How to make sure a line of code is executed after another is completed?

Question

I have a viewControl called PostViewController which has a UITableView of posts. I also have a class called PostCell which defines the UITableViewCell. I made a button function in PostCell called likeButtonClicked to favour a post similar to twitter.

@IBAction func likesButtonClicked(_ sender: Any) { NotificationCenter.default.post(name: NSNotification.Name(rawValue: "likeButtonClicked"), object: nil, userInfo: ["cell":self, "likesButton":likesButton!]) }

This is to pass the cell indexPath and the button name to PostViewController. I need indexPath to increase the likes by 1 and the button name to change its image to pink when post is favoured.

I then subscribed to the notification in viewDidLoad of PostViewController.

 NotificationCenter.default.addObserver(self, selector: #selector(postLiked), name: NSNotification.Name(rawValue: "likeButtonClicked"), object: nil)

I then wrote this function in the same viewController

  @objc func postLiked(notification: Notification){
    if let cell = notification.userInfo?["cell"] as? UITableViewCell{
        let likesButton = notification.userInfo?["likesButton"] as? SpringButton
        if let indexPath = postsTableView.indexPath(for: cell){
            let post = posts[indexPath.row]
            postId = post.id
            PostAPI.getPostById(postId: postId) { post in
                //Check if the same post were already favoured.
                if !self.liked || self.oldPostId != self.postId{
                    self.newLikes = post.likes + 1
                    self.liked = true
                    self.oldPostId = self.postId
                }else{
                    self.newLikes = self.newLikes - 1
                    self.liked = false
                }
                PostAPI.favourPost(postId: self.postId, likes: self.newLikes) {
                    PostAPI.getPostById(postId: self.postId) { postResponse in
                        let post = postResponse
                        self.posts[indexPath.row] = post
                        let cellNumber = IndexPath(row: indexPath.row, section: indexPath.section)
                        self.reloadRowData(cellNumber: cellNumber){
                            if !self.liked{
                                likesButton?.tintColor = .systemPink
                            }else{
                                likesButton?.tintColor = .darkGray
                            }
                        }
                    }
                }
            }
        }
    }
}
func reloadRowData(cellNumber: IndexPath, completion: @escaping () -> ()) {
    self.postsTableView.reloadRows(at: [cellNumber], with: .none)
    completion()
}

Please tell me why the last 4 lines of postLiked function is executed before reloadRowData function, which causes the button to change its color to pink then returns immediately to gray when it should stay pink.

Any help will be most appreciated. Thank you.

Answer 1

A table view's .reloadRows(at:...) (and .reloadData()) functions are async processes.

So your reloadRowData() func is returning before the table view actually reloads the row(s).

This is a rather unusual approach - both in using NotificationCenter for your cells to communicate with the controller, and in trying to change the button's tint color by holding a reference to the button.

The tint color really should be set in cellForRowAt, based on your data source.

---

Edit

My description of table view data reloading being asynchronous was misleading.

The point I wanted to make was this...

If I change my data, call .reloadData(), and then change my data again:

    // set myData values
    myData = ["You", "Say", "Hello"]
    
    // call .reloadData here
    tableView.reloadData()

    // change myData values
    myData = ["I", "Say", "Goodbye"]
    

the table view will not display "You Say Hello" even though we call .reloadData() when those are the values of myData.

Here's a complete, simple example to demonstrate:

class TestTableViewController: UITableViewController {
    
    var myData: [String] = ["Some", "Song", "Lyrics"]
    
    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "c")
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return myData.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let c = tableView.dequeueReusableCell(withIdentifier: "c", for: indexPath)
        c.textLabel?.text = myData[indexPath.row]
        return c
    }
    
    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        // call my func when a row is selected
        self.myReload()
    }
    
    func myReload() {
        
        // set myData values
        myData = ["You", "Say", "Hello"]
        
        // call .reloadData here
        tableView.reloadData()

        // change myData values
        myData = ["I", "Say", "Goodbye"]
        
        // table view doesn't reload itself we exit this func
        // so we'll get
        //  I
        //  Say
        //  Goodbye
        
    }
    
}

So, among the other issues described by Rob Napier in his answer, your original code was trying to change the tint color of an object in a cell before the table reloaded its data.