Swift4中闭包内TableView reloadData()无法生效的问题

Hey Augusto, let's figure out why your tableView.reloadData() isn't working as expected after fetching data from your web service. I’ve dealt with similar frustrating issues before, so here are the key checks and fixes to try:

1. First, confirm your data source is actually updated #

Before calling reloadData(), add a quick print statement to verify your data is loaded from the database correctly:

print("Loaded data count: \(yourDataSourceArray.count)")

If this count stays at 0 or doesn’t match the expected number of items, the problem isn’t with the reload—it’s that your data isn’t being saved to the database or retrieved properly. Double-check your database write/read logic first.

2. Fix the main thread call (you had a small mistake!) #

You tried using performSelector(onMainThread:), but you referenced UICollectionView.reloadData instead of UITableView.reloadData—that’s a critical typo! A cleaner, more reliable way to run UI updates on the main thread is using DispatchQueue.main.async:

DispatchQueue.main.async {
    self.tableView.reloadData()
}

Always make sure UI-related calls (like reloading table views) happen on the main thread—background threads can’t update the UI.

3. Verify your tableView’s delegate and dataSource are set #

It’s easy for these to get accidentally unset, especially if you’re reinitializing the tableView programmatically. Add assertions right before calling reloadData() to catch this:

assert(tableView.dataSource != nil, "TableView dataSource is missing!")
assert(tableView.delegate != nil, "TableView delegate is missing!")

If either assertion triggers, check your storyboard connections (make sure the tableView is linked to your view controller as dataSource/delegate) or add these lines in viewDidLoad():

tableView.dataSource = self
tableView.delegate = self

4. Ensure you’re reloading after data is fully saved #

Your trigger comes from a UITabBarButton action—make sure you’re not calling reloadData() before the web service request finishes and data is saved to the database. Wrap the reload in the completion closure of your data fetch/save logic:

@objc func refreshDataFromTabBar() {
    fetchAndSaveDataFromWebService { [weak self] in
        guard let self = self else { return }
        // Load updated data from database into your data source array
        self.yourDataSourceArray = self.loadDataFromDB()
        // Reload on main thread
        DispatchQueue.main.async {
            self.tableView.reloadData()
        }
    }
}

Using [weak self] here prevents retain cycles, and ensures you only reload once the data is ready.

5. Check if your tableView outlet is connected #

If your tableView is an IBOutlet, it might have a broken connection. Print the tableView instance to see if it’s nil:

print("TableView reference: \(tableView)")

If it prints nil, go back to your storyboard, delete the old outlet, and re-link it to your view controller.

6. Validate your dataSource methods are implemented correctly #

Even if you call reloadData(), if your dataSource methods return incorrect values, the table won’t update. Double-check these:

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    // Make sure this returns your data source array's count, not a hardcoded number
    return yourDataSourceArray.count
}

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "YourCellID", for: indexPath)
    // Ensure you're using the updated data source array to configure the cell
    let item = yourDataSourceArray[indexPath.row]
    cell.textLabel?.text = item.title
    return cell
}

7. Try forcing a layout refresh #

If all else fails, add a layout update after reloading:

DispatchQueue.main.async {
    self.tableView.reloadData()
    self.tableView.layoutIfNeeded()
}

Give these steps a shot—chances are one of them will fix the issue. If you’re still stuck, feel free to share a snippet of your data fetch/save code or the tab bar action method, and we can dig deeper!

内容的提问来源于stack exchange，提问作者Augusto