Swift:按顺序执行多个异步请求。如何等待上一个请求完成？

Question

作为我应用中身份验证过程的一部分，用户可以使用他们的 Facebook 帐户登录 - 我正在使用 Facebook iOS SDK 来处理这个过程。身份验证完成后，我向 Facebook graph api 发出请求以获取用户配置文件数据(这是第一个异步请求)。第二个异步请求也是向 Facebook graph api 请求安装了该应用程序的用户好友列表。

此函数中的最后一个也是第三个请求向我开发的 API 发出异步 POST 请求，以发布从 Facebook 收集的所有数据。最后，一旦完成，用户就可以进入该应用程序。然而事实并非如此，似乎 Facebook 请求在向 API 发出 POST 请求之前没有完成，因此它会推送空白数据。我不介意对 Facebook 的前 2 个请求按什么顺序完成，但是我需要在允许用户进入应用程序之前将数据成功发布到 API。我试过使用信号量和调度组，但是当查看控制台时，事情没有按正确的顺序运行，我可以从 API 数据库中看到正在插入空值。

认证 Controller

 // Successful login, fetch faceook profile
            let group = DispatchGroup()
            group.enter()
            // Redirect to tab bar controller should not happen until fetchProfile() has finished 
            // Redirect should not happen if fetchProfile() errors
            self.fetchProfile() 
            group.leave()

            // Redirect to tab bar controller
            let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
            let tabBarController = storyboard.instantiateViewController(withIdentifier: "tabBarController") as! UITabBarController
            self.present(tabBarController, animated: true, completion: nil)

更新的 Facebook Fetch 配置文件

 // Facebook Profile Request
    func fetchProfile() {

    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let managedContext = appDelegate.managedObjectContext
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "User")
    let user = appDelegate.user
    var facebookFriends = [String?]()

    do {
        let results = try managedContext?.fetch(fetchRequest)
        fetchedUser = results![0] as? NSManagedObject
    }
    catch {
        print("Error fetching User entity")
        return
    }


    let group = DispatchGroup()
    print("Starting Step 1")
    group.enter()

    // Facebook Profile
    let parameters = ["fields": "id, email, first_name, last_name, picture.width(500).height(500), birthday, gender"]
    FBSDKGraphRequest(graphPath: "me", parameters: parameters).start { (connection, result, error) -> Void in

        if error != nil {
            print(error)
            return
        }

        let result = result as? NSDictionary

        if let providerID = result?["id"] as? String {
            user.provider_id = providerID
            self.fetchedUser!.setValue(providerID, forKey: "provider_id")
        }

        if let firstName = result?["first_name"] as? String {
            user.first_name = firstName
            self.fetchedUser!.setValue(firstName, forKey: "first_name")
        }

        if let lastName = result?["last_name"] as? String {
            user.last_name = lastName
            self.fetchedUser!.setValue(lastName, forKey: "last_name")
        }

        if let email = result?["email"] as? String {
            user.email = email
            self.fetchedUser!.setValue(email, forKey: "email")
        }

        if let picture = result?["picture"] as? NSDictionary, let data = picture["data"] as? NSDictionary, let url = data["url"] as? String {
            user.avatar = url
            self.fetchedUser!.setValue(url, forKey: "avatar")
        }

        if let birthday = result?["birthday"] as? String {
            user.birthday = birthday
            self.fetchedUser!.setValue(sqlDate, forKey: "birthday")
        }

        if var gender = result?["gender"] as? String {
            user.gender = gender
            self.fetchedUser!.setValue(gender, forKey: "gender")
        }

        group.leave()
        print("Step 1 Done")

        group.enter()
        print("Starting Step 2")

        // Facebook Friends Request
        FBSDKGraphRequest(graphPath: "me/friends", parameters: ["fields": "id, first_name, last_name, picture"]).start { (connection, result, error) -> Void in

            if error != nil {
                print(error)
                return
            }

            let result = result as! [String:AnyObject]

            for friend in result["data"] as! [[String:AnyObject]] {
                let id = friend["id"] as! String
                facebookFriends.append(id)
            }

            group.leave()
            print("Step 2 Done")

            // User POST Request
            var dictionary = self.fetchedUser?.dictionaryWithValues(forKeys: ["provider", "provider_id", "first_name", "last_name", "email", "avatar", "birthday", "gender"])

            if facebookFriends.count > 0 {
                dictionary?["friends"] = facebookFriends
            }

            let data = NSMutableDictionary()
            data.setValuesForKeys(dictionary!)

