CountTextField.swift

import Foundation
import UIKit

class CountTextField: UITextField, UITextFieldDelegate {
    lazy var countLabel: UILabel = {
        let label = UILabel(frame: CGRect(x: 0, y: 0, width: 20, height: 40))
        label.font = UIFont.systemFont(ofSize: 12)
        label.textAlignment = .center
        label.textColor = #colorLiteral(red: 0.1098039216, green: 0.5176470588, blue: 0.8588235294, alpha: 1)
        label.text = "16"
        return label
    }()
    
    let padding = UIEdgeInsets(top: 0, left: 8, bottom: 0, right: 0)
    
    override func awakeFromNib() {
        delegate = self
        let rightView = UIView(frame: CGRect(x: 0, y: 0, width: 40, height: 40))
        rightView.addSubview(countLabel)
        
        self.rightView = rightView
        rightViewMode = .always
    }
    
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let newLength = (textField.text?.utf16.count)! + string.utf16.count - range.length
        if newLength <= 16 {
            countLabel.text = "\(16 - newLength)"
            
            return true
        } else {
            return false
        }
    }
    
    open override func textRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, padding)
    }
    
    open override func placeholderRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, padding)
    }
    
    open override func editingRect(forBounds bounds: CGRect) -> CGRect {
        return UIEdgeInsetsInsetRect(bounds, padding)
    }
}