Updating UILabel from UIAlertAction handler not wo

问题:

I have a subclass of UITableView in my iOS app (Swift 4, XCode 9). This table has one row and I want it to display an alert when it's clicked, get some input from the user, and then update a label (lblUserFromPrefs) in the table when the user clicks "OK" in the alert. Currently everything works fine except the label doesn't get updated. Here is the relevant code from my UITableView subclass:

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    print("Clicked section: \(indexPath.section) row: \(indexPath.row)")
    let alert = UIAlertController(title: "Username", message: "what is your name", preferredStyle: .alert)

    alert.addTextField { (textField) in
        textField.text = ""
    }
    alert.addAction(UIAlertAction(title: "OK", style: .default, handler: { [weak alert] (_) in
        let textField = alert!.textFields![0]
        if let text = textField.text {
            print("Text field: \(text)")
            DispatchQueue.main.async {
                self.lblUserFromPrefs.text = text
                print("label updated")
            }
        }
    }))

    DispatchQueue.main.async {
        self.present(alert, animated: true, completion: nil)
    }
}

What happens when I run this is the label's text doesn't change when the alert closes but does change immediately when the table row is clicked again. I don't know why it waits until the row is clicked again to update the text. All the print statements print when I expect (including printing "label updated" immediately when the alert's OK button is pressed) and they print the right things. I know that when you're trying to update the UI from a closure in a background thread you have to use DispatchQueue.main.async {} but I'm not sure why it's not updating even though I am using the main thread. I have tried using DispatchQueue.main.async(execute: {}) and putting self.lblUserFromPrefs.setNeedsDisplay() directly after self.lblUserFromPrefs.text = "text". Any ideas what I'm doing wrong? Thanks!!

回答1:

Add the [weak self] to the dispatch like this:

DispatchQueue.main.async { [weak self] in

}

回答2:

Try something like this:

let alertController = UIAlertController.init(title: "Enter some text", message: nil, preferredStyle: .alert)
    alertController.addTextField { (textField) in

        // Text field configuration
        textField.placeholder = "Enter..."
        textField.text = ""
    }

    alertController.addAction(UIAlertAction.init(title: "Ok", style: .default, handler: { (action) in
        if (alertController.textFields?.first?.hasText)! {
            if let text = alertController.textFields?.first?.text {
                self.label.text = text // Update the value
            }
        } else {
            self.label.text = "" // Default value
        }
    }))

    self.present(alertController, animated: true, completion: nil) // Present alert controller

回答3:

Here is the sample code for the behavior you want to achieve. Just include your alert controller code in didSelect and you should be good to go!

import UIKit
import PlaygroundSupport

class MyViewController : UITableViewController {

  override func numberOfSections(in tableView: UITableView) -> Int {
    return 1
  }

  override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return 1
  }

  override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCell(withIdentifier: "Cell")
    if cell == nil {
      cell = UITableViewCell()
    }
    cell!.textLabel?.text = "Hello World!"
    return cell!
  }

  override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    let cell = tableView.cellForRow(at: indexPath)
    cell?.textLabel?.text = "Hello Universe!"
  }
}