起初，方法是在cell.contentView上加一個label，在UITextView開始編輯時在代理方法裡隱藏label，結束編輯時如果UITextView沒文字，再把label顯示出來。相比如下方法顯得麻煩。

下面是通過runtime打印發現的UITextView裡有佔位符私有變數，可通過KVC直接設定一個佔位符，相對簡單，而且是可以釋出通過的。

// 通過執行時，發現UITextView有一個叫做“_placeHolderLabel”的私有變數
    unsigned int count = 0;
    Ivar *ivars = class_copyIvarList([UITextView class], &count);
    
    for (int i = 0; i < count; i++) {
        Ivar ivar = ivars[i];
        const char *name = ivar_getName(ivar);
        NSString *objcName = [NSString stringWithUTF8String:name];
        NSLog(@"%d : %@",i,objcName);
    }
 

static NSString *questionCellID = @"questionCellID";
            UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:questionCellID];
            if (cell == nil) {
                cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:questionCellID];
                cell.selectionStyle = UITableViewCellSelectionStyleNone;
                
                UITextView *questionTV = [[UITextView alloc] initWithFrame:CGRectMake(15, 0, WID-30, 120)];
                questionTV.font = [UIFont systemFontOfSize:12];
                questionTV.textColor = [UIColor colorWithHexString:@"323232"];
                questionTV.tag = 70;
                [cell.contentView addSubview:questionTV];
                
                // _placeholderLabel
                UILabel *placeHolderLabel = [[UILabel alloc] initWithFrame:CGRectMake(20, 12, CGRectGetWidth(questionTV.frame)-10, 14)];
                placeHolderLabel.text = @"請描述您的病史、家族史及想要諮詢的主題";
                placeHolderLabel.numberOfLines = 0;
                placeHolderLabel.textColor = [UIColor colorWithHexString:@"9a9a9a"];
                placeHolderLabel.font = [UIFont systemFontOfSize:10];
                placeHolderLabel.tag = 90;
                [cell.contentView addSubview:placeHolderLabel];
            }
            UITextView *questionTV = (UITextView *)[cell.contentView viewWithTag:70];
            questionTV.delegate = self;
            questionTV.text = _question;
            UILabel *placeHolderLabel = (UILabel *)[cell.contentView viewWithTag:90];
            placeHolderLabel.hidden = (_question.length > 0 ? YES : NO);
            
            return cell;
 

補充：經測試，上面的用KVC設定私有變數的方法，在iOS8上會崩掉，檢視runtime列印的變數名沒有_placeholderLabel，所以，還是老老實實往cell上加一個label吧。

設定textView的代理，然後加下面的程式碼

#pragma mark - UITextViewDelegate
-(void)textViewDidChange:(UITextView *)textView
{
    _question = textView.text;
    UITableViewCell *cell = [_tableView cellForRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:4]];
    UILabel *placeHolderLabel = (UILabel *)[cell.contentView viewWithTag:90];
    if (textView.text.length == 0){
        placeHolderLabel.hidden = NO;
    }else{
        placeHolderLabel.hidden = YES;
    }
}