javascript - 当 div 底部到达视口(viewport)底部时删除类

Question

我试图创建一种视差类型的效果，其中图像跟随用户直到内容结束。目前，当 div 到达视口(viewport)顶部时会添加一个类，但是当 div 内容滚动通过时我无法反转效果。

Here is a codepen example

HTML

<div class="filler">Scroll down</div>
<div class="sticky-stuff"></div>
<div class="content">
  Bacon ipsum dolor amet shankle pancetta bacon beef ribs shank strip steak beef andouille brisket ground round drumstick frankfurter. Beef rump brisket kevin ground round strip steak picanha pancetta andouille pig. Landjaeger pork meatball, turducken leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andou leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola. leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola. leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip stea leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola. leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola. leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs ando leberkas ribeye sausage frankfurter porchetta tongue fatback filet mignon pork belly beef ribs andouille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola.uille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola.k capicola flank andouille. Tongue pastrami prosciutto capicola.ille. Biltong fatback swine pancetta salami hamburger, meatloaf short ribs strip steak capicola flank andouille. Tongue pastrami prosciutto capicola. Chuck venison ball tip boudin capicola t-bone ham hock sirloin pork. Meatloaf t-bone pancetta cupim cow shankle drumstick pork chop corned beef strip steak. Pork chop beef tenderloin kielbasa. Beef ribs beef cow ham hock biltong alcatra kevin picanha fatback pig brisket ground round boudin porchetta rump. Ham hock drumstick andouille filet mignon prosciutto porchetta shoulder kielbasa sirloin, shank swine. Cow biltong jowl, short loin turkey frankfurter flank tri-tip pancetta tenderloin. Rump ground round sirloin, fatback drumstick salami prosciutto picanha. Sausage brisket porchetta, ham pancetta turducken pork chop. Short loin porchetta strip steak, frankfurter landjaeger bresaola hamburger sirloin.
</div>
<div class="filler"></div>

CSS

html, body {
  height: 100%;
  margin: 0px;
  padding: 0px;
}

.filler {
  height: 100%;
  background: green;
}

.sticky-stuff {
  width: 100%;
  height: 100vh;
  position: absolute;
  left: 0;
  background: url(http://i.imgur.com/yT65RYM.jpg) no-repeat center center / cover;
  z-index: -1;
}

.content {
  width: 400px;
  background: white;
  margin: 50px;
}

.sticky {
  position: fixed;
  top: 0;
}

JS

$(function() {

  var size = $(window).height(),
    item = $('.sticky-stuff');

  $(window).scroll(function() {

      var fromtop = $(window).scrollTop();

      if (fromtop > size) item.addClass('sticky');
      else item.removeClass('sticky');

      if (($(".content").offset().top + $(".content").height()) >= $(window).height()) {
        item.removeClass('sticky');
      }

    })
    .resize(function() {
      size = $(window).height();
    });
});

Answer 1

在以下情况下，您的 JavaScript 中存在错误。它总是正确的:

$(".content").offset().top + $(".content").height()) >= $(window).height()

应该是

$(".content").offset().top + $(".content").height()) < $(window).scrollTop()

您甚至可以通过使用更有意义的变量名称和更简单的条件来阐明您的 JavaScript。例如:

var triggerTop = $(".container").offset().top;
var triggerBottom = triggerTop + $(".container").height();
var item = $('.sticky-stuff');

$(window).scroll(function() {
    var curentScroll = $(window).scrollTop();
    var isSticky = curentScroll > triggerTop && curentScroll < triggerBottom
    item.toggleClass('sticky', isSticky);
});

你可以在这里玩http://codepen.io/tzi/pen/xZaxEv