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這裏是我的代碼:爲什麼說我的對象「球」不存在方法繪製?
<!DOCTYPE html>
<html>
<head>
<title>Touch Tracker Marker</title>
<meta name="viewport" content="width=device-width, user-scalable=no">
<style type="text/css">
body { margin: 0px; overflow: hidden; }
canvas { border: 1px solid black; }
</style>
<script type="text/javascript" src="magictouch.js"></script>
<script type="text/javascript">
var canvas;
var ctx;
var w = 0;
var h = 0;
var timer;
var updateStarted = false;
var touches = [];
var balles= [];
function ball(x,y){
this.x=x;
this.y=y;
function draw(){
ctx.beginPath();
ctx.arc(this.x, this.y, 20, 0, 2*Math.PI, true);
ctx.fillStyle = "rgba(0, 0, 200, 0.2)";
ctx.fill();
ctx.lineWidth = 2.0;
ctx.strokeStyle = "rgba(0, 0, 200, 0.8)";
ctx.stroke();
}
};
function update() {
if (updateStarted) return;
updateStarted = true;
var nw = window.innerWidth;
var nh = window.innerHeight;
if ((w != nw) || (h != nh)) {
w = nw;
h = nh;
canvas.style.width = w+'px';
canvas.style.height = h+'px';
canvas.width = w;
canvas.height = h;
}
ctx.clearRect(0, 0, w, h);
var i, len = touches.length;
for (i=0; i<len; i++) {
var touch = touches[i];
var px = touch.pageX;
var py = touch.pageY;
b = new ball(px,py);
b.draw();
console.log('drawn circle at ' + px +',' + py);
}
updateStarted = false;
}
function ol() {
canvas = document.getElementById('canvas');
ctx = canvas.getContext('2d');
timer = setInterval(update, 15);
canvas.addEventListener('touchend', function() {
ctx.clearRect(0, 0, w, h);
});
canvas.addEventListener('touchmove', function(event) {
event.preventDefault();
touches = event.touches;
});
canvas.addEventListener('touchstart', function(event) {
console.log('start');
});
};
</script>
</head>
<body onload="ol()">
<canvas id="canvas" width="300" height="300" style="top:0px; left:0px; width: 300px; height: 300px;"></canvas>
</body>
</html>
是的,謝謝你,它的工作方式。但奇怪的是,因爲他們在W3school中說過,我們可以像我所做的那樣聲明一種方法:http://www.w3schools.com/js/js_objects.asp – nonozor
他們所做的是將該方法手動添加到對象上: 'this.draw = draw' 所以方法指針被添加到ctor中的每個對象上。這真是一個黑客。方法應該附加到原型上,而不是分別添加到每個實例 – odedsh
@nonozor在創建許多對象時,向原型添加共享成員(如表現相同的函數)會節省內存和CPU。它還使從Parent繼承和擴展功能變得更加容易。有關原型和構造函數的更多信息可以在這裏找到:http://stackoverflow.com/a/16063711/1641941 – HMR