested like this:
InstagramUnoficialAPI.shared.getUserId(from: username, success: { (userId) in
InstagramUnoficialAPI.shared.fetchRecentMedia(from: userId, success: { (data) in
InstagramUnoficialAPI.shared.parseMediaJSON(from: data, success: { (media) in
guard let items = media.items else { return }
self.sortMediaToCategories(media: items, success: {
print("success")
// Error Handlers
Looks horrible, but that's not the point. I will investigate the Promise Kit once I get this working.
I need the sortMediaToCategories to wait for completion and then reload my collection view. However, in the sortMediaToCategories I have another nested function, which is async too and has a for in loop.
func sortMediaToCategories(media items: [StoryData.Items],
success: #escaping (() -> Swift.Void),
failure: #escaping (() -> Swift.Void)) {
let group = DispatchGroup()
group.enter()
for item in items {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else {return}
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: { group.notify(queue: .global(), execute: {
self.collectionView.reloadData()
group.leave()
}) },
failure: { print("error") })
//....
I can't afford the collection view to reload every time obviously, so I need to wait for loop to finish and then reload.
I'm trying to use Dispatch Groups, but struggling with it. Could you please help me with this? Any simple examples and any advice will be very appreciated.
The problem you face is a common one: having multiple asynchronous tasks and wait until all are completed.
There are a few solutions. The most simple one is utilising DispatchGroup:
func loadUrls(urls: [URL], completion: #escaping ()->()) {
let grp = DispatchGroup()
urls.forEach { (url) in
grp.enter()
URLSession.shared.dataTask(with: url) { data, response, error in
// handle error
// handle response
grp.leave()
}.resume()
}
grp.notify(queue: DispatchQueue.main) {
completion()
}
}
The function loadUrls is asynchronous and expects an array of URLs as input and a completion handler that will be called when all tasks have been completed. This will be accomplished with the DispatchGroup as demonstrated.
The key is, to ensure that grp.enter() will be called before invoking a task and grp.leave is called when the task has been completed. enter and leave shall be balanced.
grp.notify finally registers a closure which will be called on the specified dispatch queue (here: main) when the DispatchGroup grp balances out (that is, its internal counter reaches zero).
There are a few caveats with this solution, though:
All tasks will be started nearly at the same time and run concurrently
Reporting the final result of all tasks via the completion handler is not shown here. Its implementation will require proper synchronisation.
For all of these caveats there are nice solutions which should be implemented utilising suitable third party libraries. For example, you can submit the tasks to some sort of "executer" which controls how many tasks run concurrently (match like OperationQueue and async Operations).
Many of the "Promise" or "Future" libraries simplify error handling and also help you to solve such problems with just one function call.
You can reloadData when the last item calls the success block in this way.
let lastItemIndex = items.count - 1
for(index, item) in items.enumerated() {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else {return}
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: {
if index == lastItemIndex {
DispatchQueue.global().async {
self.collectionView.reloadData()
}
}
},
failure: { print("error") })
}
You have to move the group.enter() call inside your loop. Calls to enter and leave have to be balanced. If your callbacks of the mediaToStorageDistribution function for success and failure are exclusive you also need to leave the group on failure. When all blocks that called enter leave the group notify will be called. And you probably want to replace the return in your guard statement with a break, to just skip items with missing URLs. Right now you are returning from the whole sortMediaToCatgories function.
func sortMediaToCategories(media items: [StoryData.Items], success: #escaping (() -> Void), failure: #escaping (() -> Void)) {
let group = DispatchGroup()
for item in items {
if item.media_type == 1 {
guard let url = URL(string: (item.image_versions2?.candidates?.first!.url)!) else { break }
group.enter()
mediaToStorageDistribution(withImageUrl: url,
videoUrl: nil,
mediaType: .jpg,
takenAt: item.taken_at,
success: { group.leave() },
failure: {
print("error")
group.leave()
})
}
}
group.notify(queue: .main) {
self.collectionView.reloadData()
}
}
Related
Picture an image loading function with a closure completion. Let's say it returns a token ID that you can use to cancel the asynchronous operation if needed.
