(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];

    if ([txt_name.text length]>100)
    {
        return NO;
    }

    return YES;
}

斯威夫特4

import UIKit

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)
            addTarget(self, action: #selector(checkMaxLength), for: .editingChanged)
        }
    }
    
    @objc func checkMaxLength(textField: UITextField) {
        guard let prospectiveText = self.text,
            prospectiveText.count > maxLength
            else {
                return
        }
        
        let selection = selectedTextRange
        
        let indexEndOfText = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
        let substring = prospectiveText[..<indexEndOfText]
        text = String(substring)
        
        selectedTextRange = selection
    }
}

编辑:内存泄漏问题修复。

(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];

    if ([txt_name.text length]>100)
    {
        return NO;
    }

    return YES;
}

我们可以像这样设置文本框的范围。

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range  replacementString:(NSString *)string
{
     int setrange = 20;
     return !([textField.text length]>setrange && [string length] > range.length);
}

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)]

我发现这个方法又快又简单

- (IBAction)backgroundClick:(id)sender {
    if (mytext.length <= 7) {
        [mytext resignFirstResponder];
    } else {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Too Big" 
                                                        message:@"Please Shorten Name"
                                                       delegate:nil 
                                              cancelButtonTitle:@"Cancel"
                                              otherButtonTitles:nil];
        [alert show];
        [alert release];
    }
}