Listening scrollbar scrollbar (phone adapter)
1. document.documentElement.scrollHeight: html entire height of containing hidden interface level
2. window.pageYOffset: volume to be scrolled height
3. window.innerHeight: visible window height, the same
4. 2 plus 3 wherein the height is not less than the height of the bottom roll 1 represents
5. Note: listening rolling body disposed onscroll body can not be set in height: 100% can be used