我在Swift中这样做是为了在使用数字垫时限制8个字符。

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    return !(textField.text?.characters.count == MAX_LENGTH && string != "")
}

我必须测试string != ""以允许删除按钮在数字板上工作，否则它将不允许在达到最大值后删除文本字段中的字符。

有通用的解决方案设置最大长度在Swift。 通过IBInspectable，你可以在Xcode属性检查器中添加新的属性。

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:"),
                forControlEvents: UIControlEvents.EditingChanged
            )
        }
    }

    func limitLength(textField: UITextField) {
        guard let prospectiveText = textField.text
            where prospectiveText.characters.count > maxLength else {
                return
        }
        let selection = selectedTextRange
        text = prospectiveText.substringWithRange(
            Range<String.Index>(prospectiveText.startIndex ..< prospectiveText.startIndex.advancedBy(maxLength))
        )
        selectedTextRange = selection
    }

}

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
    }

}

现在你想要多少字符，只要给值就行了

 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range   replacementString:(NSString *)string {
     NSUInteger newLength = [textField.text length] + [string length] - range.length;
     return (newLength > 25) ? NO : YES;
  }

这限制了字符的数量，但也确保您可以粘贴字段，直到最大限制。

- (void)textViewDidChange:(UITextView *)textView
{
    NSString* str = [textView text];
    str = [str substringToIndex:MIN(1000,[str length])];
    [textView setText:str];

    if([str length]==1000) {
        // show some label that you've reached the limit of 1000 characters
    }
}

下面的代码类似于sickp的答案，但是正确地处理复制-粘贴操作。如果尝试粘贴超过限制的文本，下面的代码将截断文本以符合限制，而不是完全拒绝粘贴操作。

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    static const NSUInteger limit = 70; // we limit to 70 characters
    NSUInteger allowedLength = limit - [textField.text length] + range.length;
    if (string.length > allowedLength) {
        if (string.length > 1) {
            // get at least the part of the new string that fits
            NSString *limitedString = [string substringToIndex:allowedLength];
            NSMutableString *newString = [textField.text mutableCopy];
            [newString replaceCharactersInRange:range withString:limitedString];
            textField.text = newString;
        }
        return NO;
    } else {
        return YES;
    }
}