-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    if (textField.text.length >= 50) {
        return NO;
    }
    return YES;
}

稍微超出了回答原来的问题，并扩展了Frouo的答案，这里有扩展来修剪空白字符串和最大长度，并利用这些字符串扩展来修剪UITextField到最大长度:

// In String_Extensions.swift

extension String {

  func trimmedString() -> String {
    var trimmedString = self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet())
    let components = trimmedString.componentsSeparatedByCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).filter { count($0) > 0 }
    return " ".join(components)
  }

  func trimmedStringToMaxLength(maxLength: Int) -> String {
    return trimmedString().substringToIndex(advance(startIndex, min(count(self), maxLength))).trimmedString()
  }

}

// In UITextField_Extensions.swift

private var maxLengthDictionary = [UITextField : Int]()
private var textFieldMaxLength = 20

extension UITextField {

  @IBInspectable var maxLength: Int {
    get {
      if let maxLength = maxLengthDictionary[self] {
        return maxLength
      } else {
        return textFieldMaxLength
      }
    }
    set {
      maxLengthDictionary[self] = newValue < textFieldMaxLength + 1 ? newValue : textFieldMaxLength
    }
  }

  func trimAndLimitToMaxLength() {
    text = text.trimmedStringToMaxLength(maxLength)
  }

}

let someTextField = UITextField()
let someString = "   This   is   a   string   that   is longer than allowable for a text field.   "
someTextField.text = someString
someTextField.trimAndLimitToMaxLength()
println(someTextField.text) // Prints "This is a string tha"
let anotherTextField = UITextField()
anotherTextField.maxLength = 5
anotherTextField.text = someString
anotherTextField.trimAndLimitToMaxLength()
println(anotherTextField.text) // Prints "This"

trimAndLimitToMaxLength() could be used in UITextFieldDelegate's textFieldDidEndEditing(_:) so that a user could enter or paste in a longer than acceptable string and then shorten it vs. just cutting off the input at the max length. In doing this, I would also set attributed text styles to indicate any portion of the text that goes beyond the acceptable length (e.g., [NSBackgroundColorAttributeName : UIColor.redColor(), NSForegroundColorAttributeName : UIColor.whiteColor(), NSStrikethroughStyleAttributeName : NSNumber(int: 1)]

上面给出的一些答案的问题是，例如，我有一个文本字段，我必须设置输入15个字符的限制，然后它在输入第15个字符后停止。但是他们不允许删除。那就是删除按钮也不管用了。我也面临着同样的问题。提出了解决方案，如下所示。非常适合我

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
 if(textField.tag==6)
 {
    if ([textField.text length]<=30)
    {
        return YES;   
    }
    else if([@"" isEqualToString:string])
    {
        textField.text=[textField.text substringToIndex:30 ];
    }

    return NO;
 }
 else
 {
    return YES;
 }
}

我有一个文本框，我将其标签设置为“6” 并且我已经限制了最大字符限制= 30; 在任何情况下都可以正常工作

在Swift 5.2中工作:

 class AngListVC: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var angTextField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        angTextField.delegate = self
        angTextField.addTarget(self, action: #selector(textFieldDidChange(_:)), for: .editingChanged)
      
    }

   func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
           let userText =  angTextField.text ?? ""
           var newText = ""
           if range.length > 0 {
               let txt = NSString(string: userText)
               if txt.length > 0 {
                   newText = txt.replacingCharacters(in: range, with: "")
               }
           } else {
               newText = userText + ""
           }
           return newText.count <= 3
       }

@objc func textFieldDidChange(_ textField: UITextField) {
        print("textFieldDidChange")
   }

虽然UITextField类没有max length属性，但是通过设置文本字段的委托并实现以下委托方法来获得这个功能相对简单:

objective - c

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    // Prevent crashing undo bug – see note below.
    if(range.length + range.location > textField.text.length)
    {
        return NO;
    }
        
