HTML5動態創建畫布

Question

嗨，我有一個關於動態創建使用JavaScript的畫布的問題。

我創建這樣一個帆布：

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"; 

但是當我試圖找到它，我得到一個null值：

cursorLayer = document.getElementById("CursorLayer"); 

我是不是做錯了？有沒有更好的方式來使用JavaScript創建畫布？

Answer 1

問題是，您不要將您的畫布元素插入到文檔正文中。

只需做到以下幾點：

document.body.appendChild(canvas); 

例子：

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);

Answer 2

通過jQuery：

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

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

Answer 3

這是因爲您在DOM加載之前調用它。首先，創建元素並添加一些元素，然後在DOM加載之後調用它。在你的情況下，它應該看起來像：

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"); 
}