#discardableResult
func loadImage(url: URL, completion: #escaping (Result<UIImage, Error>) -> Void) -> UUID? {
if let image = loadedImages[url] {
completion(.success(image))
return nil
}
let id = UUID()
let task = URLSession.shared.dataTask(with: url) { data, response, error in
defer {
self.requests.removeValue(forKey: id)
}
if let data, let image = UIImage(data: data) {
DispatchQueue.main.async {
self.loadedImages[url] = image
completion(.success(image))
}
return
}
if let error = error as? NSError, error.code == NSURLErrorCancelled {
return
}
//TODO: Handle response errors
print(response as Any)
completion(.failure(.loadingError))
}
task.resume()
requests[id] = task
return id
}
func cancelRequest(id: UUID) {
requests[id]?.cancel()
requests.removeValue(forKey: id)
print("ImageLoader: cancelling request")
}
How would we accomplish this (elegantly) with swift concurrency? Is it even possible or practical?
I haven't done much testing on this, but I believe this is what you're looking for. It allows you to simply await an image load, but you can cancel using the URL from somewhere else. It also merges near-simultaneous requests for the same URL so you don't re-download something you're in the middle of.
actor Loader {
private var tasks: [URL: Task<UIImage, Error>] = [:]
func loadImage(url: URL) async throws -> UIImage {
if let imageTask = tasks[url] {
return try await imageTask.value
}
let task = Task {
// Rather than removing here, you could skip that and this would become a
// cache of results. Making that correct would take more work than the
// question asks for, so I won't go into it
defer { tasks.removeValue(forKey: url) }
let data = try await URLSession.shared.data(from: url).0
guard let image = UIImage(data: data) else {
throw DecodingError.dataCorrupted(.init(codingPath: [],
debugDescription: "Invalid image"))
}
return image
}
tasks[url] = task
return try await task.value
}
func cancelRequest(url: URL) {
// Remove, and cancel if it's removed
tasks.removeValue(forKey: url)?.cancel()
print("ImageLoader: cancelling request")
}
}
Calling it looks like:
let image = try await loader.loadImage(url: url)
And you can cancel a request if it's still pending using:
loader.cancelRequest(url: url)
A key lesson here is that it is very natural to access task.value multiple times. If the task has already completed, then it will just return immediately.
Return a task in a tuple or other structure.
In the cases where you don't care about the ID, do this:
try await imageTask(url: url).task.value
private var requests: [UUID: Task<UIImage, Swift.Error>] = [:]
func imageTask(url: URL) -> (id: UUID?, task: Task<UIImage, Swift.Error>) {
switch loadedImages[url] {
case let image?: return (id: nil, task: .init { image } )
case nil:
let id = UUID()
let task = Task {
defer { requests[id] = nil }
guard let image = UIImage(data: try await URLSession.shared.data(from: url).0)
else { throw Error.loadingError }
try Task.checkCancellation()
Task { #MainActor in loadedImages[url] = image }
return image
}
requests[id] = task
return (id: id, task: task)
}
}
Is it even possible or practical?
Yes to both.
As I say in a comment, I think you may be missing the fact that a Task is an object you can retain and later cancel. Thus, if you create an architecture where you apply an ID to a task as you ask for the task to start, you can use that same ID to cancel that task before it has returned.
Here's a simple demonstration. I've deliberately written it as Playground code (though I actually developed it in an iOS project).
First, here is a general TimeConsumer class that wraps a single time-consuming Task. We can ask for the task to be created and started, but because we retain the task, we can also cancel it midstream. It happens that my task doesn't return a value, but that's neither here nor there; it could if we wanted.
class TimeConsumer {
var current: Task<(), Error>?
func consume(seconds: Int) async throws {
let task = Task {
try await Task.sleep(for: .seconds(seconds))
}
current = task
_ = await task.result
}
func cancel() {
current?.cancel()
}
}
Now then. In front of my TimeConsumer I'll put a TaskVendor actor. A TimeConsumer represents just one time-consuming task, but a TaskVendor has the ability to maintain multiple time-consuming tasks, identifying each task with an identifier.
actor TaskVendor {
private var tasks = [UUID: TimeConsumer?]()
func giveMeATokenPlease() -> UUID {
let uuid = UUID()
tasks[uuid] = nil
return uuid
}
func beginTheTask(uuid: UUID) async throws {
let consumer = TimeConsumer()
tasks[uuid] = consumer
try await consumer.consume(seconds: 10)
tasks[uuid] = nil
}
func cancel(uuid: UUID) {
tasks[uuid]??.cancel()
tasks[uuid] = nil
}
}
That's all there is to it! Observe how TaskVendor is configured. I can do three things: I can ask for a token (really my actual TaskVendor needn't bother doing this, but I wanted to centralize everything for generality); I can start the task with that token; and, optionally, I can cancel the task with that token.
