使用接口构建器，您可以在任何函数中链接并获得“编辑更改”的事件。 现在你可以检查长度

- (IBAction)onValueChange:(id)sender 
{
    NSString *text = nil;
    int MAX_LENGTH = 20;
    switch ([sender tag] ) 
    {
        case 1: 
        {
            text = myEditField.text;
            if (MAX_LENGTH < [text length]) {
                myEditField.text = [text substringToIndex:MAX_LENGTH];
            }
        }
            break;
        default:
            break;
    }

}

你不能直接这样做- UITextField没有maxLength属性，但你可以设置UITextField的委托，然后使用:

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

最好的方法是设置文本更改通知。在视图控制器方法的-awakeFromNib中，你需要:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(limitTextField:) name:@"UITextFieldTextDidChangeNotification" object:myTextField];

然后在同一个类中添加:

- (void)limitTextField:(NSNotification *)note {
    int limit = 20;
    if ([[myTextField stringValue] length] > limit) {
        [myTextField setStringValue:[[myTextField stringValue] substringToIndex:limit]];
    }
}

然后链接出口myTextField到你的UITextField，它将不让你添加任何字符后，你达到限制。一定要把这个添加到你的dealloc方法中:

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"UITextFieldTextDidChangeNotification" object:myTextField];

Swift 3版本 //*****这将不工作与Swift 2.x!* * * * * / /

首先创建一个新的Swift文件:TextFieldMaxLength.swift， 然后添加下面的代码:

import UIKit

private var maxLengths = [UITextField: Int]()

extension UITextField {

   @IBInspectable var maxLength: Int {

      get {

          guard let length = maxLengths[self] 
             else {
                return Int.max
      }
      return length
   }
   set {
      maxLengths[self] = newValue
      addTarget(
         self,
         action: #selector(limitLength),
         for: UIControlEvents.editingChanged
      )
   }
}
func limitLength(textField: UITextField) {
    guard let prospectiveText = textField.text,
        prospectiveText.characters.count > maxLength
    else {
        return
    }

   let selection = selectedTextRange
   let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
   text = prospectiveText.substring(to: maxCharIndex)
   selectedTextRange = selection
   }
}

然后你会看到在故事板一个新的字段(最大长度)当你选择任何TextField

如果你还有更多问题，请查看这个链接:http://www.globalnerdy.com/2016/05/18/ios-programming-trick-how-to-use-xcode-to-set-a-text-fields-maximum-length-visual-studio-style/

我根据@Frouo给出了补充答案。我认为他的回答是最美丽的方式。因为这是一个我们可以重用的通用控件。

private var kAssociationKeyMaxLength: Int = 0

extension UITextField {

    @IBInspectable var maxLength: Int {
        get {
            if let length = objc_getAssociatedObject(self, &kAssociationKeyMaxLength) as? Int {
                return length
            } else {
                return Int.max
            }
        }
        set {
            objc_setAssociatedObject(self, &kAssociationKeyMaxLength, newValue, .OBJC_ASSOCIATION_RETAIN)
            self.addTarget(self, action: #selector(checkMaxLength), for: .editingChanged)
        }
    }

    func checkMaxLength(textField: UITextField) {

        guard !self.isInputMethod(), let prospectiveText = self.text,
            prospectiveText.count > maxLength
            else {
                return
        }

        let selection = selectedTextRange
        let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
        text = prospectiveText.substring(to: maxCharIndex)
        selectedTextRange = selection
    }

    //The method is used to cancel the check when use Chinese Pinyin input method.
    //Becuase the alphabet also appears in the textfield when inputting, we should cancel the check.
    func isInputMethod() -> Bool {
        if let positionRange = self.markedTextRange {
            if let _ = self.position(from: positionRange.start, offset: 0) {
                return true
            }
        }
        return false
    }

}

你也可以在Swift 4中使用NotificationCenter来做到这一点

NotificationCenter.default.addObserver(self, selector: #selector(self.handleTextChange(recognizer:)), name: NSNotification.Name.UITextFieldTextDidChange, object: yourTextField)

    @objc func handleTextChange(recognizer: NSNotification) {
            //max length is 50 charater max
            let textField = recognizer.object as! UITextField

            if((textField.text?.count)! > 50) {
                let newString: String? = (textField.text as NSString?)?.substring(to: 50)
                textField.text = newString

            }         
        }