jQuery - 检查元素是否可见，淡入淡出 [英] jQuery - check if elements come into view, fade in those that do

问题描述

当我知道要指定哪个元素时，我已经找到了这个问题的答案，但是我正在寻找的方法是检查滚动中是否有具有特定类的ANY元素进入了视图，并对它们进行了修改就像他们所做的那样（例如改变不透明度 - 只有那些进入视野的）。我知道代码可能看起来类似于此，但我无法完成工作：

I have found the answer to this issue when I know which element to specify, but what I am looking for is for a way to check 'on scroll' whether ANY element with a specific class has come into view, and modify them as they do (e.g. change opacity - only those that come into view). I know the code might look something similar to this, but I can't make it work:

jQuery(window).on("scroll", function() {
var difference = jQuery(window).offset().top + jQuery(window).height()/2;
if (difference > jQuery(".makeVisible").offset().top) {
     jQuery(this).animate({opacity: 1.0}, 500);

}
});

非常感谢。
注意：存在变量差异是因为我希望元素在到达屏幕中间时可见。

Thank you very much. Note: the variable difference exists because I want elements to become visible as they reach the middle of the screen.

推荐答案

借用滚动后检查元素是否可见和使用jQuery在屏幕上居中DIV 来检查元素位于屏幕的可见中心：

Borrowing from Check if element is visible after scrolling and Using jQuery to center a DIV on the screen to check if the element is in the viewable center of the screen:

function isScrolledIntoView(elem)
{
    var centerY = Math.max(0,((jQuery(window).height()-jQuery(elem).outerHeight()) / 2) 
                  + jQuery(window).scrollTop());

    var elementTop = jQuery(elem).offset().top;
    var elementBottom = elementTop + jQuery(elem).height();

    return elementTop <= centerY && elementBottom >= centerY;
}

然后，我们可以修改您的方法：

We can then modify your approach to:

jQuery(window).on("scroll resize", function() {
    jQuery(".makeVisible").each(function(index, element) {
        if (isScrolledIntoView(element)) {
           jQuery(element).animate({opacity: 1.0}, 500);
        }
    });
});

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