文/江欲行(jiqingwu@gmail.com)
这个例子里，描述了一个一直向前的对象。
每一次，对象会随机选一个出生位置和一个方向，然后一直运动下去。如果碰到边缘，就会转到画布的另一边继续沿原来的方向前进。
html5代码如下（可以从https://github.com/jollywing/nature_of_code_examples_html5/blob/master/chp1_vectors/mover.html 下载该文件）：
<!doctype html>
<html>
<head>
<title>Mover</title>
<meta charset="utf-8" />
<meta name="author" content="JollyWing(jiqingwu@gmail.com)"/>
<meta name="create" content="2014-12-17 Wed"/>
<style type="text/css">
#screen {border:1px solid #000;}
</style>
<script>
var mover = {
/* x:100,
y:100,
xspeed:11,
yspeed:10, */
radius:2,
init:function(canvasWidth, canvasHeight){
this.x = Math.floor(Math.random() * canvasWidth);
this.y = Math.floor(Math.random() * canvasHeight);
this.xspeed = 1 + Math.floor(Math.random() * 10);
this.yspeed = 1 + Math.floor(Math.random() * 10);
console.log(this.xspeed + ',' + this.yspeed);
},
move:function(canvasWidth, canvasHeight){
this.x = this.x + this.xspeed;
this.y = this.y + this.yspeed;
if(this.x > canvasWidth - this.radius){
this.x = this.radius;
}
else if(this.x < this.radius) {
this.x = canvasWidth - this.radius;
}
if(this.y > canvasHeight - this.radius){
this.y = this.radius;
}
else if(this.y < this.radius) {
this.y = canvasHeight - this.radius;
}
},
draw:function(context){
context.moveTo(this.x, this.y);
context.arc(this.x, this.y, this.radius, 0, 2*Math.PI);
context.stroke();
}
};
var timeOut;
var scr, width, height, context;
function initScr(){
scr = document.getElementById('screen');
width = scr.width;
height = scr.height;
context = scr.getContext('2d');
context.strokeStyle="red";
}
function walk(){
mover.move(width, height);
mover.draw(context);
timeOut = setTimeout('walk()', 200);
// console.log(mover.x + ',' + mover.y);
}
function stopWalk(){
clearTimeout(timeOut);
}
function init(){
initScr();
mover.init(width, height);
}
</script>
</head>
<body onload='init()'>
<p>
<canvas id="screen" width="640" height="480"></canvas>
</p>
<input type="button" value='Start' onclick='walk()'/>
<input type="button" value='Stop' onclick='stopWalk()' />
</body>
</html>
运行结果：
一直向前
2014-12-17 周三