So here's a simple test harness. Here we go!
let vendor = TaskVendor()
func test() async throws {
let uuid = await vendor.giveMeATokenPlease()
print("start")
Task {
try await Task.sleep(for: .seconds(2))
print("cancel?")
// await vendor.cancel(uuid: uuid)
}
try await vendor.beginTheTask(uuid: uuid)
print("finish")
}
Task {
try await test()
}
What you will see in the console is:
start
[two seconds later] cancel?
[eight seconds after that] finish
We didn't cancel anything; the word "cancel?" signals the place where our test might cancel, but we didn't, because I wanted to prove to you that this is working as we expect: it takes a total of 10 seconds between "start" and "finish", so sure enough, we are consuming the expected time fully.
Now uncomment the await vendor.cancel line. What you will see now is:
start
[two seconds later] cancel?
[immediately!] finish
We did it! We made a cancellable task vendor.
I'm including one possible answer to the question, for the benefit of others. I'll leave the question in place in case someone has another take on it.
The only way that I know of having a 'one-shot' async method that would return a token before returning the async result is by adding an inout argument:
func loadImage(url: URL, token: inout UUID?) async -> Result<UIImage, Error> {
token = UUID()
//...
}
Which we may call like this:
var requestToken: UUID? = nil
let result = await imageLoader.loadImage(url: url, token: &requestToken)
However, this approach and the two-shot solution by #matt both seem fussy, from the api design standpoint. Of course, as he suggests, this leads to a bigger question: How do we implement cancellation with swift concurrency (ideally without too much overhead)? And indeed, using tasks and wrapper objects seems unavoidable, but it certainly seems like a lot of legwork for a fairly simple pattern.
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"
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.
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.
I tried using DispatchQueue and DispatchGroup but its still asynchronous, I also tried both dispatchQueue.Async and dispathQueue.sync and neither have worked.
myFunc is a function called in the init() which itself calls 2 functions, getArrOneData() annd getArrTwoData([ArrOneType]).
The first function downloads ArrOneType data from firestore and returns an array which is initialised to a field.
The second function uses the downloaded data/field from the first function to initialise a field in ArrTwoType while simultaneously downloading other relevent ArrTwoType data from firestore to return as an array.
So the Problem is its still asynchronous. How do I use DispatchQueue and DispatchGroup correctly in this scenario?
thanks
note: get arrTwoData is mainly psuedocode
func myFunc(){
let group = DispatchGroup()
let dispatchQueue = DispatchQueue.global(qos: .default)
group.enter()
dispatchQueue.sync {
self.arrOne = self.getArrOneData()//getArrOneData gets data from firestore
group.leave()
}
dispatchQueue.sync {
group.enter()
self.arrTwo = self.getArrTwoData(inputArr: self.arrOne)//getArrTwoData gets data from firestore
group.leave()
}
}
//ArrOneType is an array field in ArrTwoType
func getArrTwoData(inputArr: [ArrOneType]) -> [ArrTwoType]{
var result = [ArrTwoType]()
//retrieving data from firestore, code excluded...
for document in querySnapshot!.documents {
let data = document.data()
name = data["Name"] as? String ?? "Name Unknown"
//returns an array which is a subset of inputArr filtered by name, however
//I dont think the inputArr is populated at this point when I run the program
var field2:[ArrOneType] = someFunc(name, inputArr)
var x = ArrTwoType(name: name, field2: field2)
result.append(x)
}
}
}
return result
}
If you have two requests, one which uses the response from one to prepare the next, the idea is to use #escaping completion handler closures for all asynchronous methods, e.g.
func getArrayOneData(completion: #escaping (Result<[ArrayOneType], Error>) -> Void) {
someAsynchronousMethod {
let values: [ArrayOneType] = ...
