优化之黏性吸顶(sticky)

如何正确在项目使用sticky优化吸顶用户体验

MDN是这样讲sticky的

The element is positioned according to the normal flow of the document, and then offset relative to its nearest scrolling ancestor and containing block (nearest block-level ancestor), including table-related elements, based on the values of top, right, bottom, and left. The offset does not affect the position of any other elements.
This value always creates a new stacking context. Note that a sticky element "sticks" to its nearest ancestor that has a "scrolling mechanism" (created when overflow is hidden, scroll, auto, or overlay), even if that ancestor isn't the nearest actually scrolling ancestor. This effectively inhibits any "sticky" behavior (see the Github issue on W3C CSSWG).

具体看文档 https://developer.mozilla.org/en-US/docs/Web/CSS/position

我们说说注意事项

比较坑的就是对父元素的要求，也就是 父元素一定需要滚动(父元素哦)，不然你看不到效果。比如你在前面包一层div，对不起，不会生效。

（其实这本身就不坑。好比如你使用滚动插件的时候，你需要设置高度一样，因为这样好确定位置，如果要冒泡的话，消费比较多的资源）

但是呢，兼容性不乐观，于是我们需要进行降级方案。

怎么降级，用什么呢？

当然是用JS了。先判断下是否支持

/**
* 判断是否css3中的Sticky属性
* @returns {boolean}
*/
export const isSupportSticky = function() {
    var prefixTestList = ['', '-webkit-', '-ms-', '-moz-', '-o-']
    var stickyText = ''
    for (var i = 0; i < prefixTestList.length; i++) {
        stickyText += 'position:' + prefixTestList[i] + 'sticky;'
    }
    // 创建一个dom来检查
    var div = document.createElement('div')
    var body = document.body
    div.style.cssText = 'display:none;' + stickyText
    body.appendChild(div)
    var isSupport = /sticky/i.test(window.getComputedStyle(div).position)
    body.removeChild(div)
    div = null
    return isSupport
}

不支持就JS控制就好了。如 isSupportSticky() || ceiling(父元素DOM哦)

顺便附上代码

function ceiling(obj) {
    var ot = obj.offsetTop;
    var box = obj.parentNode
    box.onscroll = function () {
        var st = box.scrollTop || box.scrollTop
        obj.style.position = st >= ot ? "fixed" : ""
    }
}

好了完美