看到的收藏一下吧，总会用到的

#define kMaxLength  500    //输入数量

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(textFiledEditChanged:)
                                                name:@"UITextViewTextDidChangeNotification"
                                              object:_contentTextView];

-(void)textFiledEditChanged:(NSNotification *)obj{
    TextView *textField = (TextView *)obj.object;
    
    NSString *toBeString = textField.text;
    NSString *lang = [[UITextInputMode currentInputMode] primaryLanguage]; // 键盘输入模式
    
    if ([lang isEqualToString:@"zh-Hans"]) { // 简体中文输入，包括简体拼音，健体五笔，简体手写
        UITextRange *selectedRange = [textField markedTextRange];
        //获取高亮部分
        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 > kMaxLength) {
                textField.text = [toBeString substringToIndex:kMaxLength];
            }
        }
        // 有高亮选择的字符串，则暂不对文字进行统计和限制
        else{
            
        }
    }
    // 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 > kMaxLength) {
            textField.text = [toBeString substringToIndex:kMaxLength];
        }
    }
    
    self.instructionsLabel.text = [NSString stringWithFormat:@"%lu/500",(unsigned long)textField.text.length];
}

最后

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter]removeObserver:self
                                                   name:@"UITextViewTextDidChangeNotification"
                                                 object:_contentTextView];
}