window.onload = function(){
var oc = document.querySelector("canvas");
if (oc.getContext){
var ctx = oc.getContext("2d");
var arr = [];
setInterval(function(){
console.log(arr);
// 在绘画之前，对数组arr内部的参数值进行改变，以达到运动的效果
ctx.clearRect(0,0,oc.width,oc.height);
for(var i =0;i<arr.length;i++){
arr[i].degree++;
arr[i].x = arr[i].startX + Math.sin(arr[i].degree* Math.PI/180)*arr[i].step*2;
arr[i].y = arr[i].startY - (arr[i].degree* Math.PI/180)*arr[i].step*5;
console.log(arr[i].y);
if (arr[i].y <= arr[i].r){
arr.splice(i,1);
}
arr[i].alp-=0.001;
}
//对已经更改好的数组对应的圆进行绘画。
for (var i = 0; i<arr.length; i++){
ctx.save();
ctx.fillStyle = "rgba("+arr[i].red+","+arr[i].green+","+arr[i].blue+","+arr[i].alp+")";
ctx.beginPath();
ctx.arc(arr[i].x,arr[i].y,arr[i].r,0,2*Math.PI);
ctx.fill();
ctx.restore();
}
},1000/60);
// 在arr中注入随机圆信息
setInterval(function(){
var x = Math.random()*oc.width;
var r = Math.random()*10+5;
var y = oc.height -r ;
var red = Math.round(Math.random()*255);
var green = Math.round(Math.random()*255);
var blue = Math.round(Math.random()*255);
var alp = 1;
var degree = 0;
var startX = x;
var startY = y;
var step = 10;
arr.push({
x: x,
y: y,
r: r,
red: red,
green: green,
blue: blue,
alp : alp,
degree : degree,
startX: startX,
startY: startY,
step: step
});
},200)
}