HTML5动态创建画布

Question 1

嗨，我有一个关于使用JavaScript动态创建画布的问题。

我创建一个这样的画布：

var canvas = document.createElement('canvas');
canvas.id     = "CursorLayer";
canvas.width  = 1224;
canvas.height = 768;
canvas.style.zIndex   = 8;
canvas.style.position = "absolute";
canvas.style.border   = "1px solid";

但是当我尝试找到它时，我得到了一个null值：

cursorLayer = document.getElementById("CursorLayer");

我做错了吗？有没有更好的方法使用JavaScript创建画布？

Question 2

问题是您没有在文档主体中插入canvas元素。

只需执行以下操作：

document.body.appendChild(canvas);

例：

var canvas = document.createElement('canvas');

canvas.id = "CursorLayer";
canvas.width = 1224;
canvas.height = 768;
canvas.style.zIndex = 8;
canvas.style.position = "absolute";
canvas.style.border = "1px solid";


var body = document.getElementsByTagName("body")[0];
body.appendChild(canvas);

cursorLayer = document.getElementById("CursorLayer");

console.log(cursorLayer);

// below is optional

var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgba(255, 0, 0, 0.2)";
ctx.fillRect(100, 100, 200, 200);
ctx.fillStyle = "rgba(0, 255, 0, 0.2)";
ctx.fillRect(150, 150, 200, 200);
ctx.fillStyle = "rgba(0, 0, 255, 0.2)";
ctx.fillRect(200, 50, 200, 200);

展开摘要

Question 3

通过Jquery：

$('<canvas/>', { id: 'mycanvas', height: 500, width: 200});

http://jsfiddle.net/8DEsJ/736/

Question 4

发生这种情况是因为在DOM加载之前就调用了它。首先，创建元素并向其添加属性，然后在DOM加载后调用它。在您的情况下，它应如下所示：

var canvas = document.createElement('canvas');
canvas.id     = "CursorLayer";
canvas.width  = 1224;
canvas.height = 768;
canvas.style.zIndex   = 8;
canvas.style.position = "absolute";
canvas.style.border   = "1px solid";
window.onload = function() {
    document.getElementById("CursorLayer");
}

Question 5

 <html>
 <head></head>
 <body>
 <canvas id="canvas" width="300" height="300"></canvas>
 <script>
  var sun = new Image();
  var moon = new Image();
  var earth = new Image();
  function init() {
  sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
  moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
  earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
  window.requestAnimationFrame(draw);
  }

  function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');

  ctx.globalCompositeOperation = 'destination-over';
  ctx.clearRect(0, 0, 300, 300);

  ctx.fillStyle = 'rgba(0, 0, 0, 0.4)';
  ctx.strokeStyle = 'rgba(0, 153, 255, 0.4)';
  ctx.save();
  ctx.translate(150, 150);

  // Earth
  var time = new Date();
  ctx.rotate(((2 * Math.PI) / 60) * time.getSeconds() + ((2 * Math.PI) / 60000) * 
  time.getMilliseconds());
  ctx.translate(105, 0);
  ctx.fillRect(10, -19, 55, 31); 
  ctx.drawImage(earth, -12, -12);

   // Moon
  ctx.save();
  ctx.rotate(((2 * Math.PI) / 6) * time.getSeconds() + ((2 * Math.PI) / 6000) * 
  time.getMilliseconds());
  ctx.translate(0, 28.5);
  ctx.drawImage(moon, -3.5, -3.5);
  ctx.restore();

  ctx.restore();

   ctx.beginPath();
   ctx.arc(150, 150, 105, 0, Math.PI * 2, false);
   ctx.stroke();

   ctx.drawImage(sun, 0, 0, 300, 300);

   window.requestAnimationFrame(draw);
    }

   init();
   </script>
   </body>
   </html>

Question 6

另类

使用元素.innerHTML=这是相当快的现代浏览器

document.body.innerHTML = "<canvas width=500 height=150 id='CursorLayer'>";


// TEST

var ctx = CursorLayer.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(100, 100, 50, 50);

canvas { border: 1px solid black }

展开摘要