Firebase 查询未按正确的顺序执行

Question

我获取数据并显示在表格视图中，问题是数据没有以正确的顺序执行。

我试过：

            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()
                    }
                })
            }
        })
    }

但它也没有奏效，第二个火力基地调用的执行顺序不正确。

Answer 1

您应该在设置DispatchGroup时设置notify闭包。而且，您无需为函数使用完成闭包loadComments。

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

当调用leave的次数与enter相同时，将调用notify。在您的代码中，最后一次leave调用发生在您设置要通知的任何内容之前。

Answer 2

我用这个解决了：

    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()
                        }
                    })
                }

不过，我会在这里使用调度组，因此不必检查它是否不必要地完成。