wxml:

<scroll-view  style="height: {{windowHeight}}px; width: {{windowWidth}}px;" scroll-y="true" bindscrolltoupper="pullDownRefresh" bindscrolltolower="pullUpLoad" bindtap="click_blank">

<view wx:if="{{comment}}" class="comment-input"><input type="text" name="content" auto-focus placeholder-style="margin-top:-8rpx;" placeholder="{{comment_placeholder}}" bindchange="comment_input" /><button>发送</button></view>

js:

// 评论（点击空白处隐藏input，点击评论出现）
    click_blank:function(e){
        var flag = "";
        var comment_placeholder ="";
        if(e.target.dataset.comment==1){
            flag = true;
            comment_placeholder = e.target.dataset.username;
        }else{
            flag = false;
            comment_placeholder = "";
        }
        this.setData({
            comment:flag,
            comment_placeholder:"评论"+comment_placeholder+"动态"
        })
    },