    NSUInteger newLength = [textField.text length] + [string length] - range.length;
    return newLength <= 25;
}

斯威夫特

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    
    let currentCharacterCount = textField.text?.count ?? 0
    if range.length + range.location > currentCharacterCount {
        return false
    }
    let newLength = currentCharacterCount + string.count - range.length
    return newLength <= 25
}

在文本字段更改之前，UITextField会询问委托是否应该更改指定的文本。此时文本字段还没有改变，所以我们获取它的当前长度和我们插入的字符串长度(通过粘贴复制的文本或使用键盘输入单个字符)，减去范围长度。如果该值太长(本例中超过25个字符)，则返回NO以禁止更改。

当在文本字段的末尾输入单个字符时，范围。Location是当前字段的长度和范围。Length将为0，因为我们没有替换或删除任何东西。插入到文本字段的中间只是意味着一个不同的范围。位置，粘贴多个字符意味着字符串中有多个字符。

删除单个字符或删除多个字符由一个长度非零的范围和一个空字符串指定。替换只是一个非空字符串的范围删除。

关于崩溃的“撤消”错误的说明

正如评论中提到的，UITextField有一个bug，可能导致崩溃。

如果粘贴到字段，但验证实现阻止了粘贴，则粘贴操作仍会记录在应用程序的撤销缓冲区中。如果你随后触发一个撤销(通过摇动设备并确认撤销)，UITextField将尝试用一个空字符串替换它认为它粘贴到自己的字符串。这将会崩溃，因为它从未将字符串粘贴到自身。它将尝试替换字符串中不存在的部分。

幸运的是，你可以保护UITextField不像这样杀死自己。您只需要确保它建议替换的范围确实存在于其当前字符串中。这就是上面最初的健全性检查所做的工作。

Swift 3.0与复制和粘贴工作良好。

func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
        let str = (textView.text + text)
        if str.characters.count <= 10 {
            return true
        }
        textView.text = str.substring(to: str.index(str.startIndex, offsetBy: 10))
        return false
    }

希望对你有帮助。

我已经实现了一个UITextField扩展，以添加一个maxLength属性。

它基于Xcode 6 IBInspectables，所以你可以在接口构建器上设置maxLength限制。

实现如下:

UITextField + MaxLength。h

#import <UIKit/UIKit.h>

@interface UITextField_MaxLength : UITextField<UITextFieldDelegate>

@property (nonatomic)IBInspectable int textMaxLength;
@end

UITextField + MaxLength。m

#import "UITextField+MaxLength.h"

@interface UITextField_MaxLength()

@property (nonatomic, assign) id <UITextFieldDelegate> superDelegate;

@end

@implementation UITextField_MaxLength

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

    //validate the length, only if it's set to a non zero value
    if (self.textMaxLength>0) {
        if(range.length + range.location > textField.text.length)
            return NO;

        if (textField.text.length+string.length - range.length>self.textMaxLength) {
            return NO;
        }
    }

    //if length validation was passed, query the super class to see if the delegate method is implemented there
    if (self.superDelegate && [self.superDelegate respondsToSelector:@selector(textField:shouldChangeCharactersInRange:replacementString:)]) {
        return [self.superDelegate textField:textField shouldChangeCharactersInRange:range replacementString:string];
    }
    else{
        //if the super class does not implement the delegate method, simply return YES as the length validation was passed
        return YES;
    }
}

- (void)setDelegate:(id<UITextFieldDelegate>)delegate {
    if (delegate == self)
        return;
    self.superDelegate = delegate;
    [super setDelegate:self];
}

//forward all non overriden delegate methods
- (id)forwardingTargetForSelector:(SEL)aSelector {
    if ([self.superDelegate  respondsToSelector:aSelector])
        return self.superDelegate;

    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([self.superDelegate respondsToSelector:aSelector])
        return YES;

    return [super respondsToSelector:aSelector];
}
@end