javascript – 图像预加载器如何工作？

我很难理解图像预加载器在java脚本中的工作方式.因此,如果有人能够解释他们如何使用一个可以帮助很多的例子.
没有jquery

解决方法:

加载单个图像

浏览器将异步加载图像…这意味着当浏览器被赋予图像的.src时,它将开始在后台加载该图像,但也将继续在.src之后直接处理javascript代码

// 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.

那么你怎么知道图像什么时候完全加载并准备好使用？

答：完成加载图像后,您可以告诉浏览器“回电”.通过在图像对象上添加“img.onload”函数,可以“回调”.每当浏览器完成加载图像时,浏览器将触发“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进行：

// 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

要预加载多个图像：

>创建一个数组来保存所有图像URL,并将图像URL添加到此数组中.

// 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=[];

>将新图像元素添加到图像对象数组(为URL数组中的每个URL添加一个新图像).

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

>将每个图像对象的.onload回调设置为相同的函数.

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

>使用图像URL数组将每个图像的.src设置为单个URL.

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

>在theImageHasFinishedLoading回调中,每次成功加载新图像时递增计数器.

// 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,您将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!');
        }
    } 

这是一个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>

标签：html,preloading,javascript,image,asynchronous
来源： https://codeday.me/bug/20190917/1809429.html