在在网上查了资料，�很多都是下面这个方法，但是在原生键盘上是有问题的，当当在手机上英文九宫格连点的时候，会替换掉最后的文字

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

-(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)
        {
            if (toBeString.length > MAX_STARWORDS_LENGTH)
            {
                textField.text = [toBeString substringToIndex:MAX_STARWORDS_LENGTH];
            }
        }

    }
    // 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_STARWORDS_LENGTH)
        {
            NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_STARWORDS_LENGTH];
            if (rangeIndex.length == 1)
            {
                textField.text = [toBeString substringToIndex:MAX_STARWORDS_LENGTH];
            }
            else
            {
                NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_STARWORDS_LENGTH)];
                textField.text = [toBeString substringWithRange:rangeRange];
            }
        }
    }
}

经过多次测试后，终于解决了，首先要设置UITextField的代理，实现下面的代理方法，代码如下：

static const int nameMaxLength = 6;
@interface FCEditNameViewController ()<UITextFieldDelegate>
@property (strong, nonatomic) IBOutlet UITextField *nameTextField;
@property (nonatomic, strong) NSString     *oldStr;//记录修改之前的值
@property (nonatomic, strong) NSString     *positionStr;记录高亮的文字
@end

//当前光标位置 range.location 
//已选文字长度 range.length 
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
    if ([string isEqualToString:@""]) {
        return YES;
    }
    //获取高亮部分
    UITextRange *selectedRange = [textField markedTextRange];
    UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
    if (!position) {
        self.oldStr = textField.text;
        if (range.length + range.location > textField.text.length) {
            return NO;
        }
        NSInteger newStrLength = [textField.text length] + [string length] - range.length;
        if (newStrLength <= nameMaxLength) {
            return YES;
        }else{
            return NO;
        }
       
    }else{
        if ([self IsChinese:string]) {
            NSString *str = [NSString stringWithFormat:@"%@%@",self.oldStr,string];
            self.oldStr = @"";
            if (str.length > nameMaxLength) {
                return NO;
            }else{
                return YES;
            }
        }
    }
    return YES;
}

//判断是否有中文
- (BOOL)IsChinese:(NSString *)str {
    for(int i=0; i< [str length];i++){
        int a = [str characterAtIndex:i];
        if( a > 0x4e00 && a < 0x9fff)
        {
            return YES;
        }
    }
    return NO;
}