几秒钟后画布性能下降

我编写了以下 1000 个边界方块演示来测试 HTML5 画布的功能。一开始运行良好，但几秒钟后 fps 明显下降。我不知道为什么。任何指示将不胜感激。

var c = document.getElementById("canvas");
var context = c.getContext("2d");

var WIDTH = 600;
var HEIGHT = 800;

c.width = WIDTH;
c.height = HEIGHT;

image = loadImage("square.png");

function loadImage(imageName){
    var i = new Image();
    i.src = imageName;
    return i;
}


function clear(){
    context.fillStyle = "#d0e7f9";
    context.rect(0,0,WIDTH,HEIGHT);
    context.fill();
}
clear();

var SpriteList = [];

var Sprite = (function() { //javascript class(?)... shredders
    function Sprite(){  //constructor
        this.x = Math.random()*WIDTH;
        this.y = Math.random()*HEIGHT;
        this.vx = Math.random()*10;
        this.vy = Math.random()*10;
        SpriteList.push(this);
    }

    Sprite.prototype.update = function(){
        this.x += this.vx;
        this.y += this.vy;

        if (this.x<0 || this.x>WIDTH){
            this.vx *= -1;
        }
        if (this.y<0 || this.y>HEIGHT){
            this.vy *= -1;
        }
    };

    return Sprite;
})();

for (var i = 0;i<1000;i++){
    new Sprite();
}

function draw(){
    clear();
    for (i in SpriteList)
    {
        var s = SpriteList[i];
        s.update();
        context.drawImage(image, s.x, s.y);
    }
}

setInterval(draw,1000/60);

代码存在一些问题，但发生这种情况的主要原因是以下代码：

此代码将要求您使用beginPath():

function clear(){
    context.fillStyle = "#d0e7f9";
    context.beginPath();
    context.rect(0,0,WIDTH,HEIGHT); /// this will require beginPath();
    context.fill();
}

或者为了避免这种情况，您可以简单地修改代码来执行此操作：

function clear(){
    context.fillStyle = "#d0e7f9";
    context.fillRect(0,0,WIDTH,HEIGHT); /// this does not require beginPath();
}

这里现场演奏小提琴 http://jsfiddle.net/AbdiasSoftware/trSe4/

/// use a var here    
var image = loadImage("square.png");

/// your image loader is missing - image may not show up
function loadImage(imageName){
    var i = new Image();
    i.onload = nextStep; /// something like this
    i.src = imageName;
    return i;
}

var SpriteList = [];

/// create this as an object
function Sprite(){  //constructor
    this.x = Math.random()*WIDTH;
    this.y = Math.random()*HEIGHT;
    this.vx = Math.random()*10;
    this.vy = Math.random()*10;
    return this;
}

Sprite.prototype.update = function(){
    this.x += this.vx;
    this.y += this.vy;

    if (this.x<0 || this.x>WIDTH){
        this.vx *= -1;
    }
    if (this.y<0 || this.y>HEIGHT){
        this.vy *= -1;
    }
};

/// separate pushing of the instances
for (var i = 0;i<1000;i++){
    SpriteList.push(new Sprite());
}

var oldTime = 0;

function draw(timeElapsed){ /// in milliseconds
    clear();

    var diffTime = timeElapsed - oldTime;

    /// use vars here too
    for (var i = 0, s; s = SpriteList[i]; i++ )
    {
        s.update();
        context.drawImage(image, s.x, s.y);
    }

    oldTime = timeElapsed;

    /// use rAF here
    requestAnimationFrame(draw);
}

draw(0); /// start

The setInterval如果浏览器在您给出的时间预算内处理精灵的速度不够快，可能会导致整个事情堆栈调用。

通过使用 rAF，浏览器只会在可以的情况下请求帧，即使这意味着较低的帧速率 - 至少不会锁定/减慢浏览器的速度。

（因为您没有提供指向您正在使用的图像的链接，所以我用临时画布替换了它 - 您仍然需要考虑onload实际图像的事件处理程序）。