How to tell if a DOM element is visible in the cur

2018-12-30 23:42发布

站内问答 / JavaScript
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Is there an efficient way to tell if a DOM element (in an HTML document) is currently visible (appears in the viewport)?

(The question regards Firefox)

22条回答
不流泪的眼
2楼-- · 2018-12-31 00:00

Now most browsers support getBoundingClientRect method, which has become the best practice. Using an old answer is very slow, not accurate and has several bugs.

The solution selected as correct is almost never precise. You can read more about its bugs.

This solution was tested on IE7+, iOS5+ Safari, Android2+, Blackberry, Opera Mobile, and IE Mobile 10.

function isElementInViewport (el) {

    //special bonus for those using jQuery
    if (typeof jQuery === "function" && el instanceof jQuery) {
        el = el[0];
    }

    var rect = el.getBoundingClientRect();

    return (
        rect.top >= 0 &&
        rect.left >= 0 &&
        rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /*or $(window).height() */
        rect.right <= (window.innerWidth || document.documentElement.clientWidth) /*or $(window).width() */
    );
}

How to use:

You can be sure that the function given above returns correct answer at the moment of time when it is called, but what about tracking element's visibility as an event?

Place the following code at the bottom of your <body> tag:

function onVisibilityChange(el, callback) {
    var old_visible;
    return function () {
        var visible = isElementInViewport(el);
        if (visible != old_visible) {
            old_visible = visible;
            if (typeof callback == 'function') {
                callback();
            }
        }
    }
}

var handler = onVisibilityChange(el, function() {
    /* your code go here */
});


//jQuery
$(window).on('DOMContentLoaded load resize scroll', handler); 

/* //non-jQuery
if (window.addEventListener) {
    addEventListener('DOMContentLoaded', handler, false); 
    addEventListener('load', handler, false); 
    addEventListener('scroll', handler, false); 
    addEventListener('resize', handler, false); 
} else if (window.attachEvent)  {
    attachEvent('onDOMContentLoaded', handler); // IE9+ :(
    attachEvent('onload', handler);
    attachEvent('onscroll', handler);
    attachEvent('onresize', handler);
}
*/

If you do any DOM modifications, they can change your element's visibility of course.

Guidelines and common pitfalls:

Maybe you need to track page zoom / mobile device pinch? jQuery should handle zoom/pinch cross browser, otherwise first or second link should help you.

If you modify DOM, it can affect the element's visibility. You should take control over that and call handler() manually. Unfortunately, we have no cross browser onrepaint event. On the other hand that allows us to make optimizations and perform re-check only on DOM modifications that can change element's visibility.

Never Ever use it inside jQuery $(document).ready() only, because there is no warranty CSS has been applied in this moment. Your code can work locally with your CSS on hard drive, but once put on remote server it will fail.

After DOMContentLoaded is fired, styles are applied, but the images are not loaded yet. So, we should add window.onload event listener.

We can't catch zoom/pinch event yet.

The last resort could be the following code:

/* TODO: this looks like a very bad code */
setInterval(handler, 600);

You can use awesome feature pageVisibiliy HTML5 API if you care if the tab with your web page is active and visible.

TODO: this method does not handle two situations:

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伤终究还是伤i
3楼-- · 2018-12-31 00:01

I use this function (it only checks if the y is inscreen since most of the time the x is not needed) 

function elementInViewport(el) {
    var elinfo = {
        "top":el.offsetTop,
        "height":el.offsetHeight,
    };

    if (elinfo.top + elinfo.height < window.pageYOffset || elinfo.top > window.pageYOffset + window.innerHeight) {
        return false;
    } else {
        return true;
    }

}
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低头抚发
4楼-- · 2018-12-31 00:02

Here's my solution, it will work if an element is hidden inside a scroll-able container.

Here's a demo (try re-sizing the window to)

var visibleY = function(el){
    var top = el.getBoundingClientRect().top, rect, el = el.parentNode;
    do {
        rect = el.getBoundingClientRect();
        if (top <= rect.bottom === false)
            return false;
        el = el.parentNode;
    } while (el != document.body);
    // Check its within the document viewport
    return top <= document.documentElement.clientHeight;
};

I only needed to check if it's visible in the Y axis (for a scrolling ajax load more records feature).

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余生请多指教
5楼-- · 2018-12-31 00:04

my shorter and faster version.

function isElementOutViewport(el){
    var rect = el.getBoundingClientRect();
    return rect.bottom < 0 || rect.right < 0 || rect.left > window.innerWidth || rect.top > window.innerHeight;
}

add jsFiddle as required https://jsfiddle.net/on1g619L/1/

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余生无你
6楼-- · 2018-12-31 00:04

For a similar challenge i really enjoyed this gist which exposes a polyfill for scrollIntoViewIfNeeded().

All the necessary Kung Fu needed to answer is within this block:

var parent = this.parentNode,
    parentComputedStyle = window.getComputedStyle(parent, null),
    parentBorderTopWidth = parseInt(parentComputedStyle.getPropertyValue('border-top-width')),
    parentBorderLeftWidth = parseInt(parentComputedStyle.getPropertyValue('border-left-width')),
    overTop = this.offsetTop - parent.offsetTop < parent.scrollTop,
    overBottom = (this.offsetTop - parent.offsetTop + this.clientHeight - parentBorderTopWidth) > (parent.scrollTop + parent.clientHeight),
    overLeft = this.offsetLeft - parent.offsetLeft < parent.scrollLeft,
    overRight = (this.offsetLeft - parent.offsetLeft + this.clientWidth - parentBorderLeftWidth) > (parent.scrollLeft + parent.clientWidth),
    alignWithTop = overTop && !overBottom;

this refers to the element that you want to know if it is i.e. overTop or overBottom - just should get the drift...

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残风、尘缘若梦
7楼-- · 2018-12-31 00:05

There is jQuery plugin called inview that does the job

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