美国小妞Xamarin:

YourTextField.ShouldChangeCharacters = 
delegate(UITextField textField, NSRange range, string replacementString)
        {
            return (range.Location + replacementString.Length) <= 4; // MaxLength == 4
        };

斯威夫特4

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    guard let text = textField.text else { return true }
    let newLength = text.count + string.count - range.length
    return newLength <= 10
}

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);
}

这个简单的方法怎么样?它对我来说很好。

extension UITextField {

    func  charactersLimit(to:Int) {

        if (self.text!.count > to) {
            self.deleteBackward()
        }
    }
}

然后:

someTextField.charactersLimit(to:16)

Swift 2.1+

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

    if (range.length + range.location > textField.text!.characters.count )
    {
        return false;
    }

    let newLength = textField.text!.characters.count + string.characters.count - range.length
    return newLength <= 25
}

希望能有所帮助

这应该足以解决问题(用你想要的极限替换4)。只要确保在IB中添加委托。

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
     NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
     return (newString.length<=4);
}