HTML5画布&x2B;javascript类=错误
我试图为一个区域编写一个javascript类,该区域包含2个画布元素,1个用于背景,1个用于前景。 但是当我点击画布时,我得到了一个错误: “未捕获的TypeError:无法读取未定义的属性'drawImage'” 任何帮助都将不胜感激HTML5画布&x2B;javascript类=错误,javascript,html,class,canvas,drawimage,Javascript,Html,Class,Canvas,Drawimage,我试图为一个区域编写一个javascript类,该区域包含2个画布元素,1个用于背景,1个用于前景。 但是当我点击画布时,我得到了一个错误: “未捕获的TypeError:无法读取未定义的属性'drawImage'” 任何帮助都将不胜感激 function GAMEAREA(canvasElement, bgElement) { this.canvas = document.getElementById(canvasElement); this.context = this.ca
function GAMEAREA(canvasElement, bgElement)
{
this.canvas = document.getElementById(canvasElement);
this.context = this.canvas.getContext("2d");
this.bgCanvas = document.getElementById(bgElement);
this.bgContext = this.bgCanvas.getContext("2d");
this.pawnImg = document.getElementById("pawnImg");
this.init = function()
{
this.adjustSize();
this.canvas.addEventListener("mousedown", this.onClick, false);
};
this.onClick = function(e)
{
this.context.drawImage(this.pawnImg, 0, 0);
};
this.adjustSize = function()
{
this.canvas.height = window.innerHeight - ($("#headerDiv").outerHeight() + $("#footerDiv").outerHeight());
if (this.canvas.height > window.innerWidth) {
this.canvas.height = window.innerWidth;
this.canvas.width = this.canvas.height;
}
else this.canvas.width = this.canvas.height;
this.bgCanvas.height = this.canvas.height;
this.bgCanvas.width = this.canvas.width;
this.bgCanvas.style.display = "block";
this.canvas.style.display = "block";
};
this.init();
}
您的问题在于以下几行:
this.canvas = document.getElementById(canvasElement);
this.context = this.canvas.getContext("2d");
此上下文未定义。确保上面的行返回了正确的对象。问题在于,此
不是您认为的单击处理程序中的对象,这意味着此。上下文
是未定义的
并且未定义的
没有drawImage()
方法。尝试使用:
…强制将此
设置为所需的值。或者你可以这样做:
function GAMEAREA(canvasElement, bgElement)
{
var self = this;
...
this.onClick = function(e)
{
self.context.drawImage(self.pawnImg, 0, 0);
};
...
}
函数中此
的值取决于该函数的调用方式。有关如何设置此的详细信息,请访问。,这解释了此如何在JavaScript中工作。是的,我知道,但我无法理解为什么它没有定义。我认为画布是可以访问的,因为它的大小调整正确。
function GAMEAREA(canvasElement, bgElement)
{
var self = this;
...
this.onClick = function(e)
{
self.context.drawImage(self.pawnImg, 0, 0);
};
...
}