completion(.success(values))
}
}
func getArrayTwoData(for typeOneValues: [ArrayOneType], completion: #escaping (Result<[ArrayTwoType], Error>) -> Void) {
someAsynchronousMethod(for: typeOneValues) {
let values: [ArrayTwoType] = ...
completion(.success(values))
}
}
Then you can do things like:
func getEverything(completion: #escaping (Result<[ArrayTwoType], Error>) -> Void) {
getArrayOneData { result in
switch result {
case .failure(let error):
print(error)
case .success(let typeOneValues):
getArrayTwoData(for: typeOneValues) { result in
switch result {
case .failure(let error):
print(error)
case .success(let typeTwoValues):
completion(.success(typeTwoValues))
}
}
}
}
}
Note, no dispatch groups needed. Having added completion handlers to the asynchronous methods, we can just call the second method from the completion handler of the first.
I have using a dispatch group wait() that block my a for loop from completing the code until a set of urlsession tasks (in another loop with completion handler) to be completed before appending new element to my array
the current code will finish the first loop before the second loop of urlClass.selectfoodURL is completed
I want to append the array in meal history after my urlfood for loop is completed
on of the problem in my approach of using dispatch groups is the wait(), when my select food is called the urlsession stuck and doesn’t complete with group.wait
func userSnackHistoryArray() {
let group = DispatchGroup()
let Arrays // array of dictionary
for array in Arrays {
var generateMeal = MealDetails() // struct type
do {
let aa = try JSONDecoder().decode(userSnack.self, from: array)
generateMeal.names = convertToJsonFile.type
for name in generateMeal.names!{
group.enter()
urlClass.selectfoodURL(foodName: name){ success in
generateMeal.units!.append(allVariables.selectedUnit)
group.leave()
}
}
// my select food is called but the urlsession stuck and doesnt complete with group.wait is active
// group.wait()
mealHistory.append(generateMeal)
} catch { }
}
group.notify(queue: .main){
print("complete")
}
}
I have shortened my code to focus on the problem ,, I can split my code into two functions and solve the problem , but I want to use only one function
any suggestions or ideas ?
Rather than waiting, you should just create a local array of values to be added, and then add them when it’s done:
func retrieveSnacks() {
var snacksToAdd: [Snack] = []
let group = DispatchGroup()
...
for url in urls {
group.enter()
fetchSnack(with: url) { result in
dispatchPrecondition(condition: .onQueue(.main)) // note, I’m assuming that this closure is running on the main queue; if not, dispatch this appending of snacks (and `leave` call) to the main queue
if case .success(let snack) = result {
snacksToAdd.append(snack)
}
group.leave()
}
}
// when all the `leave` calls are called, only then append the results
group.notify(queue: .main) {
self.snacks += snacksToAdd
// trigger UI update, or whatever, here
}
}
Note, the above does not assure that the objects are added in the original order. If you need that, you can use a dictionary to build the temporary results and then append the results in sorted order:
func retrieveSnacks() {
var snacksToAdd: [URL: Snack] = [:]
let group = DispatchGroup()
...
for url in urls {
group.enter()
fetchSnack(with: url) { result in
if case .success(let snack) = result {
snacksToAdd[url] = snack
}
group.leave()
}
}
group.notify(queue: .main) {
let sortedSnacks = urls.compactMap { snacksToAdd[$0] }
self.snacks += sortedSnacks
// trigger UI update, or whatever, here
}
}
Finally, I might suggest adopting a completion handler pattern:
func retrieveSnacks(completion: #escaping ([Snack]) -> Void) {
var snacksToAdd: [URL: Snack] = [:]
let group = DispatchGroup()
...
for url in urls {
group.enter()
fetchSnack(with: url) { result in
if case .success(let snack) = result {
snacksToAdd[url] = snack
}
group.leave()
}
}
group.notify(queue: .main) {
let sortedSnacks = urls.compactMap { snacksToAdd[$0] }
completion(sortedSnacks)
}
}
retrieveSnacks { addedSnacks in
self.snacks += addedSnacks
// update UI here
}
This pattern ensures that you don’t entangle your network-related code with your UI code.
I apologize that the above is somewhat refactored from your code snippet, but there wasn’t enough there for me to illustrate what precisely it would look like. But hopefully the above illustrates the pattern and you can see how you’d apply it to your code base. So, don’t get lost in the details, but focus on the basic pattern of building records to be added in a local variable and only update the final results in the .notify block.
