您不能扩展xmlextFiled的高度。
但是你可以使用ExtView来实现，你要做的是
使用extextView代替extFiled，并将ScrollEnabled设置为false，给予extView top、bottom、leading和trailing约束。
给予高度约束给extView，并将其内容优先级设置为低。
现在你的文本视图将根据插入的文本展开。
是的，您必须手动添加和管理占位符逻辑

下面是以编程方式创建多行扩展UTextField的完整工作代码：

import UIKit
class ExpandableTextFieldViewController: UIViewController, UITextViewDelegate {
    // Create a UITextView
    let textView: UITextView = {
        let textView = UITextView()
        textView.translatesAutoresizingMaskIntoConstraints = false
        textView.isScrollEnabled = false // Disable scrolling
        textView.font = UIFont.systemFont(ofSize: 16) // Set your preferred font size
        textView.layer.borderColor = UIColor.gray.cgColor
        textView.layer.borderWidth = 1.0
        textView.layer.cornerRadius = 5.0
        textView.textContainerInset = UIEdgeInsets(top: 8, left: 8, bottom: 8, right: 8) // Adjust padding
        return textView
    }()
    override func viewDidLoad() {
        super.viewDidLoad()
        // Add the UITextView to the view
        view.addSubview(textView)
        // Set delegates
        textView.delegate = self
        // Add constraints
        NSLayoutConstraint.activate([
            textView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20),
            textView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
            textView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -20),
            textView.bottomAnchor.constraint(lessThanOrEqualTo: view.bottomAnchor, constant: -20), // Ensure it doesn't go beyond the bottom of the screen
            // Set an initial height constraint (you can change this to your preferred initial height)
            textView.heightAnchor.constraint(equalToConstant: 100).withPriority(.defaultLow) // Set content hugging priority to low
        ])
        
        // Customize placeholder text and color
        textView.text = "Enter text here"
        textView.textColor = UIColor.lightGray
    }
    // MARK: - UITextViewDelegate methods
    func textViewDidChange(_ textView: UITextView) {
        // Adjust the height of the UITextView based on its content
        let fixedWidth = textView.frame.size.width
        let newSize = textView.sizeThatFits(CGSize(width: fixedWidth, height: .greatestFiniteMagnitude))
        textView.heightAnchor.constraint(equalToConstant: newSize.height).isActive = true
    }
    func textViewDidBeginEditing(_ textView: UITextView) {
        // Remove placeholder text when the user starts editing
        if textView.textColor == UIColor.lightGray {
            textView.text = nil
            textView.textColor = UIColor.black
        }
    }
    func textViewDidEndEditing(_ textView: UITextView) {
        // Add placeholder text if the user ends editing with an empty field
        if textView.text.isEmpty {
            textView.text = "Enter text here"
            textView.textColor = UIColor.lightGray
        }
    }
}