jQuery，如果滚动是一定数量的像素

问题描述：

是否有内置的jQuery函数来确定滚动的长度?

Is there a built in jQuery function to determine the length of a scroll?

我正在尝试编写一个函数，如果用户从顶部滚动了50个像素，则会向div中添加一个类.

I'm trying to write a function that will add a class to a div if the user has scrolled 50 pixels from the top.

所以代码将如下所示:

if(userHasScrolled) {
     $('#myDiv').addClass('hasScrolled');

}

答

您可以在滚动处理程序内部检查$(document).scrollTop():

You can check $(document).scrollTop() inside of a scroll handler:

var $document = $(document),
    $element = $('#some-element'),
    className = 'hasScrolled';

$document.scroll(function() {
  if ($document.scrollTop() >= 50) {
    // user scrolled 50 pixels or more;
    // do stuff
    $element.addClass(className);
  } else {
    $element.removeClass(className);
  }
});

如果只需要添加类名(无需其他操作)，则可以简化为:

If adding the class name is all you want (no other actions needed), this could be shortened to:

var $document = $(document),
    $element = $('#some-element'),
    className = 'hasScrolled';

$document.scroll(function() {
  $element.toggleClass(className, $document.scrollTop() >= 50);
});

请参见 http://api.jquery.com/scrollTop/.

请注意，使用scroll事件处理程序的效率非常低.

Note that it is very inefficient to use scroll event handlers.

jQuery，如果滚动是一定数量的像素

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