            //let semaphore = DispatchSemaphore(value: 2)
            group.enter()
            print("Starting Step 3")

            do {
                // Here "jsonData" is the dictionary encoded in JSON data
                let jsonData = try JSONSerialization.data(withJSONObject: data, options: .prettyPrinted)

                // Here "decoded" is of type `Any`, decoded from JSON data
                let decoded = try JSONSerialization.jsonObject(with: jsonData, options: [])

                // Final dict
                if let dictFromJSON = decoded as? [String:String] {

                    let endpoint = "http://endpoint.com/user"
                    let url = URL(string: endpoint)
                    let session = URLSession.shared
                    var request = URLRequest(url: url!)

                    request.httpMethod = "POST"
                    request.httpBody = try JSONSerialization.data(withJSONObject: dictFromJSON, options: [])
                    request.addValue("application/json", forHTTPHeaderField: "Accept")
                    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
                    session.dataTask(with: request, completionHandler: { (data, response, error) -> Void in

                        if error != nil {
                            //semaphore.signal()
                            group.leave()
                            print(error)
                            return
                        }

                        do {
                            // Save response
                            let json = try(JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? [String: AnyObject])

                            if let userID = json?["user_id"] {
                                user.user_id = userID as? Int
                                self.fetchedUser!.setValue(userID, forKey: "user_id")
                            }

                            if let friends = json?["friends"] , !(friends is NSNull){
                                user.friends = friends as? [String]
                                self.fetchedUser!.setValue(friends, forKey: "friends")
                            }

                            group.leave()
                            //semaphore.signal()

                        } catch let jsonError {
                            print(jsonError)
                            return
                        }

                    }).resume()

                }
            } catch {
                print(error.localizedDescription)
            }

            // Wait to async task to finish before moving on
            //_ = semaphore.wait(timeout: DispatchTime.distantFuture)
            print("Step 3 Done")
        }
    }
}

Answer 1

将每个闭包之后的代码移动到闭包本身内部，以便它在运行之前等到它之前的代码:

// Facebook Profile Request
func fetchProfile() {

    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let managedContext = appDelegate.managedObjectContext
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "User")
    let user = appDelegate.user
    var facebookFriends = [String?]()

    do {
        let results = try managedContext?.fetch(fetchRequest)
        fetchedUser = results![0] as? NSManagedObject
    }
    catch {
        print("Error fetching User entity")
        return
    }


    let group = DispatchGroup()
    print("Starting Step 1")
    group.enter()

    // Facebook Profile
    let parameters = ["fields": "id, email, first_name, last_name, picture.width(500).height(500), birthday, gender"]
    FBSDKGraphRequest(graphPath: "me", parameters: parameters).start { (connection, result, error) -> Void in

        if error != nil {
            print(error)
            return
        }

        let result = result as? NSDictionary

        if let providerID = result?["id"] as? String {
            user.provider_id = providerID
            self.fetchedUser!.setValue(providerID, forKey: "provider_id")
        }

        if let firstName = result?["first_name"] as? String {
            user.first_name = firstName
            self.fetchedUser!.setValue(firstName, forKey: "first_name")
        }

        if let lastName = result?["last_name"] as? String {
            user.last_name = lastName
            self.fetchedUser!.setValue(lastName, forKey: "last_name")
        }

        if let email = result?["email"] as? String {
            user.email = email
            self.fetchedUser!.setValue(email, forKey: "email")
        }

