定义两个UITextView，一个用于输入文本，一个用于显示提示信息，当输入文本为空时显示提示信息，否则不显示提示信息。

        //6.3.1文字内容提示
        _contentTextViewTip = [[UITextView alloc] initWithFrame:CGRectMake(12, 5, WIDTH_SCREEN - 12 - 12, 105)];
        if (_contentTextViewTip != nil)
        {
            _contentTextViewTip.text = POST_CONTENT_TIP;
            _contentTextViewTip.font = [UIFont systemFontOfSize:16.0];
            _contentTextViewTip.userInteractionEnabled = NO;
            _contentTextView.delegate = self;
            _contentTextViewTip.textColor = [[UIColor alloc] initWithRed:160/255.0 green:160/255.0 blue:160/255.0 alpha:1.0];
            [scrollView addSubview:_contentTextViewTip];
        }
        
        //6.3.2文字内容文本框
        _textViewRect = CGRectMake(12, 5, WIDTH_SCREEN - 12 * 2, 105);
        if(_contentTextView != nil)
        {
            _contentTextView = [[UITextView alloc]initWithFrame:_textViewRect];
            _contentTextView.backgroundColor = [UIColor clearColor];
            _contentTextView.font = [UIFont systemFontOfSize:16.0];
            //_contentTextView.textColor = [[UIColor alloc] initWithRed:160/255.0 green:160/255.0 blue:160/255.0 alpha:1.0];
            //_contentTextView.text = @"说点什么~";
            [_contentTextView setScrollEnabled:YES];
            _contentTextView.userInteractionEnabled = YES;
            _contentTextView.showsVerticalScrollIndicator = YES;
            CGSize size = CGSizeMake(WIDTH_SCREEN - 12 * 2, 600.0f);
            [_contentTextView setContentSize:size];
            _contentTextView.returnKeyType = UIReturnKeyDone;
            _contentTextView.keyboardType = UIKeyboardTypeDefault;
            _contentTextView.delegate = self;
            [scrollView addSubview:_contentTextView];
        }

#pragma mark _contentTextView Delegate
/**
 *  文字改变时触发的事件
 */
- (void)textViewDidChange:(UITextView *)textView
{
    NSLog(@"textViewDidChange：%@", textView.text);
    if (textView == _contentTextView)
    {
        
        if ([textView.text isEqualToString:@""])
        {
            _contentTextViewTip.alpha = 1;
        }
        else
        {
            _contentTextViewTip.alpha = 0;
        }
    }
}

ios UITextView 提示文本