过去，要检测一个元素是否可见或者两个元素是否相交并不容易，很多解决办法不可靠或性能很差。然而，随着互联网的发展，这种需求却与日俱增，比如，下面这些情况都需要用到相交检测：

过去，相交检测通常要用到事件监听，并且需要频繁调用 {{domxref("Element.getBoundingClientRect()")}} 方法以获取相关元素的边界信息。事件监听和调用 {{domxref("Element.getBoundingClientRect()")}} 都是在主线程上运行，因此频繁触发、调用可能会造成性能问题。这种检测方法极其怪异且不优雅。

假如有一个无限滚动的网页，开发者使用了一个第三方库来管理整个页面的广告，又用了另外一个库来实现消息盒子和点赞，并且页面有很多动画（译注：动画往往意味着较高的性能消耗）。两个库都有自己的相交检测程序，都运行在主线程里，而网站的开发者对这些库的内部实现知之甚少，所以并未意识到有什么问题。但当用户滚动页面时，这些相交检测程序就会在页面滚动回调函数里不停触发调用，造成性能问题，体验效果让人失望。

Intersection Observer API 会注册一个回调函数，每当被监视的元素进入或者退出另外一个元素时(或者 {{Glossary("viewport")}} )，或者两个元素的相交部分大小发生变化时，该回调方法会被触发执行。这样，我们网站的主线程不需要再为了监听元素相交而辛苦劳作，浏览器会自行优化元素相交管理。

注意 Intersection Observer API 无法提供重叠的像素个数或者具体哪个像素重叠，他的更常见的使用方式是——当两个元素相交比例在 N% 左右时，触发回调，以执行某些逻辑。

Intersection observer 的概念和用法

Intersection Observer API 允许你配置一个回调函数，当以下情况发生时会被调用

通常，您需要关注文档最接近的可滚动祖先元素的交集更改，如果元素不是可滚动元素的后代，则默认为设备视窗。如果要观察相对于根(root)元素的交集，请指定根(root)元素为null。

无论您是使用视口还是其他元素作为根，API都以相同的方式工作，只要目标元素的可见性发生变化，就会执行您提供的回调函数，以便它与所需的交叉点交叉。

目标(target)元素与根(root)元素之间的交叉度是交叉比(intersection ratio)。这是目标(target)元素相对于根(root)的交集百分比的表示，它的取值在0.0和1.0之间。

创建一个 intersection observer

创建一个 IntersectionObserver 对象，并传入相应参数和回调用函数，该回调函数将会在目标(target)元素和根(root)元素的交集大小超过阈值(threshold)规定的大小时候被执行。

阈值为1.0意味着目标元素完全出现在 root 选项指定的元素中可见时，回调函数将会被执行。

Intersection observer options

传递到 {{domxref("IntersectionObserver.IntersectionObserver", "IntersectionObserver()")}} 构造函数的 options 对象，允许您控制观察者的回调函数的被调用时的环境。它有以下字段：

每当目标满足该 IntersectionObserver 指定的 threshold 值，回调被调用。

只要目标满足为 IntersectionObserver 指定的阈值，就会调用回调。回调接收 {{domxref("IntersectionObserverEntry")}} 对象和观察者的列表：

请留意，你注册的回调函数将会在主线程中被执行。所以该函数执行速度要尽可能的快。如果有一些耗时的操作需要执行，建议使用 {{domxref("Window.requestIdleCallback()")}} 方法。

How intersection is calculated -- 交集的计算

所有区域均被 Intersection Observer API 当做一个矩形看待。如果元素是不规则的图形也将会被看成一个包含元素所有区域的最小矩形，相似的，如果元素发生的交集部分不是一个矩形，那么也会被看作是一个包含他所有交集区域的最小矩形。

上述解释有助于理解IntersectionObserverEntry 提供的属性，其用于描述 target 和 root 的交集。

The intersection root and root margin

在我们开始跟踪 target 元素和容器元素之前，我们要先知道什么是容器 (root) 元素。容器元素又称为 intersection root，或 root element。这个既可以是 target 元素祖先元素也可以是指定 null 则使用浏览器视口做为容器(root)。

root intersection rectangle 是用来对目标元素进行相交检测的矩形，它的大小有以下几种情况:

用作描述 intersection root 元素边界的矩形可以使用 root margin 来调整矩形大小，即 rootMargin 属性，在我们创建 IntersectionObserver 对象的时候使用。rootMargin 的属性值将会做为 margin 偏移值添加到 intersection root 元素的对应的 margin 位置，并最终形成 root 元素的矩形边界 (在执行回调函数时可由 {{domxref("IntersectionObserverEntry.rootBounds")}} 得到)。

Thresholds

IntersectionObserver API 并不会每次在元素的交集发生变化的时候都会执行回调。相反它使用了 thresholds 参数。当你创建一个 observer 的时候，你可以提供一个或者多个 number 类型的数值用来表示 target 元素在 root 元素的可见程序的百分比，然后，API的回调函数只会在元素达到 thresholds 规定的阈值时才会执行。

