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javascript - [js] Warum erscheinen keine Bilder auf der Leinwand? Online warten

<body>
    <p id="main"></p>
</p>
<script type="text/javascript">
function GED(ele) {return document.getElementById(ele);}
function load_source(url, w, h) {
    this.canvas = document.createElement('canvas');
    this.canvas.width = w;
    this.canvas.height = h;
    this.ctx = this.canvas.getContext('2d');
    this.img = new Image();
    this.img.src = url;
    this.img.onload = function () {
        this.ctx.drawImage(this.img, 0, 0);
    }.bind(this);
    return this.canvas;
}
source = new load_source('http://htmljs.b0.upaiyun.com/uploads/1382542991440-2angles.png', 300, 100);
canvas = document.createElement('canvas')
canvas.id = 'ff'
canvas.width = 300;
canvas.height = 100;
GED('main').appendChild(canvas);
ctxs = GED('ff').getContext('2d'); 
ctxs.drawImage(source, 110, 110);
</script>

欧阳克2763 Tage vor679

Antworte allen(2)Ich werde antworten

淡淡烟草味2017-07-05 10:48:45

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
    <p id="main"></p>
<script type="text/javascript">
function GED(ele){return document.getElementById(ele);}
function load_source(url,w,h){
    this.canvas = document.createElement('canvas');
    this.canvas.width = w;
    this.canvas.height = h;  
    this.ctx = this.canvas.getContext('2d');
    this.img = new Image();
    this.img.src = url;
    this.img.onload = function () {
        this.ctx.drawImage(this.img,0,0);
        window.ctxs.drawImage(source,110,110);
    }.bind(this);
    return this.canvas;
}
source = new load_source('http://htmljs.b0.upaiyun.com/uploads/1382542991440-2angles.png',300,100);
GED('main').appendChild(source);
canvas = document.createElement('canvas')
canvas.id='ff'
canvas.width = 300;
canvas.height = 100;
GED('main').appendChild(canvas);
ctxs= GED('ff').getContext('2d'); 
</script>
</body>
</html>

上面这段代码是正常的，因为你的图片在load_source的时候，是通过img.onload异步画到load_source里面的canvas上的，然而，在那个时间之前，img上是没有图像的，所以load_source里面的canvas也是没图像的。

但是，在那个时间之前，DOM里的canvas已经就绪，而且执行了ctxs.drawImage(source,110,110)。由于此时的load_source里的canvas还是空的（里面的图还没加载完毕，里面的画布也就没有内容），所以source也就是空的，所以ctx.drawImage(source,110,110)画上去的东西也是空的。

Antwort

为情所困2017-07-05 10:48:45

试一哈下面这段代码，感觉是在做课程设计的样子


<!DOCTYPE html>
<html>
<head>
</head>
<body>

<p id="main">
    <canvas id="myCanvas" width="400px" height="300px" style="border: 1px solid red;">
    </canvas>
</p>
<script type="text/javascript">

   var canvas = document.getElementById("myCanvas");

   if(canvas.getContext){  
    //获取对应的CanvasRenderingContext2D对象(画笔)
        var ctx = canvas.getContext("2d");
        
        //创建新的图片对象
        var img = new Image();
        //指定图片的URL
        img.src = "http://htmljs.b0.upaiyun.com/uploads/1382542991440-2angles.png";
        //浏览器加载图片完毕后再绘制图片
        img.onload = function(){
            //以Canvas画布上的坐标(10,10)为起始点，绘制图像
            ctx.drawImage(img, 10, 10);             
        };
    }
</script>
</html>

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