Firebase query not executing in the correct order?

问题

I fetch data and display in a tableView, the problem is the data is not executing in the correct order.

I have tried:

            for case let child as DataSnapshot in data!.children.reversed() {
                let newDispatchGroup = DispatchGroup()
                let commentID = child.key
                let uid = child.childSnapshot(forPath: "UID").value as! String
                let commentText = child.childSnapshot(forPath: "Comment").value!
                let timeStamp = child.childSnapshot(forPath: "timeStamp").value!
                let date = ConvertDate(mediaTimestamp: timeStamp as! Double).getDate!
                //print(date, "dsfsdafdasfdsafdsahjkfhfdsafsajkadhffdsfsafsasjkfhsdajkhfdsajkhfjklads")

                newDispatchGroup.enter()

                ref.child("users2").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
                    print(snapshot, "dshjkfhjkadhfsjkfhsdajkhfdsajkhfjklads")
                    print(date, "dsfsdafdasfdsafdsahjkfhfdsafsajkadhffdsfsafsasjkfhsdajkhfdsajkhfjklads")

                    let username = snapshot.childSnapshot(forPath: "username").value
                    let profileImage = snapshot.childSnapshot(forPath: "profileImage").value

                    let newUser = User(theuserID: uid, theUsername: username as! String, theprofImage: profileImage as! String)

                    let newComment = Comment(newUser: newUser, text: commentText as! String, timeStamp: date, NcommentID: commentID)
                    self.commentsVC1.arrayOfComments.append(newComment)
                    newDispatchGroup.leave()
                    //completion()
                })
                newDispatchGroup.notify(queue: .main, execute: {
                    print(self.totalComments, "COgfdsdfgfdsgdsfgdfsgfdsgdfsgdskj", self.commentsVC1.arrayOfComments.count)
                    if self.totalComments == self.commentsVC1.arrayOfComments.count {
                        print("COmejkfbdshkafdsagfhksdagfdsakj")
                        self.commentsVC1.tableView.reloadData()
                    }
                })
            }
        })
    }

But it did not work either, the order in which the second firebase calls execute is incorrect.

回答1:

You should set up your notify closure when you set up your DispatchGroup. And you would not need to use a completion closure for you loadComments function.

let dispatchGroup = DispatchGroup()
dispatchGroup.notify(queue: .main, execute: {
    if self.totalComments == self.commentsVC1.arrayOfComments.count {
        print("COmejkfbdshkafdsagfhksdagfdsakj")
        self.commentsVC1.tableView.reloadData()
    }
})

loadComments()

notify will be called, when leave has been called the same amount of times as enter. In your code the last leave call is happening before you have set the anything to be notified about.

回答2:

I solved with this:

    for case let child as DataSnapshot in snap.children.reversed() {
                    let commentID = child.key
                    let uid = child.childSnapshot(forPath: "UID").value as! String
                    let commentText = child.childSnapshot(forPath: "Comment").value!
                    let timeStamp = child.childSnapshot(forPath: "timeStamp").value!
                    let date = ConvertDate(mediaTimestamp: timeStamp as! Double).getDate!
                    let newUser = User(theuserID: uid)
                    let newComment = Comment(newUser: newUser, text: commentText as! String, timeStamp: date, NcommentID: commentID)
                    self.commentsVC1.arrayOfComments.append(newComment)
                    ref.child("users2").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in

                        let username = snapshot.childSnapshot(forPath: "username").value
                        let profileImage = snapshot.childSnapshot(forPath: "profileImage").value

                        let newUserIner = User(theuserID: uid, theUsername: username as! String, theprofImage: profileImage as! String)
                        newComment.user = newUserIner
                        if self.totalComments == self.commentsVC1.arrayOfComments.count {
                            self.commentsVC1.tableView.reloadData()
                        }
                    })
                }

I would use dispatch group here though so one does not have have to check if its done unnecessarily.

来源：https://stackoverflow.com/questions/57067730/firebase-query-not-executing-in-the-correct-order