FWIW, this is the method signature for the method that the above snippets are using to asynchronously fetch the objects in question.
func fetchSnack(with url: URL, completion: #escaping (Result<Snack, Error>) -> Void) {
...
// if async fetch not successful
DispatchQueue.main.async {
completion(.failure(error))
}
// if successful
DispatchQueue.main.async {
completion(.success(snack))
}
}
In my model have function to fetch data which expects completion handler as parameter:
func fetchMostRecent(completion: (sortedSections: [TableItem]) -> ()) {
self.addressBook.loadContacts({
(contacts: [APContact]?, error: NSError?) in
// 1
if let unwrappedContacts = contacts {
for contact in unwrappedContacts {
// handle constacts
...
self.mostRecent.append(...)
}
}
// 2
completion(sortedSections: self.mostRecent)
})
}
It's calling another function which does asynchronous loading of contacts, to which I'm forwarding my completion
The call of fetchMostRecent with completion looks like this:
model.fetchMostRecent({(sortedSections: [TableItem]) in
dispatch_async(dispatch_get_main_queue()) {
// update some UI
self.state = State.Loaded(sortedSections)
self.tableView.reloadData()
}
})
This sometimes it works, but very often the order of execution is not the way as I would expect. Problem is, that sometimes completion() under // 2 is executed before scope of if under // 1 was finished.
Why is that? How can I ensure that execution of // 2 is started after // 1?
A couple of observations:
It will always execute what's at 1 before 2. The only way you'd get the behavior you describe is if you're doing something else inside that for loop that is, itself, asynchronous. And if that were the case, you'd use a dispatch group to solve that (or refactor the code to handle the asynchronous pattern). But without seeing what's in that for loop, it's hard to comment further. The code in the question, alone, should not manifest the problem you describe. It's got to be something else.
Unrelated, you should note that it's a little dangerous to be updating model objects inside your asynchronously executing for loop (assuming it is running on a background thread). It's much safer to update a local variable, and then pass that back via the completion handler, and let the caller take care of dispatching both the model update and the UI updates to the main queue.
In comments, you mention that in the for loop you're doing something asynchronous, and something that must be completed before the completionHandler is called. So you'd use a dispatch group to do ensure this happens only after all the asynchronous tasks are done.
Note, since you're doing something asynchronous inside the for loop, not only do you need to use a dispatch group to trigger the completion of these asynchronous tasks, but you probably also need to create your own synchronization queue (you shouldn't be mutating an array from multiple threads). So, you might create a queue for this.
Pulling this all together, you end up with something like:
func fetchMostRecent(completionHandler: ([TableItem]?) -> ()) {
addressBook.loadContacts { contacts, error in
var sections = [TableItem]()
let group = dispatch_group_create()
let syncQueue = dispatch_queue_create("com.domain.app.sections", nil)
if let unwrappedContacts = contacts {
for contact in unwrappedContacts {
dispatch_group_enter(group)
self.someAsynchronousMethod {
// handle contacts
dispatch_async(syncQueue) {
let something = ...
sections.append(something)
dispatch_group_leave(group)
}
}
}
dispatch_group_notify(group, dispatch_get_main_queue()) {
self.mostRecent = sections
completionHandler(sections)
}
} else {
completionHandler(nil)
}
}
}
And
model.fetchMostRecent { sortedSections in
guard let sortedSections = sortedSections else {
// handle failure however appropriate for your app
return
}
// update some UI
self.state = State.Loaded(sortedSections)
self.tableView.reloadData()
}
Or, in Swift 3:
func fetchMostRecent(completionHandler: #escaping ([TableItem]?) -> ()) {
addressBook.loadContacts { contacts, error in
var sections = [TableItem]()
let group = DispatchGroup()
let syncQueue = DispatchQueue(label: "com.domain.app.sections")
if let unwrappedContacts = contacts {
for contact in unwrappedContacts {
group.enter()
self.someAsynchronousMethod {
// handle contacts
syncQueue.async {
let something = ...
sections.append(something)
group.leave()
}
}
}
group.notify(queue: .main) {
self.mostRecent = sections
completionHandler(sections)
}
} else {
completionHandler(nil)
}
}
}