问题描述
我正在使用 HTML5 中的 canvas 元素，我注意到一种特殊的行为.在初始加载时，我正在显示的图像不显示.但是，当我刷新浏览器时，它会正确显示.我用过 IE9 和 Chrome.两者的行为相同.JavaScript 代码如下所示:
I'm playing with the canvas element in HTML5 and I have noticed a peculiar behavior. On initial load, an image I'm displaying does not show. However, when I refresh the browser, it displays appropriately. I've used IE9 and Chrome. Both behave identically. The JavaScript code looks like this:
window.onload = load;
function load() {
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
context.fillRect(0, 0, 640, 400);
var image = new Image();
image.src = "Images/smiley.png";
context.drawImage(image, 50, 50);
}
矩形两次都正确绘制，它是只在浏览器刷新时显示的笑脸.
The rectangle draws correctly both times, it's the smiley that only shows on a browser refresh.
我正在学习 HTML5 和 JavaScript.我确定我只是在做一些愚蠢的事情，但我无法弄清楚.
I'm in the process of learning HTML5 and JavaScript. I'm sure I'm just doing something stupid, but I can't figure it out.
推荐答案
图片是异步加载的，所以只有在刷新后才加载足够早，因为它是缓存的.通常在您调用 drawImage 时它还没有加载.使用onload:
Images load asynchronously, so only after refresh it loads early enough because it's cached. Normally it isn't loaded yet at the time you call drawImage. Use onload:
var image = new Image();
image.src = "Images/smiley.png";
image.onload = function() {
context.drawImage(image, 50, 50);
};
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