Xcode开发问题：TextField仅接受整数并启用按钮及报错功能实现遇阻

Hey there! Let's work through this problem step by step to get your app behaving exactly how you want it to.

First, Fix the Core Setup Issues #

That "Unexpectedly found nil" error you hit earlier is almost certainly from one of two easy-to-fix problems:

• Your button outlet wasn't properly connected in Interface Builder
• You commented out FirstBox.delegate = self, so the text field's validation logic never runs at all

Let's start by cleaning up the setup:

class ViewController: UIViewController, UITextFieldDelegate { // Add the text field delegate protocol
    @IBOutlet weak var FirstBox: UITextField!
    @IBOutlet weak var button: UIButton! // Double-check this is connected in your storyboard!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        FirstBox.delegate = self // Uncomment this line to activate the delegate methods
        button.isEnabled = false // Start with the button disabled by default
    }
}

Fix the Text Field Validation Logic #

Your current code only checks if the single character being typed is an integer, which misses key edge cases:

• Pressing backspace (the replacement string is empty, which would fail your Int(string) check)
• Pasting a multi-character string that's not a valid integer
• Combining existing text with new characters to form a valid integer

Instead, we need to validate the full text that will exist after the user's change:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    // Get the current text in the field (or empty string if it's nil)
    let currentText = textField.text ?? ""
    // Calculate what the text will be after the user's edit
    guard let validRange = Range(range, in: currentText) else { return false }
    let updatedText = currentText.replacingCharacters(in: validRange, with: string)
    
    // Validate the updated text
    if let _ = Int(updatedText), !updatedText.isEmpty {
        // Valid integer input: enable the button and clear any error state
        button.isEnabled = true
        // If you have an error label, hide it here: errorLabel.isHidden = true
    } else {
        // Invalid input: disable the button and show your error message
        button.isEnabled = false
        showInputError()
    }
    
    // Allow the text change to happen (we're just validating, not blocking input)
    return true
}

// Helper method to show your error feedback
func showInputError() {
    // Option 1: Show a pop-up alert
    let alert = UIAlertController(title: "Invalid Input", message: "Must enter an integer value to continue.", preferredStyle: .alert)
    alert.addAction(UIAlertAction(title: "OK", style: .default))
    present(alert, animated: true)
    
    // Option 2 (less intrusive): Use a label below the text field
    // errorLabel.isHidden = false
    // errorLabel.text = "Must enter an integer value to continue."
    // errorLabel.textColor = .red
}

Why This Works #

• We validate the entire input after each edit, so edge cases like backspace or pasting are handled correctly
• The button starts disabled and only enables when a valid integer is entered
• You get clear error feedback for users, either via a pop-up or a persistent label (pick whichever fits your app's flow better)

Quick Note on Error Throwing #

Your original code defined an error enum but didn't use it. Throwing errors here isn't necessary for UI feedback—displaying an alert or label is much more user-friendly for input validation. If you need error handling for internal calculation logic, you can add that later in your segue preparation or calculation code.

内容的提问来源于stack exchange，提问作者Robert