DispatchGroup 等待 Firestore 查询完成

Question

我在单个用户 tapGesture 中运行多个 Firestore 查询，这要求我确保应用程序中同时进行的 Firestore 查询最少甚至没有运行。我已经阅读了关于这个问题的多个答案（），但我的查询并没有像我预期的那样运行在所需的序列中。

非常感谢您帮助指导我使用 DispatchGroup 来确保代码和查询的逻辑顺序。

我想确保 Firestore 查询 在函数 checkAndCreateUserWorkoutProfile 在 DispatchGroup.notify(...) 中的 2 个打印语句之前完成，从“Firestore”开始序列”方法启动。相反，查询在两个打印语句启动后完成。

下面是我的 Xcode 调试器的代码和屏幕截图。如屏幕截图所示，问题是使用“DispatchGroup.notify(...)”不会等到函数 checkAndCreateUserWorkoutProfile 中的 Firestore 查询已完成。

alert.addAction(UIAlertAction(title: "Select", style: .default, handler: { [self]_ in
            
            let dispatchGroup = DispatchGroup()
            
            dispatchGroup.enter()
            
            DispatchQueue.main.async {
                print("Firestore Sequence 2 Initiated")
                startActivityIndicator()
                
                if let user = Auth.auth().currentUser {
                    let userID = user.uid
                    
                    db.collection("user").whereField("author_uid", isEqualTo: userID).getDocuments { snapshot, error in
                        
                        if error == nil && snapshot != nil {
                            
                            for document in snapshot!.documents {
                            
                            let docID = document.documentID
                            
                            db.collection("user")
                                .document(docID)
                                .setData(["selectedWorkoutID" : workoutRow.workoutId], merge: true)
                                    
                            print("Firestore Sequence 2 Success - user selectedWorkoutID updated")
                                
                            }
                        }
                        dispatchGroup.leave()
                        
                        if let user = Auth.auth().currentUser {
                            let userID = user.uid
                            checkAndCreateUserWorkoutProfile(selectedWorkout: workoutRow, userID: userID)
                        }
                    }
                }
            }

            dispatchGroup.notify(queue: .main) {
                print("Firestore Sequence 4 Initiated")
                print("Firestore Sequence 5 Initiated - Create/Read User Specific Dayprogram data")
                }

        }))

func checkAndCreateUserWorkoutProfile(selectedWorkout: Workout, userID: String) {

        print("Firestore Sequence 3 Initiated - createORread user specific workout profile")
        
        let dispatchGroup = DispatchGroup()
        
        dispatchGroup.enter()
        
        db.collection("Workout")
            .whereField("workoutId", isEqualTo: selectedWorkout.workoutId)
            .whereField("userID", isEqualTo: userID)
            .getDocuments() { (querySnapshot, err) in
                
                if querySnapshot?.count == 0 {

                    var ref: DocumentReference? = nil
                  
                    ref = self.db.collection("Workout").addDocument(data:
                        [
                        "author_uid": selectedWorkout.author_uid!,
                        "workoutId": selectedWorkout.workoutId,
                        "userID": userID
                        ])

                     { err in
                        if let err = err {
                            print("Error adding user specific workout profile: \(err)")
                            dispatchGroup.leave()
                        } else {
                            print("Firestore Sequence 3 Success - User specific workout profile added/created with ID: \(ref!.documentID)")
                            dispatchGroup.leave()
                        }
                    }
                    
                    
                } 
                
            }

        }

根据@Kiril S. 的回答，代码已更正如下所示。

class WorkoutViewController: UIViewController {

let dispatchGroup = DispatchGroup()


alert.addAction(UIAlertAction(title: "Select", style: .default, handler: { [self]_ in
            
            dispatchGroup.enter()
            
            DispatchQueue.main.async {
                print("Firestore Sequence 2 Initiated")
                startActivityIndicator()
                
                if let user = Auth.auth().currentUser {
                    let userID = user.uid
                    
                    db.collection("user").whereField("author_uid", isEqualTo: userID).getDocuments { snapshot, error in
                        
                        if error == nil && snapshot != nil {
                            
                            for document in snapshot!.documents {
                            
                            let docID = document.documentID
                            
                            db.collection("user")
                                .document(docID)
                                .setData(["selectedWorkoutID" : workoutRow.workoutId], merge: true)
                                    
                            print("Firestore Sequence 2 Success - user selectedWorkoutID updated")
                                
                            }
                        }
                        
                        
                        if let user = Auth.auth().currentUser {
                            let userID = user.uid
                            checkAndCreateUserWorkoutProfile(selectedWorkout: workoutRow, userID: userID)
                         dispatchGroup.leave()
                        }
                    }
                }
            }

            dispatchGroup.notify(queue: .main) {
                print("Firestore Sequence 4 Initiated")
                print("Firestore Sequence 5 Initiated - Create/Read User Specific Dayprogram data")
                }

        }))

func checkAndCreateUserWorkoutProfile(selectedWorkout: Workout, userID: String) {

        print("Firestore Sequence 3 Initiated - createORread user specific workout profile")
        
        dispatchGroup.enter()
        
        db.collection("Workout")
            .whereField("workoutId", isEqualTo: selectedWorkout.workoutId)
            .whereField("userID", isEqualTo: userID)
            .getDocuments() { (querySnapshot, err) in
                
                if querySnapshot?.count == 0 {

                    var ref: DocumentReference? = nil
                  
                    ref = self.db.collection("Workout").addDocument(data:
                        [
                        "author_uid": selectedWorkout.author_uid!,
                        "workoutId": selectedWorkout.workoutId,
                        "userID": userID
                        ])

                     { err in
                        if let err = err {
                            print("Error adding user specific workout profile: \(err)")
                            self.dispatchGroup.leave()
                        } else {
                            print("Firestore Sequence 3 Success - User specific workout profile added/created with ID: \(ref!.documentID)")
                            self.dispatchGroup.leave()
                        }
                    }     
                }      
            }
        }

Xcode 控制台显示 codes/queries 的更正序列。

Answer 1

所以基本上现在发生了什么：

警报行动开始
创建let dispatchGroup = DispatchGroup()
进入那个组
离开小组 ==> dispatchGroup.notify 解雇
checkAndCreateUserWorkoutProfile启动，创建自己的调度组等等（已经错了，无所谓）

所以你有两个问题：

您必须有 1 个调度组，而不是每个函数单独的组。所以将 let dispatchGroup = DispatchGroup() 移动到 class 成员级别，这样两个函数都在同一个调度组中。

而你的checkAndCreateUserWorkoutProfile需要进入调度组 before 之前的调用离开了它。所以顺序应该改为

if let user = Auth.auth().currentUser {
    let userID = user.uid
    checkAndCreateUserWorkoutProfile(selectedWorkout: workoutRow, userID: userID)
}
dispatchGroup.leave()

这样：

警报行动开始
进入定义为 class 成员的组
调用 checkAndCreateUserWorkoutProfile，它进入 相同的 调度组并开始其异步调用
离开群组的警报操作（但不会触发通知，因为 checkAndCreateUserWorkoutProfile 尚未离开群组）
checkAndCreateUserWorkoutProfile 最终离开小组 ==> dispatchGroup.notify 开火

DispatchGroup 等待 Firestore 查询完成

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