为什么实例变量在闭包内部具有不同的值？

Question

func loadYelpComments(){
        guard let business = business else {return}
        guard let _ = tableView else {return}
        guard let businessID = business.id else {return}
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .full

        HTTPHelper.getYelpComments(for: businessID, completionHandler: { data in
            let json = JSON(data)
            let reviews = json["reviews"].arrayValue.map({return $0.dictionaryValue})
            var commentDate: Date?


            for (index, review) in reviews.enumerated(){
                let userDictionary = review["user"]?.dictionary
                if let dateString = review["time_created"]?.stringValue{
                    commentDate = dateFormatter.date(from: dateString)
                }

                let yelpComment = YelpComment(rating: review["rating"]?.intValue, userImageURL: userDictionary?["image_url"]?.stringValue, userName: userDictionary?["name"]?.stringValue, comment: review["text"]?.stringValue, commentDate: commentDate, commentPageURL: review["url"]?.stringValue)

                self.comments.append(yelpComment)
            }

            print("Number of comments: \(self.comments.count)") //Prints: Number of comments: 3"
            self.tableView.reloadData()

        })

            print("Number of comments: \(self.comments.count)") //This prints firt "Number of comments: 0"

    }

The getYelpComments(for:completionHandler:) class method is responsible for fetching JSON data from the Yelp API. To my surprise even though the comments instance array is being updated by appending new YelpComment objects to it, its count has different values inside and outside of the closure.

These comments are supposed to be displayed in a table view. What further confuses me is the fact that when I add tableView.reloadData() within the closure I get an index out of bounds for an empty array error but when I add the same line of code: tableView.reloadData() out side of the closure I get no error and comments.count equates zero. Any help would be much appreciated!

PS I don't know if mentioning this is going to make any difference but data is @escaping . I am also using SwiftyJSON and Alamofire .

UPDATE:

My table view data source methods are:

   override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

        if section == 0 {
            return super.tableView(tableView, numberOfRowsInSection: section)
        } else {
            return comments.count
        }

    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        if indexPath.section == 0 {
            return super.tableView(tableView, cellForRowAt: indexPath)
        } else {
            let cell = tableView.dequeueReusableCell(withIdentifier: CustomCellTypeIdentifiers.YelpCommentCell, for: indexPath) as! YelpCommentCell
            cell.configureYelpCell(with: comments[indexPath.row])
            return cell
        }
    }

Answer 1

Because the job of the completion block is to report to you when the network call is done. HTTPHelper is making a call to a server, which can take some considerable amount of time. It does not stop your program from executing, and just wait at that line: it goes to the next line, immediately, and calls print("Number of comments: \\(self.comments.count)") , and you get 0, because the network call isn't done yet. Then, some time later, that whole completion block happens.

This is called an asynchronous function , and it commonly trips up people who haven't seen it before, but you see lots of it eventually, so it's worth reading up on.

The fact that you say "when I add tableView.reloadData() within the closure I get an index out of bounds for an empty array error", it sounds like one of your UITableViewDataSource functions that configures the table ( cellForRowAt , numberOfRowsInSection ), has an error in it somewhere.

Answer 2

Closures are not necessarily executed immediately at the point where you see them.

In your code, the print outside the closure will be executed before the print inside the closure. This is because getYelpComments is what is called an "asynchronous method". While you are waiting for the Yelp servers to respond, the code doesn't just sit there doing nothing. It continues to execute the code after the closure, printing that the count is 0.

After Yelp responds, the closure gets executed. And count, being 3, is printed.

As for why putting tableView.reloadData() causes it to crash, there is probably something wrong in your table view datasource methods. It is most likely an off-by-1.

Edit:

I think the fact that you write

if section == 0 {
    return super.tableView(tableView, numberOfRowsInSection: section)

is weird. I don't know why you want to return the super implementation if the section is 0. Does your table view have multiple sections? If not, you should probably remove the check. If yes, then properly return a value for the first section.