例如，当你想要在 target 在 root 元素中中的可见性每超过25%或者减少25%的时候都通知一次。你可以在创建 observer 的时候指定 thresholds 属性值为 [0, 0.25, 0.5, 0.75, 1]，你可以通过检测在每次交集发生变化的时候的都会传递回调函数的参数 {{domxref("IntersectionObserverEntry.isIntersecting", "isIntersecting")}} 的属性值来判断 target 元素在 root 元素中的可见性是否发生变化。如果 isIntersecting 为真，target 元素的至少已经达到 thresholds 属性值当中规定的其中一个阈值，如果为假，则 target 元素不在给定的阈值范围内可见。

Note that it's possible to have a non-zero intersection rectangle, which can happen if the intersection is exactly along the boundary between the two or the area of {{domxref("IntersectionObserverEntry.boundingClientRect", "boundingClientRect")}} is zero. This state of the target and root sharing a boundary line is not considered enough to be considered transitioning into an intersecting state.

为了让我们感受下 thresholds 是如何工作的，尝试滚动以下的例子，每一个 colored box 的四个边角都会展示自身在 root 元素中的可见程度百分比，所以在你滚动 root 的时候你将会看到四个边角的数值一直在发生变化。每一个 box 都有不同的 thresholds：

<template id="boxTemplate">
  <div class="sampleBox">
    <div class="label topLeft"></div>
    <div class="label topRight"></div>
    <div class="label bottomLeft"></div>
    <div class="label bottomRight"></div>
  </div>
</template>

<main>
  <div class="contents">
    <div class="wrapper">
    </div>
  </div>
</main>

.contents {
  position: absolute;
  width: 700px;
  height: 1725px;
}

.wrapper {
  position: relative;
  top: 600px;
}

.sampleBox {
  position: relative;
  left: 175px;
  width: 150px;
  background-color: rgb(245, 170, 140);
  border: 2px solid rgb(201, 126, 17);
  padding: 4px;
  margin-bottom: 6px;
}

#box1 {
  height: 200px;
}

#box2 {
  height: 75px;
}

#box3 {
  height: 150px;
}

#box4 {
  height: 100px;
}

.label {
  font: 14px "Open Sans", "Arial", sans-serif;
  position: absolute;
  margin: 0;
  background-color: rgba(255, 255, 255, 0.7);
  border: 1px solid rgba(0, 0, 0, 0.7);
  width: 3em;
  height: 18px;
  padding: 2px;
  text-align: center;
}

.topLeft {
  left: 2px;
  top: 2px;
}

.topRight {
  right: 2px;
  top: 2px;
}

.bottomLeft {
  bottom: 2px;
  left: 2px;
}

.bottomRight {
  bottom: 2px;
  right: 2px;
}

let observers = [];

startup = () => {
  let wrapper = document.querySelector(".wrapper");

  // Options for the observers

  let observerOptions = {
    root: null,
    rootMargin: "0px",
    threshold: []
  };

  // An array of threshold sets for each of the boxes. The
  // first box's thresholds are set programmatically
  // since there will be so many of them (for each percentage
  // point).

  let thresholdSets = [
    [],
    [0.5],
    [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0],
    [0, 0.25, 0.5, 0.75, 1.0]
  ];

  for (let i=0; i<=1.0; i+= 0.01) {
    thresholdSets[0].push(i);
  }

  // Add each box, creating a new observer for each

  for (let i=0; i<4; i++) {
    let template = document.querySelector("#boxTemplate").content.cloneNode(true);
    let boxID = "box" + (i+1);
    template.querySelector(".sampleBox").id = boxID;
    wrapper.appendChild(document.importNode(template, true));

    // Set up the observer for this box

    observerOptions.threshold = thresholdSets[i];
    observers[i] = new IntersectionObserver(intersectionCallback, observerOptions);
    observers[i].observe(document.querySelector("#" + boxID));
  }

  // Scroll to the starting position

  document.scrollingElement.scrollTop = wrapper.firstElementChild.getBoundingClientRect().top + window.scrollY;
  document.scrollingElement.scrollLeft = 750;
}

intersectionCallback = (entries) => {
  entries.forEach((entry) => {
    let box = entry.target;
    let visiblePct = (Math.floor(entry.intersectionRatio * 100)) + "%";

    box.querySelector(".topLeft").innerHTML = visiblePct;
    box.querySelector(".topRight").innerHTML = visiblePct;
    box.querySelector(".bottomLeft").innerHTML = visiblePct;
    box.querySelector(".bottomRight").innerHTML = visiblePct;
  });
}

startup();

Clipping and the intersection rectangle

The browser computes the final intersection rectangle as follows; this is all done for you, but it can be helpful to understand these steps in order to better grasp exactly when intersections will occur.

Intersection change callbacks

When the amount of a target element which is visible within the root element crosses one of the visibility thresholds, the {{domxref("IntersectionObserver")}} object's callback is executed. The callback receives as input an array of all of {{domxref("IntersectionObserverEntry")}} objects, one for each threshold which was crossed, and a reference to the IntersectionObserver object itself.

