记录一下限制输入字符的判断。不仅局限于中文或英文

• 首先在ViewDidLoad中注册通知

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(textFieldEditChanged:)
    name:@"UITextFieldTextDidChangeNotification" object:self.userTF];

• 下面是判断逻辑 由于需求有中文键盘下的字母数字输入，所以其中那部分判断如不需要可以去除

// 输入字符判断
-(void)textFieldEditChanged:(NSNotification *)obj {
    UITextField *textField = (UITextField *)obj.object;
    NSString *toBeString = textField.text;
    NSString *lang = [textField.textInputMode primaryLanguage];
    if ([lang isEqualToString:@"zh-Hans"])// 简体中文输入
        {
        //获取高亮部分
        UITextRange *selectedRange = [textField markedTextRange];
        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
     
        // 没有高亮选择的字，则对已输入的文字进行字数统计和限制
        
        if (!position || !selectedRange)
        {
            if (toBeString.length > MAX_CHINESE_LENGTH)
            {
                NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_CHINESE_LENGTH];
                if (rangeIndex.length == 1)
                {
                    
                    textField.text = [toBeString substringToIndex:MAX_CHINESE_LENGTH];
                }
                else
                {
                    NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_CHINESE_LENGTH)];
                    textField.text = [toBeString substringWithRange:rangeRange];
                }
            }
            if (toBeString.length > MAX_ENGLISH_LENGTH)
            {
                NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_ENGLISH_LENGTH];
                if (rangeIndex.length == 1)
                {
                    textField.text = [toBeString substringToIndex:MAX_ENGLISH_LENGTH];
                }
                else
                {
                    NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_ENGLISH_LENGTH)];
                    textField.text = [toBeString substringWithRange:rangeRange];
                }
            }
        } 
    }
        // 中文输入法以外的直接对其统计限制即可，不考虑其他语种情况
        else
        {
            if (toBeString.length > MAX_ENGLISH_LENGTH)
                {
                    NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_ENGLISH_LENGTH];
                    if (rangeIndex.length == 1)
                    {
                        textField.text = [toBeString substringToIndex:MAX_ENGLISH_LENGTH];
                    }
                    else
                    {
                NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_ENGLISH_LENGTH)];
                textField.text = [toBeString substringWithRange:rangeRange];
                    }
                }
            }
        } 

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