如何在声明闭包外修改UITextField的placeholder，实现空输入提示？

要实现点击"Add Item"按钮时，若输入为空就修改文本框的placeholder，你只需要在代码的else分支中直接修改已捕获的UITextField的placeholder属性即可，还可以添加震动动画增强用户提示效果。

修改后的核心代码 #

在addButtonPressed方法的UIAlertAction闭包的else分支中添加以下代码：

else {
    // 修改placeholder文本
    textField.placeholder = "Enter something here"
    // 可选：添加文本框震动动画，提示用户输入
    let shakeAnimation = CABasicAnimation(keyPath: "position")
    shakeAnimation.duration = 0.05
    shakeAnimation.repeatCount = 5
    shakeAnimation.autoreverses = true
    shakeAnimation.fromValue = NSValue(cgPoint: CGPoint(x: textField.center.x - 5, y: textField.center.y))
    shakeAnimation.toValue = NSValue(cgPoint: CGPoint(x: textField.center.x + 5, y: textField.center.y))
    textField.layer.add(shakeAnimation, forKey: "shake")
}

完整的修改后代码 #

import UIKit

class TodoListViewController: UITableViewController {

    var itemArray = ["A", "B", "C"]
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.navigationController?.navigationBar.titleTextAttributes = [.foregroundColor: UIColor.white, .font: UIFont.systemFont(ofSize: 25, weight: .bold)]
        self.navigationController?.view.backgroundColor = UIColor(red: 0.29, green: 0.63, blue: 0.82, alpha: 1)
        
    }
    
    //MARK: - TableView DataSource Methods
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return itemArray.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "ToDoItemCell", for: indexPath)
        cell.textLabel?.text = itemArray[indexPath.row]
        return cell
    }
    
    //MARK: - TableView Delegate Methods
    
    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        
        if tableView.cellForRow(at: indexPath)?.accessoryType == .checkmark{
            tableView.cellForRow(at: indexPath)?.accessoryType = .none
        } else {
            tableView.cellForRow(at: indexPath)?.accessoryType = .checkmark
        }
        
        tableView.deselectRow(at: indexPath, animated: true)
    }
    
    //MARK: - Add New Items
    
    
    @IBAction func addButtonPressed(_ sender: UIBarButtonItem) {
        
        var textField = UITextField()
        let alert = UIAlertController(title: "Add New Item:", message: "", preferredStyle: .alert)
        let action = UIAlertAction(title: "Add Item", style: .default) { action in
            if let inputText = textField.text, !inputText.isEmpty {
                self.itemArray.append(inputText)
                self.tableView.reloadData()
            } else {
                // 修改placeholder并添加震动提示
                textField.placeholder = "Enter something here"
                let shakeAnimation = CABasicAnimation(keyPath: "position")
                shakeAnimation.duration = 0.05
                shakeAnimation.repeatCount = 5
                shakeAnimation.autoreverses = true
                shakeAnimation.fromValue = NSValue(cgPoint: CGPoint(x: textField.center.x - 5, y: textField.center.y))
                shakeAnimation.toValue = NSValue(cgPoint: CGPoint(x: textField.center.x + 5, y: textField.center.y))
                textField.layer.add(shakeAnimation, forKey: "shake")
            }
        }
        alert.addTextField { alertTextField in
            alertTextField.placeholder = "Create new item"
            textField = alertTextField
        }
        alert.addAction(action)
        present(alert, animated: true, completion: nil)
    }
    
}

关键说明 #

1. 我们通过提前捕获的textField引用，直接修改其placeholder属性，由于alert已经处于显示状态，修改后会立即更新界面
2. 新增的震动动画是可选增强效果，能更直观地提醒用户需要输入内容
3. 把原代码中的textField.text != ""改成了更安全的if let inputText = textField.text, !inputText.isEmpty，避免强制解包的潜在问题

内容的提问来源于stack exchange，提问作者Twisha Solgama