Thanks for introducing dispatch group concept, its cool.

dopee broooo

El profesor!

Brian, is there an elegant way to include the dispatchGroup inside the fetchAppGroup() helper as an optional? Then perform the dispatchGroup.enter() and dispatchGroup.leave() inside the fetchAppGroup(). I tried to do this, but trying to unwrap an optional using guard let statement, it requires to return, so the code stops from executing in case we don't want to use dispatchGroups. What i am trying to achieve here is: func fetchAppGroup(dispatchGroup: DispatchGroup? ,urlString: String, completion: @escaping (AppGroup?, Error?) -> Void) { var hasDispatchGroup = true guard let dispatchGroup = dispatchGroup else { hasDispatchGroup = false // this will throw an error since the compiler will ask you to either return or throw an error } if(hasDispatchGroup) { dispatchGroup.enter() } ..... and inside the "do" block: if(hasDispatchGroup) { dispatchGroup.leave() } } I am trying to make the method as flexible as possible to achieve DRY.

All, any good suggestions/books to learn more about DispatchGroup() please? Thank you!

I used eumn in the Service.swift just to make the code a little cleaner and more dynamic enum FeedType: String { case TopFree = "top-free" case TopGrossing = "top-grossing" case NewGames = "new-games-we-love" } func createUrlForFeedType(type:FeedType) -> URL? { let urlString = "https://rss.itunes.apple.com/api/v1/us/ios-apps/\(type.rawValue)/all/50/explicit.json" return URL(string: urlString) } func fetchByType(type: FeedType , completion: @escaping (AppGroup?, Error?) -> ()) { guard let url = createUrlForFeedType(type: type) else { return } URLSession.shared.dataTask(with: url) { (data, resp, err) in if let err = err { print ("Failed to fetch", err) completion(nil, err) return } guard let data = data else { return } do { let appGroup = try JSONDecoder().decode(AppGroup.self, from: data) completion(appGroup, nil) } catch { print ("faild to decode error", error) completion(nil,error) }}.resume() }

Hi! Mine is taking a long time to appear.Actually I am getting an error at console http load failed error code -999

Why do we put dispatchGroup.leave() inside the Service function's code? How come group1 and group2 variables are being initialized despite these being set after dispatchGroup.leave()?

hi Brian why you add .leave() in the begin of completion , not the end of it

Hi Brian, thanks for this video. Came here looking for one thing and walked away knowing about DispatchGroups which solved one of my other problems at the same time! Excellent stuff! Keep up the good work, it's much appreciated! -- G

Something you might find interesting in the fetchData function is the use of compactMap in dispatchGroup.notify: var groups = [AppGroup]() func fetchData() { var group1, group2, group3: AppGroup? let dispatchGroup = DispatchGroup() ... dispatchGroup.notify(queue: .main) { self.groups = [group1, group2, group3].compactMap {$0} // compact out any nil valued group // The above statement has the same effect as // if let group = group1 { // self.groups.append(group) // } // if let group = group2 { // self.groups.append(group) // } // if let group = group3 { // self.groups.append(group) // } self.collectionView.reloadData() } }

hey brian why we need to create group1, group2 and group3 we can direct in every fetch API append them to groups and dispatcherGroup.notify reload collection view

$ work for me. var urlStrings = ["url1", "url2", "url3", "url4", "url5"] // FIXME: fetchedData() fileprivate func fetchedData() { // Use for Fetched data together DispatchGroup() let disPatchGroup = DispatchGroup() // Looping for urlString in urlStrings { disPatchGroup.enter() // start include APIService.shared.fetchedAppGroup(urlString: urlString) { (appGroup, err) in disPatchGroup.leave() // stop include if let error = err { print("Have Error", error) return } guard let appGroup = appGroup else { return } self.groups.append(appGroup) } } disPatchGroup.notify(queue: DispatchQueue.main) { _ = self.groups.compactMap({$0}) self.collectionView.reloadData() } }

Don't know if I'm using this right but can use DispatchSemaphores to keep the order fileprivate func fetchGroup() { let fetchDisPatchSemaphore = DispatchSemaphore(value: 0) for urlString in urlStrings { Service.shared.fetchAppGroup(url: urlString) { (appGroup, err) in fetchDisPatchSemaphore.signal() if let err = err { print("Failed to fetch games data", err) return } self.editorsGameGroup = appGroup self.appGroups.append(appGroup) } fetchDisPatchSemaphore.wait() DispatchQueue.main.async { self.collectionView.reloadData() } }