转自：http://m.2cto.com/kf/201608/534005.html

流程：
1.创建textView
2.给textView添加一个UILabel子控件，作为placeholder
3.在文本改变的代理方法里面显示/隐藏UILabel

特点：
该方法同样也可以实现类似于placeholder的功能。相比较方法一，方法二可以实现动态监听文本的改变，并非弹出键盘就立即清除placeholder，只有当用户开始输入文本的时候。placeholder才会消失。同样，当用户清空文本的时候，placeholder又会重新显示出来。

代理：<UITextViewDelegate>

pragma mark - 绘制 发布 界面

• (void)createPublishView{

  UITextView * textView = [[UITextView alloc]init];
  self.textView = textView;
  [self.backgroundView addSubview:_textView];

  [_textView mas_makeConstraints:^(MASConstraintMaker *make) {

    make.left.right.top.mas_equalTo(0);
    make.height.mas_equalTo(74);
  

  }];

  _textView.backgroundColor = [UIColor redColor];
  _textView.delegate = self;
  [self setupPlaceHolder];

}

// 给textView添加一个UILabel子控件

• (void)setupPlaceHolder {
  UILabel *placeHolder = [[UILabel alloc] init];
  self.placeHolder = placeHolder;
  [self.textView addSubview:placeHolder];

  [_placeHolder mas_makeConstraints:^(MASConstraintMaker *make) {

    make.left.mas_equalTo(15);
    make.top.mas_equalTo(12);
    make.height.mas_equalTo(20);
  

  }];

  placeHolder.backgroundColor = [UIColor yellowColor];

placeHolder.text = @"说点什么吧...";
placeHolder.textColor = [UIColor lightGrayColor];
placeHolder.numberOfLines = 0;
placeHolder.contentMode = UIViewContentModeTop;
placeHolder.font = [UIFont jk_systemFontOfPxSize:14];

}

pragma mark - TextView 代理 & 数据源

• (void)textViewDidChange:(UITextView *)textView {
  if (!textView.text.length) {
  self.placeHolder.alpha = 1;
  } else {
  self.placeHolder.alpha = 0;
  }
  }
  //关闭键盘
  -(void) dismissKeyBoard{

  [self.textView resignFirstResponder];

}