Each entry in the list of thresholds is an {{domxref("IntersectionObserverEntry")}} object describing one threshold that was crossed; that is, each entry describes how much of a given element is intersecting with the root element, whether or not the element is considered to be intersecting or not, and the direction in which the transition occurred.

The code snippet below shows a callback which keeps a counter of how many times elements transition from not intersecting the root to intersecting by at least 75%. For a threshold value of 0.0 (default) the callback is called approximately upon transition of the boolean value of {{domxref("IntersectionObserverEntry.isIntersecting", "isIntersecting")}}. The snippet thus first checks that the transition is a positive one, then determines whether {{domxref("IntersectionObserverEntry.intersectionRatio", "intersectionRatio")}} is above 75%, in which case it increments the counter.

Interfaces

A simple example

This simple example causes a target element to change its color and transparency as it becomes more or less visible. At Timing element visibility with the Intersection Observer API, you can find a more extensive example showing how to time how long a set of elements (such as ads) are visible to the user and to react to that information by recording statistics or by updating elements..

HTML

The HTML for this example is very short, with a primary element which is the box that we'll be targeting (with the creative ID "box") and some contents within the box.

CSS

The CSS isn't terribly important for the purposes of this example; it lays out the element and establishes that the {{cssxref("background-color")}} and {{cssxref("border")}} attributes can participate in CSS transitions, which we'll use to affect the changes to the element as it becomes more or less obscured.

JavaScript

Finally, let's take a look at the JavaScript code that uses the Intersection Observer API to make things happen.

Setting up

We call {{domxref("EventTarget.addEventListener", "Window.addEventListener()")}} to start listening for the {{event("load")}} event; once the page has finished loading, we get a reference to the element with the ID "box" using {{domxref("Document.querySelector", "querySelector()")}}, then call the createObserver() method we'll create in a moment to handle building and installing the intersection observer.

Creating the intersection observer

The createObserver() method is called once page load is complete to handle actually creating the new {{domxref("IntersectionObserver")}} and starting the process of observing the target element.

This begins by setting up an options object containing the settings for the observer. We want to watch for changes in visibility of the target element relative to the document's viewport, so root is null. We need no margin, so the margin offset, rootMargin, is specified as "0px". This causes the observer to watch for changes in the intersection between the target element's bounds and those of the viewport, without any added (or subtracted) space.

The list of visibility ratio thresholds, threshold, is constructed by the function buildThresholdList(). The threshold list is built programmatically in this example since there are a number of them and the number is intended to be adjustable.

Once options is ready, we create the new observer, calling the {{domxref("IntersectionObserver.IntersectionObserver", "IntersectionObserver()")}} constructor, specifying a function to be called when intersection crosses one of our thresholds, handleIntersect(), and our set of options. We then call {{domxref("IntersectionObserver.observe", "observe()")}} on the returned observer, passing into it the desired target element.

We could opt to monitor multiple elements for visibility intersection changes with respect to the viewport by calling observer.observe() for each of those elements, if we wanted to do so.

Building the array of threshold ratios

The buildThresholdList() function, which builds the list of thresholds, looks like this:

This builds the array of thresholds—each of which is a ratio between 0.0 and 1.0, by pushing the value i/numSteps onto the thresholds array for each integer i between 1 and numSteps. It also pushes 0 to include that value. The result, given the default value of numSteps (20), is the following list of thresholds:

We could, of course, hard-code the array of thresholds into our code, and often that's what you'll end up doing. But this example leaves room for adding configuration controls to adjust the granularity, for example.

Handling intersection changes

#	Ratio	#	Ratio
1	0.05	11	0.55
2	0.1	12	0.6
3	0.15	13	0.65
4	0.2	14	0.7
5	0.25	15	0.75
6	0.3	16	0.8
7	0.35	17	0.85
8	0.4	18	0.9
9	0.45	19	0.95
10	0.5	20	1.0

When the browser detects that the target element (in our case, the one with the ID "box") has been unveiled or obscured such that its visibility ratio crosses one of the thresholds in our list, it calls our handler function, handleIntersect():

For each {{domxref("IntersectionObserverEntry")}} in the list entries, we look to see if the entry's {{domxref("IntersectionObserverEntry.intersectionRatio", "intersectionRatio")}} is going up; if it is, we set the target's {{cssxref("background-color")}} to the string in increasingColor (remember, it's "rgba(40, 40, 190, ratio)"), replaces the word "ratio" with the entry's intersectionRatio. The result: not only does the color get changed, but the transparency of the target element changes, too; as the intersection ratio goes down, the background color's alpha value goes down with it, resulting in an element that's more transparent.

Similarly, if the intersectionRatio is going down, we use the string decreasingColor and replace the word "ratio" in that with the intersectionRatio before setting the target element's background-color.

Finally, in order to track whether the intersection ratio is going up or down, we remember the current ratio in the variable prevRatio.

Result

Below is the resulting content. Scroll this page up and down and notice how the appearance of the box changes as you do so.