ios - 完成问题

Question

所以我正在尝试为我的初始 View Controller 实现一个 revelaingSplashView。我最初的 vc 有几个数据库调用似乎以奇怪的顺序完成。经过一些跟踪后，我将其缩小到完成 block 最后发生的特定功能。我在这里列出的

@objc func grabFriendsEvents(){
    print("Attempting to see where your friends are going")
    UserService.following { (user) in
        for following in user {
            print(following.username as Any)
            PostService.showFollowingEvent(for: following.uid, completion: { (event) in
                self.friendsEvents.append(event)
               // self.friendsEvents.append(contentsOf: event)
                // leave here
                self.friendsEvents = self.friendsEvents.removeDuplicates()
                print("ending in friends events")
                self.collectionView?.reloadData()
            })

        }  

    }
}

下面列出了我实现的服务方法中的两个支持函数。

static func following(for user: User = User.current, completion: @escaping ([User]) -> Void) {
    // 1
    let followingRef = Database.database().reference().child("following").child(user.uid)
    followingRef.observeSingleEvent(of: .value, with: { (snapshot) in
        // 2
        guard let followingDict = snapshot.value as? [String : Bool] else {
            return completion([])
        }

        // 3
        var following = [User]()
        let dispatchGroup = DispatchGroup()

        for uid in followingDict.keys {
            dispatchGroup.enter()

            show(forUID: uid) { user in
                if let user = user {
                    following.append(user)
                }

                dispatchGroup.leave()
            }
        }

        // 4
        dispatchGroup.notify(queue: .main) {
            completion(following)
        }
    })
}

第二个在这里列出

static func showFollowingEvent(for followerKey: String,completion: @escaping (Event) -> Void) {
    //getting firebase root directory
    let ref = Database.database().reference()

    ref.child("users").child(followerKey).child("Attending").observeSingleEvent(of: .value, with: { (attendingSnapshot) in
        print(attendingSnapshot)
        guard var eventKeys = attendingSnapshot.children.allObjects as? [DataSnapshot] else{return}
        for event in eventKeys{
            let dispatchGroup = DispatchGroup()
            dispatchGroup.enter()
            EventService.show(forEventKey: event.key, completion: { (event) in
                dispatchGroup.leave()
                completion(event!)
            })
        }
    }) { (err) in
        print("couldn't grab event info",err)

    }
}

现在 grabFriendsEvents 返回几次，具体取决于您拥有的 friend 数量和他们参加的事件。我试图找出知道什么时候完成的方法，这样我就可以忽略揭示性的观点。我认为调度组可能是可行的方法，但我不确定将它放在哪里。非常感谢任何帮助。

Answer 1

1. 在 grabFriendsEvents 中:PostService.showFollowingEvent 也是一个循环调用的异步函数，因此您也需要为其添加一个调度队列。

   let dispatchGroup = DispatchGroup()
   for following in user {
       print(following.username as Any)
       dispatchGroup.enter()
       PostService.showFollowingEvent(for: following.uid, completion: { (event) in
           self.friendsEvents.append(event)
          // self.friendsEvents.append(contentsOf: event)
           // leave here
           self.friendsEvents = self.friendsEvents.removeDuplicates()
           print("ending in friends events")
           self.collectionView?.reloadData()
           dispatchGroup.leave()
       })
   
   } 
   dispatchGroup.notify(queue: .main) {
       // dismiss the revealing view
   }
   

2. 在 showFollowingEvent 中你永远不会调用 dispatchGroup.notify，这就是它多次触发的原因

3. 我知道可能不行，但最好重组API调用