        if let picture = result?["picture"] as? NSDictionary, let data = picture["data"] as? NSDictionary, let url = data["url"] as? String {
            user.avatar = url
            self.fetchedUser!.setValue(url, forKey: "avatar")
        }

        if let birthday = result?["birthday"] as? String {
            user.birthday = birthday
            self.fetchedUser!.setValue(sqlDate, forKey: "birthday")
        }

        if var gender = result?["gender"] as? String {
            user.gender = gender
            self.fetchedUser!.setValue(gender, forKey: "gender")
        }

        group.leave()
        print("Step 1 Done")

        group.enter()
        print("Starting Step 2")

        // Facebook Friends Request
        FBSDKGraphRequest(graphPath: "me/friends", parameters: ["fields": "id, first_name, last_name, picture"]).start { (connection, result, error) -> Void in

            if error != nil {
                print(error)
                return
            }

            let result = result as! [String:AnyObject]

            for friend in result["data"] as! [[String:AnyObject]] {
                let id = friend["id"] as! String
                facebookFriends.append(id)
            }

            group.leave()
            print("Step 2 Done")

            // User POST Request
            var dictionary = self.fetchedUser?.dictionaryWithValues(forKeys: ["provider", "provider_id", "first_name", "last_name", "email", "avatar", "birthday", "gender"])

            if facebookFriends.count > 0 {
                dictionary?["friends"] = facebookFriends
            }

            let data = NSMutableDictionary()
            data.setValuesForKeys(dictionary!)

            //let semaphore = DispatchSemaphore(value: 2)
            group.enter()
            print("Starting Step 3")

            do {
                // Here "jsonData" is the dictionary encoded in JSON data
                let jsonData = try JSONSerialization.data(withJSONObject: data, options: .prettyPrinted)

                // Here "decoded" is of type `Any`, decoded from JSON data
                let decoded = try JSONSerialization.jsonObject(with: jsonData, options: [])

                // Final dict
                if let dictFromJSON = decoded as? [String:String] {

                    let endpoint = "http://endpoint.com/user"
                    let url = URL(string: endpoint)
                    let session = URLSession.shared
                    var request = URLRequest(url: url!)

                    request.httpMethod = "POST"
                    request.httpBody = try JSONSerialization.data(withJSONObject: dictFromJSON, options: [])
                    request.addValue("application/json", forHTTPHeaderField: "Accept")
                    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
                    session.dataTask(with: request, completionHandler: { (data, response, error) -> Void in

                        if error != nil {
                            //semaphore.signal()
                            group.leave()
                            print(error)
                            return
                        }

                        do {
                            // Save response
                            let json = try(JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? [String: AnyObject])

                            if let userID = json?["user_id"] {
                                user.user_id = userID as? Int
                                self.fetchedUser!.setValue(userID, forKey: "user_id")
                            }

                            if let friends = json?["friends"] , !(friends is NSNull){
                                user.friends = friends as? [String]
                                self.fetchedUser!.setValue(friends, forKey: "friends")
                            }

                            group.leave()
                            //semaphore.signal()

                        } catch let jsonError {
                            print(jsonError)
                            return
                        }

                    }).resume()

                }
            } catch {
                print(error.localizedDescription)
            }

            // Wait to async task to finish before moving on
            //_ = semaphore.wait(timeout: DispatchTime.distantFuture)
            print("Step 3 Done")
        }
    }
}

解释:当你做异步网络请求时，闭包就是所谓的转义，这意味着它们在函数返回后运行。例如，FBSDKGraphRequest.start 采用转义闭包，保存它，返回，在它返回后，在请求完成后运行闭包。这是故意的。否则，它将是同步的并阻塞您的代码，这将导致您的应用程序卡住(除非您使用 GCD 自己异步运行代码)。

TL;DR 在诸如 FBSDKGraphRequest.start 之类的函数返回后调用闭包，导致下一组在其完成之前开始。这可以通过放置它们使它们一个接一个地运行来解决。