canvas是html5中非常重要而且有用的东西，我们可以在canvas上绘制任意的元素，就像你制作flash一样。今天我们就在canvas上来制作一款火焰发射的效果。就像古代的火球炮一样，而且可以在浏览器边缘反弹，感觉会比较屌。来看看效果图：
我们可以在这里查看火焰球的DEMO演示
当然，我们要来分析一下源代码，主要是一些JS代码。
首先很简单地在页面上放一个canvas标签，并且给它点简单的样式：
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canvas{
position: absolute;
height: 100%;
width: 100%;
left: 0;
top: 0;
cursor: crosshair;
}接下来就来分析一下JS代码。我们来逐步分解JS。
由于这个是二维动画，所以我们利用canvas的getContext方法来返回一个对象，这个对象包含我们对二维动画操作的API，代码如下：
canvas = document.querySelector('canvas');
ctx = canvas.getContext('2d');下面我们来定义粒子：
particles = {};
newParticle = (function(){ var nextIndex = 0; return function(x,y,r,o,c,xv,yv,rv,ov){
particles[++nextIndex] = {
index: nextIndex,
x: x,
y: y,
r: r,
o: o,
c: c,
xv: xv,
yv: yv,
rv: rv,
ov: ov
};
};
})();然后我们来定义火球：
fireballs = {};
newFireball = (function(){ var nextIndex = 0; return function(x,y,xv,yv,life){
fireballs[++nextIndex] = {
index: nextIndex,
x: x,
y: y,
xv: xv,
yv: yv,
life: life
};
};
})();这里life表示火球的生命周期，下面我们可以看到，life值会随着火球发射力度的改变而改变。
接下来是定义鼠标拖动弹弓，准备发射火球：
mouse = {x:0,y:0,d:0};
onmousemove = function(e){
mouse.x = e.clientX-o.x;
mouse.y = e.clientY-o.y; var dx = mouse.x - pos1.x,
dy = mouse.y - pos1.y;
mouse.d = Math.sqrt(dx*dx+dy*dy);
};
charging = false;
pos1 = {x:0,y:0};
showInstructions = true;
onmousedown = function(e){
pos1.x = mouse.x;
pos1.y = mouse.y;
charging = true;
showInstructions = false;
};
onmouseup = function(){ if(charging){
newFireball(
mouse.x,
mouse.y,
(pos1.x-mouse.x)*0.03,
(pos1.y-mouse.y)*0.03, 600
);
charging = false;
}
};可以看到，当鼠标按键弹起时，新建一个火球，并初始化life值。
下面是火球运动时的动画执行代码，包括碰到浏览器边缘时的反射效果：
time = 0;
requestAnimationFrame(loop = function(){
ctx.setTransform(1,0,0,1,0,0);
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = 1;
ctx.fillStyle = bgColor;
ctx.fillRect(0,0,width,height);
ctx.translate(o.x,o.y);
if(charging){ var c = Math.floor(30+mouse.d/2);
ctx.strokeStyle = 'rgba('+c+','+c+','+c+',1)';
ctx.lineWidth = 4;
ctx.beginPath();
ctx.moveTo(pos1.x,pos1.y);
ctx.lineTo(mouse.x,mouse.y);
ctx.lineCap = 'round';
ctx.stroke();
}
if(showInstructions){
pos1.x = -70;
pos1.y = -35;
if(time<10){ var x = -70,
y = -35,
r = 30-time*2,
a = time/10;
}else if(time<80){ var x = (time-10)*2-70,
y = (time-10)-35,
r = 10,
a = 1;
}else if(time<90){ var x = 70,
y = 35,
r = 10+(time-80)*2,
a = 1-(time-80)/10;
}else if(time<140){ var x = 70,
y = 35,
r = 30,
a = 0;
} var dx = pos1.x-x,
dy = pos1.y-y,
d = Math.sqrt(dx*dx+dy*dy); if(time<80&&time>10){
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = 1; var c = Math.floor(30+d/2);
ctx.strokeStyle = 'rgba('+c+','+c+','+c+',1)';
ctx.lineWidth = 4;
ctx.beginPath();
ctx.moveTo(pos1.x,pos1.y);
ctx.lineTo(x,y);
ctx.lineCap = 'round';
ctx.stroke();
} if(time<140){
ctx.globalCompositeOperation = 'source-over';
ctx.globalAlpha = a;
ctx.beginPath();
ctx.arc(x,y,r,0,Math.PI*2);
ctx.lineWidth = 2;
ctx.strokeStyle = '#aaa';
ctx.stroke();
} if(time==80){
newFireball(
x,
y,
dx*0.03,
dy*0.03, 240
);
}
time = (time+1)%180;
}
ctx.globalCompositeOperation = 'lighter';
for(var i in particles){
var p = particles[i];
ctx.beginPath();
ctx.arc(p.x,p.y,p.r,0,Math.PI*2);
ctx.globalAlpha = p.o;
ctx.fillStyle = p.c;
ctx.fill();
}
for(var i in particles){
var p = particles[i];
p.x += p.xv;
p.y += p.yv;
p.r += p.rv;
p.o += p.ov;
if(p.r<0)delete particles[p.index];
if(p.o<0)delete particles[p.index];
}
for(var i in fireballs){
f = fireballs[i];
var numParticles = Math.sqrt(f.xv*f.xv+f.yv*f.yv)/5;
if(numParticles<1)numParticles=1;
var numParticlesInt = Math.ceil(numParticles),
numParticlesDif = numParticles/numParticlesInt;
for(var j=0;j(boundary = edge.bottom-7)){
f.y = boundary;
f.yv *= -1;
} if(f.x>(boundary = edge.right-7)){
f.x = boundary;
f.xv *= -1;
}else if(f.x<(boundary = edge.left+7)){
f.x = boundary;
f.xv *= -1;
} if(--f.life<0)delete fireballs[f.index];
}
requestAnimationFrame(loop);
}); 
