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xxx.swift

//
//  ViewController.swift
//  text
//
//  Created by willard on 2018/4/2.
//  Copyright © 2018年 willard. All rights reserved.
//

import UIKit


class ViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    @IBOutlet weak var textfield: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()
        textfield.addTarget(self, action: #selector(textDidChanged), for: .editingChanged)
    }

    @objc
    func textDidChanged() {
        if let text = textfield.text {
            if text.count > 5 {
                let start = text.index(text.startIndex, offsetBy: 0)
                let end = text.index(text.startIndex, offsetBy: 5)
                textfield.text = String(text[start..<end])
            }
        }
        
        label.text = textfield.text
    }
}