预加载背景图片

Question

我正在构建一个循环通过 3 个不同背景的页面，每 750 毫秒更改一次。为此，我在正文中添加了一个带有相关背景图像的类，并随着 JS 的变化而变化。对于第一个循环，它们会闪烁，因为图像必须加载，所以它不会立即出现。因此有什么方法可以用来预加载图像吗？

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);

Answer 1

我会做这样的事情。 loadImages 返回一个 Promise，一旦加载了所有图像，它将解析。附加到它的.then 调用cycleImages，它启动了间隔。由于无论如何您都需要 JS 中的 URL 来进行预加载，而不是类切换，我直接操作 background-image，这样您就可以从 CSS 中删除图像 URL 并节省一些冗余字节。这也使得将来扩展图像列表也变得更加容易，您只需向图像数组中添加一个项目，而不是维护复杂的 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： 考虑到Just a student的cmets，这里有一个版本，它只会在加载后切换图像，但如果加载则立即切换。它还会跳过无法加载的图像。由于cycleImages 不再通过.then 调用，我还对其进行了更改，以便它接受一个目标元素（作为一个jQuery 对象）以及一组图像承诺。这样，如果您愿意，您可以轻松地在具有不同图像集的页面上的多个位置使用它。

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>

您也可以将其作为参数传入，而不是对图像更改之间的间隔进行硬编码。不过到那时，我会重构它以使用配置对象来传递除图像承诺数组之外的所有内容：cycleImages(myImages, {target: $('#backgrounds'), interval: 1000});

Answer 2

您可以在 javascript 中加载它们。喜欢：

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

用您自己的图片网址更改“url1”、“url2”、“url3”。浏览器将加载图像，但它们不会在任何地方可见。

Answer 3

有趣的想法。

您能否在后台加载它们，例如：

<div class="hidden-images">
    <img src="img1.jpg" />
    <img src="img2.jpg" />
    <img src="img3.jpg" />
</div>

然后在 CSS 中

.hidden-images {
    position: relative;
    z-index: 1;
 }

那么无论你的主容器 div 是什么类型的，在其上添加以下内容？

 .main-container {
     position: relative;
     z-index: 2; // Or higher, whatever is needed
 }

Answer 4

你也可以像这样使用不同的 html 和 css 结构来做到这一点。

html

<div class="backgrounds">
  <div class="background freddy"></div>
</div>

css

.backgrounds{
    background-color: transparent;
    background-position: center bottom;
    background-repeat: no-repeat;
    background-size: 130%;
    height: 250px;
    width; 100%;
}

.backgrounds .freddy{
        height: 100%;
        width: 100%;
        background-image: url('http://adrianweb.net/includes/images/hero.jpg');
    }

.backgrounds .irene{
         height: 100%;
        width: 100%;
        background-image: url('http://adrianweb.net/includes/images/projects-blur.png');
    }

.backgrounds .joe{
         height: 100%;
        width: 100%;
        background-image: url('http://adrianweb.net/includes/images/contacts-blur.png');
    }

JS

$(document).ready(function(){
setInterval(function() {
    if ( $('.background').hasClass('freddy') ){
        $('.background').removeClass('freddy').addClass('irene');

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

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

    }
}, 750);
});

也许您可以添加过渡/动画以使其看起来淡入淡出。

下面的示例代码笔：

http://codepen.io/adrianrios/pen/yMEpEv