UITextField 和 UITextView 限制最大输入长度

maxLength.swift

import UIKit

class ViewController: UIViewController {
    
    var textField: UITextField!
    var textView: UITextView!
    private let maxLength = 20

    override func viewDidLoad() {
        super.viewDidLoad()
        
        textField = UITextField()
        textField.addTarget(self, action: #selector(textFieldDidChanged(_:)), for: .editingChanged)
        view.addSubview(textField)
        
        textView = UITextView()
        textView.delegate = self
        view.addSubview(textView)
    }
}

extension ViewController {
    @objc func textFieldDidChanged(_ textField: UITextField) {
        guard textField.markedTextRange == nil else { return }
        textField.text = textField.text?.prefix(maxLength).toString()
    }
}

extension ViewController: UITextViewDelegate {
    func textViewDidChange(_ textView: UITextView) {
        guard textView.markedTextRange == nil else { return }
        textView.text = textView.text?.prefix(maxLength).toString()
    }
}

extension Substring {
    func toString() -> String {
        return String(self)
    }
}