tableView Firebase Swift中的异步数据

Question

I have an issue with my least favourite part in Firebase. I want to pull a post from user's following list (every user has one and only one post). First, I created a completion handler to get a list of all followers from Firebase and store it in userArray array of strings:

func GetUsersInFollowing(completion: @escaping (Bool) -> ()) {
    ref.child("following").queryOrdered(byChild: FIRAuth.auth()!.currentUser!.uid).observeSingleEvent(of: .value, with: { (snapshot) in
        for group in snapshot.children {
            self.userArray.append((group as AnyObject).key)
        }
        completion(true)
    })
}

Now the plan is to pull a post from every element of userArray. Here is where the problem starts. I call CreatePosts() immediately after GetUsersInFollowing() completes.

func CreatePosts() {
    for x in userArray {
        var thePost = Post()
        print("1")
        self.ref.child("users").child(x).observeSingleEvent(of: .value, with: { (snapshot) in
            let value = snapshot.value as? NSDictionary
            thePost.fullName = value?["fullname"] as? String ?? ""
            thePost.username = value?["username"] as? String ?? ""
            thePost.profileImageURL = value?["photourl"] as? String ?? ""
            print("2")
        })

        self.ref.child("posts").child(x).observeSingleEvent(of: .value, with: { (snapshot) in
            let value = snapshot.value as? NSDictionary
            thePost.description = value?["description"] as? String ?? ""
            thePost.info = value?["location"] as? String ?? ""
            thePost.postImageURL = value?["photoURL"] as? String ?? ""
            thePost.timePost = value?["timestamp"] as? NSDate
            thePost.upVotes = value?["upvotes"] as? Int ?? 0
        })

        self.postArray.append(thePost)
        self.tableView.reloadData()
    }
}

Everything looks ok to me, but it surely isn't. Here's how I create cells:

func configureCell(post: Post) {
    self.post = post
    self.username.text = post.username
    self.profileImage = post.profileImageURL
    print("3")
    self.fullname.text = post.fullName
    self.timestamp.text = post.timePost
    self.upvotes.text = post.upVotes
    self.location.text = post.location
    self.descriptionText.text = post.description
}

The output in the console varies, but usually I get: 1 1 3 3 2 2

The idea is to first retrieve all data from Firebase, add it to post object, append the object to the array and then create cell for that object with data downloaded. Cell is already created even though data is not retrieved. I think that is the problem. Thank you, every suggestion is appreciated.

Answer 1

You need to inner query for combining both user profile data and post data. Like this -

    func CreatePosts() {

        //Using userPostArrayObjFetched as a counter to check the number of data fetched.
        //Remove this code, if you don't want to wait till all the user data is fetched.
        var userPostArrayObjFetched = 0

        for (index,userID) in userArray.enumerated() {

            print("1" + userID)

            self.ref.child("users").child(userID).observeSingleEvent(of: .value, with: { (snapshot) in

                var thePost = Post()

                let value = snapshot.value as? NSDictionary
                thePost.fullName = value?["fullname"] as? String ?? ""
                thePost.username = value?["username"] as? String ?? ""
                thePost.profileImageURL = value?["photourl"] as? String ?? ""
                print("2" + userID)

                self.ref.child("posts").child(userID).observeSingleEvent(of: .value, with: { (snapshot) in
                    let value = snapshot.value as? NSDictionary
                    thePost.description = value?["description"] as? String ?? ""
                    thePost.info = value?["location"] as? String ?? ""
                    thePost.postImageURL = value?["photoURL"] as? String ?? ""
                    thePost.timePost = value?["timestamp"] as? NSDate
                    thePost.upVotes = value?["upvotes"] as? Int ?? 0
                    print("3" + userID)

                    self.postArray.append(thePost)

//                      Uncomment if you want to reload data as fetched from Firebase without waiting for all the data to be fetched.
//                    self.tableView.reloadData()

                    userPostArrayObjFetched += 1

                    if userPostArrayObjFetched == userArray.count{
                         self.tableView.reloadData()
                    }
                })
            })
        }
    }