javascript - 预加载背景图片

我正在构建一个页面，该页面循环显示三种不同的背景，每750毫秒更换一次。为此，我在<body>上添加一个包含相关背景图像的类，通过JS更换。在第一次循环时，由于图像需要加载，它们会闪烁，因此不会立即显示。因此，有没有什么方法可以预加载这些图像？

CSS ：

&.backgrounds{
    background-position: center bottom;
    background-repeat: no-repeat;
    background-size: 130%;

    &.freddy{
        background-image: url(/img/illustrations/snapchat/snapchat_holding_page_freddy.jpg);
    }

    &.irene{
        background-image: url(/img/illustrations/snapchat/snapchat_holding_page_irene.jpg);
    }

    &.joe{
        background-image: url(/img/illustrations/snapchat/snapchat_holding_page_joe.jpg);
    }
}

JS :

setInterval(function() {
    if ( $('.backgrounds').hasClass('freddy') ){
        $('.backgrounds').removeClass('freddy').addClass('irene');

    } else if ( $('.backgrounds').hasClass('irene') ){
        $('.backgrounds').removeClass('irene').addClass('joe');

    } else if ( $('.backgrounds').hasClass('joe') ){
        $('.backgrounds').removeClass('joe').addClass('freddy');

    }
}, 750);

答案＃1

我会这样做。loadImages 返回一个 Promise，一旦所有图片加载完毕就会被解决。附加在它上的 .then 会调用 cycleImages，从而启动定时器。由于你无论如何都需要在 JS 中使用 URL 来进行预加载，所以我不是通过切换类，而是直接操作 background-image，这样你可以从 CSS 中移除图片 URL，节省一些冗余的字节。这也使将来扩展图片列表更容易，你只需向图片数组中添加一个项目，而不需要维护复杂的 if 语句。

function loadImages (images) {
  // each image will be loaded by this function.
  // it returns a Promise that will resolve once
  // the image has finished loading
  let loader = function (src) {
    return new Promise(function (resolve, reject) {
      let img = new Image();
      img.onload = function () {
        // resolve the promise with our url so it is
        // returned in the result of Promise.all
        resolve(src);
      };
      img.onerror = function (err) {
        reject(err);
      };
      img.src = src;
    });
  };

  // create an image loader for each url
  let loaders = [];
  images.forEach(function (image) {
    loaders.push(loader(image));
  });

  // Promise.all will return a promise that will resolve once all of of our
  // image loader promises resolve
  return Promise.all(loaders);
}


// the images we are going to display
let myImages = [
  'http://www.gifpng.com/400x200',
  'http://www.gifpng.com/400x200/ffffff/000000',
  'http://www.gifpng.com/400x200/000000/ffffff'
];

// $(document).ready(fn) is deprecated,
// use the $(fn) form instead
$(function() {

  // after the images are loaded this will be called with an array of the loaded images
  function cycleImages (images) {
    let index = 0;
    setInterval(function() {
      // since we need an array of the image names to preload them anyway,
      // just load them via JS instead of class switching so you can cut them
      // out of the CSS and save some space by not being redundant
      $('#backgrounds').css('backgroundImage', 'url("' + images[index] + '")');
      // increment, roll over to 0 if at length after increment
      index = (index + 1) % images.length;
    }, 750);
  }


  // load the images and start cycling through them after they are loaded
  loadImages(myImages).then(cycleImages).catch(function (err) {
    console.error(err);
  });
});

#backgrounds {
  height: 200px;
  width: 400px;
  border: 1px solid #000;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="backgrounds"></div>

编辑： 仅考虑一名学生的评论，这里有一个版本，它只会在图像加载完成后切换，但如果图像已经加载，则会立即切换。它还会跳过加载失败的图像。由于 cycleImages 不再通过 .then 调用，我还修改了它，使其接受一个目标元素（作为 jQuery 对象）以及一个图像 promise 数组。这样，如果你愿意，可以在页面的多个地方轻松使用不同的图像集。

function loadImages (images) {
  // each image will be loaded by this function.
  // it returns a Promise that will resolve once
  // the image has finished loading
  let loader = function (src) {
    return new Promise(function (resolve, reject) {
      let img = new Image();
      img.onload = function () {
        // resolve the promise with our url
        resolve(src);
      };
      img.onerror = function (err) {
        reject(err);
      };
      img.src = src;
    });
  };

  // return an array of image-loading promises
  return images.map(function (image) {
    return loader(image);
  });
}


// the images we are going to display
let myImages = [
  'http://www.gifpng.com/400x200',
  'http://www.invalid-domain-name.foo/this-url-certainly-does-not-exist.jpg',
  'http://www.gifpng.com/400x200/ffffff/000000',
  'http://www.gifpng.com/400x200/000000/ffffff'
];

// $(document).ready(fn) is deprecated,
// use the $(fn) form instead
$(function() {

  // this receives an array of the promises for each image
  function cycleImages ($target, images) {
    let index = 0,
      interval = 750; // how many ms to wait before attempting to switch images

    function nextImage () {
      // p is the promise for the current image
      let p = images[index],
        next = function (wait) {
          // increment our counter and wait to display the next one
          index = (index + 1) % images.length;
          setTimeout(nextImage, wait);
        };

      // wait for this image to load or fail to load
      p.then(function (src) {
        // it loaded, display it
        $target.css('backgroundImage', 'url("' + src + '")');
        next(interval);
      }).catch(function (err) {
        // this one failed to load, skip it
        next(0);
      });

    }

    // start cycling
    nextImage();
  }


  // load the images and start cycling through them as they are loaded
  cycleImages($('#backgrounds'), loadImages(myImages));
});

#backgrounds {
  height: 200px;
  width: 400px;
  border: 1px solid #000;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="backgrounds"></div>

你也可以将图像切换的间隔作为参数传入，而不是硬编码此间隔。不过在那时，我会重构它以使用配置对象来传递除了图像 Promise 数组之外的所有内容：cycleImages(myImages, {target: $ ('#backgrounds'), interval: 1000});

答案＃2

你可以在 JavaScript 中加载它们。像这样：

(new Image()).src = url1;
(new Image()).src = url2;
(new Image()).src = url3;

将“url1”、“url2”、“url3”替换为你自己的图片网址。浏览器会加载图片，但它们不会在任何地方可见。