我正在尝试使用canvas元素在Javascript中跟随光标的球。我将对canvas对象的引用传递给名为followMouse的函数,但是当我尝试获取它的上下文时,firebug给了我这个错误:
TypeError:canvas.getContext不是函数
当我将画布引用记录到控制台时,它会显示canvas元素。我不知道为什么然后我不能在函数中获取上下文。有谁知道发生了什么事?
这是我的代码:
function drawCircle(x, y, canvas) {
var context = canvas.getContext('2d');
context.beginPath();
context.arc(x, y, 40, 0, 2 * Math.PI);
context.fill();
}
function followMouse(canvas, mousePos) {
var context = canvas.getContext("2d");
console.log(canvas);
context.clearRect(0, 0, 700, 700);
var xPos = xPos + (mousePos.x - xPos) / 100;
var yPos = yPos + (mousePos.y - yPos) / 100;
drawCircle(xPos, yPos, canvas);
window.requestAnimFrame(followMouse, canvas);
}
window.onload = function() {
var canvas = document.getElementById("main");
var mousePos = {
x: 0,
y: 0
};
canvas.addEventListener('mousemove', function(evt) {
var pos = getMousePos(canvas, evt);
mousePos.x = pos.x;
mousePos.y = pos.y;
}, false);
followMouse(canvas, mousePos);
}
答案 0 :(得分:1)
您的热线window.requestAnimFrame(followMouse, canvas);
调用followMouse(timestamp)
,其中时间戳是号在requestAnimationFrame
触发时传递给回调。这会导致调用timestamp.getContext
,这显然无效。
相反,将您的调用包装在匿名函数中:
window.requestAnimFrame(function() { followMouse(canvas, mousePos) }, canvas);