如何知道浏览器何时完成加载后处理图像？

Question

创建图像对象时,可以使用"完整"属性或"onload"方法知道何时完全加载,然后,此图像已使用一段时间处理(例如调整大小),在大文件中可能需要几秒钟.
如何知道浏览器何时完成加载后处理图像？ 编辑:在示例中可以看到"完整"消息和图像外观之间的滞后,我想避免这种情况. 使用"onload"方法的示例:

var BIGimage;
	putBIGimage();
	function putBIGimage(){
		BIGimage=document.createElement("IMG");
		BIGimage.height=200;
		BIGimage.src="http://orig09.deviantart.net/5e53/f/2013/347/f/d/i_don_t_want_to_ever_leave_the_lake_district_by_martinkantauskas-d6xrdch.jpg";
		BIGimage.onload=function(){waitBIGimage();};
	}
	function waitBIGimage(){
			console.log("Complete.");
			document.body.appendChild(BIGimage);
	}

使用"完整"属性的示例:

var BIGimage;
	putBIGimage();
	function putBIGimage(){
		BIGimage=document.createElement("IMG");
		BIGimage.height=200;
		BIGimage.src="http://orig09.deviantart.net/5e53/f/2013/347/f/d/i_don_t_want_to_ever_leave_the_lake_district_by_martinkantauskas-d6xrdch.jpg";
		waitBIGimage();
	}
	function waitBIGimage(){
		if (!BIGimage.complete){
			console.log("Loading...");
			setTimeout(function(){
				waitBIGimage();
			},16);
		} else {
			console.log("Complete.");
			document.body.appendChild(BIGimage);
		}
	}

编辑:谢谢@Kaiido的回应我做了这个sollution等待图像处理.

var imagesList=["https://omastewitkowski.files.wordpress.com/2013/07/howard-prairie-lake-oregon-omaste-witkowski-owfotografik-com-2-2.jpg",
		"http://orig03.deviantart.net/7b8d/f/2015/289/0/f/0ffd635880709fb39c2b69f782de9663-d9d9w6l.jpg",
		"http://www.livandiz.com/dpr/Crater%20Lake%20Pano%2016799x5507.JPG"];
var BIGimages=loadImages(imagesList);
onLoadImages(BIGimages,showImages);

function loadImages(listImages){
	var image;
	var list=[];
	for (var i=0;i<listImages.length;i++){
		image=document.createElement("IMG");
		image.height=200;
		image.src=listImages[i]+"?"+Math.random();
		list.push(image);
	}
	return list;		
}

function showImages(){
	loading.style.display="none";
	for (var i=0; i<BIGimages.length;i++){
		document.body.appendChild(BIGimages[i]);
	}
};

function onLoadImages(images,callBack,n) {
	if (images==undefined) return null;
	if (callBack==undefined) callBack=function(){};
	else if (typeof callBack!="function") return null;
	var list=[];
	if (!Array.isArray(images)){
		if (typeof images =="string") images=document.getElementById(images);
		if (!images || images.tagName!="IMG") return null;
		list.push(images);
	} else list=images;
	if (n==undefined || n<0 || n>=list.length) n=0;
	for (var i=n; i<list.length; i++){
		if (!list[i].complete){
			setTimeout(function(){onLoadImages(images,callBack,i);},16);
			return false;
		}
		var ctx = document.createElement('canvas').getContext('2d');
		ctx.drawImage(list[i], 0, 0);
	}
	callBack();
	return true;
}

<DIV id="loading">Loading some big images...</DIV>

Answer 1

HTMLImageElement 接口有一个decode()方法，它允许我们等待图像准备好绘制，就像您想要的那样。

var BIGimage;
putBIGimage();

function putBIGimage() {
  BIGimage = document.createElement("IMG");
  BIGimage.height = 200;
  BIGimage.src = "https://upload.wikimedia.org/wikipedia/commons/c/cf/Black_hole_-_Messier_87.jpg?r=" + Math.random();
  BIGimage.onload = e => console.log('load event', performance.now());
  BIGimage.decode().then(waitBIGimage);
  BIGimage.onerror = console.error;
}

function waitBIGimage() {
  // uses the synchronous testing method to check it works fine
  var start = performance.now();
  // only to see if it worked fine
  var ctx = document.createElement('canvas').getContext('2d');
  ctx.drawImage(BIGimage, 0, 0);
  ctx.drawImage(ctx.canvas, 0, 0);

  var end = performance.now();
  console.log(`it took ${end - start}ms to draw`)

  // do your stuff
  document.body.appendChild(BIGimage);
}

drawImage另一种方法是利用 CanvasContext2D方法可以同步的事实。一些浏览器可能会尝试延迟下一个绘画帧的实际绘画，但是通过将画布绘制到自身上，我们可以强制大多数当前的 UA 同步渲染我们的图像。

所以你可以在你的方法中使用它作为等待方法waitBIGimage。

var BIGimage;
putBIGimage();

function putBIGimage() {
  BIGimage = document.createElement("IMG");
  BIGimage.height = 200;
  BIGimage.src = "https://upload.wikimedia.org/wikipedia/commons/c/cf/Black_hole_-_Messier_87.jpg?r=" + Math.random();
  BIGimage.onload = waitBIGimage;
  BIGimage.onerror = console.error;
}

function waitBIGimage() {
  // only for demo
  // we've got to also log the time since even the console.log method will be blocked during processing
  var start = performance.now();
  console.log('waiting', start);

  // this is all needed
  var ctx = document.createElement('canvas').getContext('2d');
  ctx.drawImage(BIGimage, 0, 0);
  // on some system the drawing of the image may be delayed to the next painting frame
  // we can try force the synchronous rendering by drawing the canvas over itself
  ctx.drawImage(ctx.canvas, 0, 0);
  // demo only
  var end = performance.now();
  console.log("Complete.", end);
  console.log(`it took ${end - start}ms`)

  // do your stuff
  document.body.appendChild(BIGimage);
}

在我的 Firefox 上，处理图像大约需要 1 秒，而在我的 Chrome 上，速度要快一些。

但该方法的一个大问题是它是同步的，因此会在处理图像的整个过程中阻塞您的脚本。

另一种方法是使用该createImageBitmap()方法，该方法应在 GPU 中分配图像位图，准备绘制。