(function (root, factory) {
if (typeof define === 'function' && define.amd) {
define(function() {
return factory(root);
});
} else if (typeof exports === 'object') {
module.exports = factory;
} else {
root.echo = factory(root);
}
})(this, function (root) {
'use strict';
var echo = {};
var callback = function () {};
var offset, poll, delay, useDebounce, unload;
var isHidden = function (element) {
return (element.offsetParent === null);
};
var inView = function (element, view) {
if (isHidden(element)) {
return false;
}
var box = element.getBoundingClientRect();
return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
};
var debounceOrThrottle = function () {
if(!useDebounce && !!poll) {
return;
}
clearTimeout(poll);
poll = setTimeout(function(){
echo.render();
poll = null;
}, delay);
};
echo.init = function (opts) {
opts = opts || {};
var offsetAll = opts.offset || 0;
var offsetVertical = opts.offsetVertical || offsetAll;
var offsetHorizontal = opts.offsetHorizontal || offsetAll;
var optionToInt = function (opt, fallback) {
return parseInt(opt || fallback, 10);
};
offset = {
t: optionToInt(opts.offsetTop, offsetVertical),
b: optionToInt(opts.offsetBottom, offsetVertical),
l: optionToInt(opts.offsetLeft, offsetHorizontal),
r: optionToInt(opts.offsetRight, offsetHorizontal)
};
delay = optionToInt(opts.throttle, 250);
useDebounce = opts.debounce !== false;
unload = !!opts.unload;
callback = opts.callback || callback;
echo.render();
if (document.addEventListener) {
root.addEventListener('scroll', debounceOrThrottle, false);
root.addEventListener('load', debounceOrThrottle, false);
} else {
root.attachEvent('onscroll', debounceOrThrottle);
root.attachEvent('onload', debounceOrThrottle);
}
};
echo.render = function () {
var nodes = document.querySelectorAll('img[data-echo], [data-echo-background]');
var length = nodes.length;
var src, elem;
var view = {
l: 0 - offset.l,
t: 0 - offset.t,
b: (root.innerHeight || document.documentElement.clientHeight) + offset.b,
r: (root.innerWidth || document.documentElement.clientWidth) + offset.r
};
for (var i = 0; i < length; i++) {
elem = nodes[i];
if (inView(elem, view)) {
if (unload) {
elem.setAttribute('data-echo-placeholder', elem.src);
}
if (elem.getAttribute('data-echo-background') !== null) {
elem.style.backgroundImage = "url(" + elem.getAttribute('data-echo-background') + ")";
}
else {
elem.src = elem.getAttribute('data-echo');
}
if (!unload) {
elem.removeAttribute('data-echo');
elem.removeAttribute('data-echo-background');
}
callback(elem, 'load');
}
else if (unload && !!(src = elem.getAttribute('data-echo-placeholder'))) {
if (elem.getAttribute('data-echo-background') !== null) {
elem.style.backgroundImage = "url(" + src + ")";
}
else {
elem.src = src;
}
elem.removeAttribute('data-echo-placeholder');
callback(elem, 'unload');
}
}
if (!length) {
echo.detach();
}
};
echo.detach = function () {
if (document.removeEventListener) {
root.removeEventListener('scroll', debounceOrThrottle);
} else {
root.detachEvent('onscroll', debounceOrThrottle);
}
clearTimeout(poll);
};
return echo;
});
如何使用
1、引入文件
- <script src="js/echo.min.js"></script>
2、HTML结构
- <img src="img/blank.gif" alt="Photo" data-echo="img/photo.jpg" />
blank.gif 是一个 1 x 1 的图片，用做默认图片，data-echo 的属性值是图片的真实地址。你可以给图片设置宽度和高度，或者在 CSS 中设置，否则似乎很底部很底部的图片才会延迟加载。
3、JavaScript
echo.init({
offset: 100,
throttle: 250,
unload: false,
callback: function (element, op) {
console.log('loaded ok.');
}
});
4、常用参数及方法说明
| 参数 | 描述 | 默认值 |
| offset | 离可视区域多少像素的图片可以被加载 | 0 |
| throttle | 图片延迟多少毫秒加载 | 250 |
| debounce | 防抖动 | true |
| unload | 告诉echo是加载还是卸载视图中的图片，当图片离开视图区域时触发 | false |
| callback | 回调函数，用来检测图片是否加载 | function() |
最后echo.js还提供了一个.render()方法，用法如下：
echo.render();
应用场景：当你的页面没有发生滚动，而你想加载即将要显示的图片，如图片轮播，当第一张图片显示完，接着滑动展示第二张图片，这个时候使用echo.render()
提前加载第二张图片，就不会出现图片加载卡顿白屏等现象。