javascript - 图像预加载器如何工作？

Question

我很难理解图像预加载器在 java 脚本中的工作原理。因此，如果有人能用一个例子来解释他们如何工作，那将会有很大帮助。 没有jquery

Answer 1

加载单个图像

浏览器将异步加载图像...这意味着当浏览器获得图像的 .src 时，它将开始在后台加载该图像，但也会继续处理 JavaScript 代码直接在 .src

之后

// create a new image object
var img=new Image();

// set the image object's image source
img.src='myImage.png';

// do some stuff while the image is loading in the background
alert('Your image has started loading, but is not yet complete');

即使在图像完全加载并准备使用之前，警报也会显示。

那么您如何知道图像何时已完全加载并可供使用？

回答:您可以告诉浏览器在完成加载图像后“给您回电”。您可以通过在图像对象上添加“img.onload”函数来“回调”。每当浏览器完成加载图像时，浏览器就会触发“img.onload”函数中的代码。

img.onload = theImageHasFinishedLoading;

function theImageHasFinishedLoad(){
    alert('Your image has finished loading...you can use it now');
}

完整的图像加载过程将按以下顺序发生:

// happens 1st
var img=new Image();

// happens 2nd: this callback is ASSIGNED, but NOT TRIGGERED yet
//     until the image has fully loaded
img.onload = theImageHasFinishedLoading;

// happens 3rd
img.src='myImage.png';

// happens 4th
alert('Your image has started loading, but is not yet complete');

// happens 5th after the browser has fully loaded the image
//     (The browser will call this function automatically because .onload was set)
function theImageHasFinishedLoad(){
    alert('Your image has finished loading...you can use it now');
}

预加载多个图像

预加载多个图像:

• 创建一个数组来保存所有图像 URL，并将图像 URL 添加到该数组中。

  // For example
  var imageURLs=[];
  imageURLs[0]='myImage.png';
  

• 创建一个数组来保存所有图像对象(==您的实际图像)。

  // For example
  var imgs=[];
  

• 将 new Image 元素添加到图像对象数组(为 URL 数组中的每个 URL 添加一个 new Image)。

  //For example
  imgs[0] = new Image();
  

• 将每个图像对象的 .onload 回调设置为同一函数。

  // For example
  imgs[0].onload = theImageHasFinishedLoading;
  

• 使用图像 URL 数组将每个图像的 .src 设置为单独的 URL。

  // For example
  imgs[0].src = imageURLs[0];
  

• 在 theImageHasFinishedLoading 回调中，每次成功加载新图像时都会增加一个计数器。

  // For example
  var counter=0;
  
  function theImageHasFinishedLoading(){
      counter++;
  }
  

当计数器达到与图像 URL 数组相同的长度时，您就会知道所有图像都已完全加载。

    function theImageHasFinishedLoading(){
        counter++;
        if(counter>=imageURLs.length){
            alert('All the images have successfully preloaded. Go use them now!');
        }
    } 

这是完整的示例代码和演示:

window.onload=(function(){


  // canvas related variables
  var canvas=document.getElementById("canvas");
  var ctx=canvas.getContext("2d");
  var cw=canvas.width;
  var ch=canvas.height;

  // put the paths to your images in imageURLs[]
  var imageURLs=[];  
  imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/face1.png");
  imageURLs.push("https://dl.dropboxusercontent.com/u/139992952/multple/face2.png");

  // the loaded images will be placed in imgs[]
  var imgs=[];
  var imagesOK=0;
  startLoadingAllImages(imagesAreNowLoaded);

  // Create a new Image() for each item in imageURLs[]
  // When all images are loaded, run the callback (==imagesAreNowLoaded)
  function startLoadingAllImages(callback){

    // iterate through the imageURLs array and create new images for each
    for (var i=0; i<imageURLs.length; i++) {
      // create a new image an push it into the imgs[] array
      var img = new Image();
      // Important! By pushing (saving) this img into imgs[],
      //     we make sure the img variable is free to
      //     take on the next value in the loop.
      imgs.push(img);
      // when this image loads, call this img.onload
      img.onload = function(){ 
        // this img loaded, increment the image counter
        imagesOK++; 
        // if we've loaded all images, call the callback
        if (imagesOK>=imageURLs.length ) {
          callback();
        }
      };
      // notify if there's an error
      img.onerror=function(){alert("image load failed");} 
      // set img properties
      img.src = imageURLs[i];
    }      
  }

  // All the images are now loaded
  // Do drawImage & fillText
  function imagesAreNowLoaded(){

    // the imgs[] array now holds fully loaded images
    // the imgs[] are in the same order as imageURLs[]

    ctx.font="30px sans-serif";
    ctx.fillStyle="#333333";

    // drawImage the first image (face1.png) from imgs[0]
    // and fillText its label below the image
    ctx.drawImage(imgs[0],0,10);
    ctx.fillText("face1.png", 0, 135);

    // drawImage the first image (face2.png) from imgs[1]
    // and fillText its label below the image
    ctx.drawImage(imgs[1],200,10);
    ctx.fillText("face2.png", 210, 135);

  }



}); // end window.onload

body{ background-color: ivory; }
#canvas{border:1px solid red;}

<canvas id="canvas" width=400 height=200></canvas>