UiTextField对输入的长度进行限制并提示用户还可输入的长度

@property (weak, nonatomic) IBOutlet UITextField *nickName;
@property(nonatomic,strong)UILabel *canEditSizeLabel;//可编辑数的label

 //给textfield添加一个点击事件用来监听字数的变化
    self.nickName.userInteractionEnabled = YES;
    [self.nickName addTarget:self action:@selector(limitLength:) forControlEvents:UIControlEventEditingChanged];

//监听textfield的字数长度的变化
-(void)limitLength:(UITextField *)sender
{
    //判断当前输入法是否是中文
    bool isChinese;
    if ([[[UITextInputMode currentInputMode] primaryLanguage] isEqualToString: @"en-US"]) {
        isChinese = false;
    }
    else
    {
        isChinese = true;
    }
    
    
    if(sender == self.nickName) {//判断当前输入框是我想要的输入框
        // 8位
        NSString *str = [[self.nickName text] stringByReplacingOccurrencesOfString:@"?" withString:@""];
        if (isChinese) { //中文输入法下
            UITextRange *selectedRange = [self.nickName markedTextRange];
            //获取高亮部分
            UITextPosition *position = [self.nickName 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) {
//                NSLog(@"汉字");
                if ( str.length>8) {//长度大于8时进行截取
                    NSString *strNew = [NSString stringWithString:str];
                    [self.nickName setText:[strNew substringToIndex:8]];
                }
                
                //改变提示信息
                _canEditSizeLabel.text = [NSString stringWithFormat:@"还可输入%ld个字",8-(self.nickName.text.length)];
            }
            else
            {
//                NSLog(@"输入的英文还没有转化为汉字的状态");
                
            }
        }else{//非中文输入法下
//            NSLog(@"str=%@; 本次长度=%d",str,[str length]);
            if ([str length]>8) {
                NSString *strNew = [NSString stringWithString:str];
                [self.nickName setText:[strNew substringToIndex:8]];
            }
            
            
            _canEditSizeLabel.text = [NSString stringWithFormat:@"还可输入%ld个字",8-(self.nickName.text.length)];
        }
    }
}