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javascript – Canvas’fps无意中加速了？

2019-08-27 04:36:24 作者：互联网

我正在使用基于在线教程的canvas元素搞乱,并构建了以下页面,here.

这是我的标记：

<!DOCTYPE html>

<html lang="en">
    <head>
        <meta charset="utf-8" />
        <title>Canvas Game</title>

        <link rel="stylesheet" href="style.css">

        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
        <script src="script.js"></script>
    </head>
    <body>
    <header>
        <h1>Cool Canvas Bouncing Effect!</h1>
        <p>Which would you like to see bounce around?</p>
        <input id="beachBallButton" type="button" value="Beach Ball" />
        <input id="avatarButton" type="button" value="Avatar" />
    </header>
    <br />
    <canvas id="myCanvas" width="600" height="400">
        Your browser does not support canvas!
    </canvas>
    </body>
</html>

这是我的JavaScript：

$(document).ready(function() {

const FPS;
var x = 0;
var y = 0;
var xDirection = 1;
var yDirection = 1;
var image = new Image();
image.src = null;
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext('2d');

$('#avatarButton').click(function() {
    x = 0;
    y = 0;
    FPS = 30;
    image.src = 'avatar.png';

    setInterval(draw, 1000 / FPS);

    function draw() {
        ctx.clearRect(0,0,canvas.width,canvas.height);
        ctx.fillRect(0,0,canvas.width,canvas.height);
        ctx.drawImage(image, x, y);

        x += 1 * xDirection;
        y += 1 * yDirection;

        if (x >= 500)
        {
            x = 500;
            xDirection = -1;
        }
        else if (x <= 0)
        {
            x = 0;
            xDirection = 1;
        }

        if (y >= 300)
        {
            y = 300;
            yDirection = -1;
        }
        else if (y <= 0)
        {
            y = 0;
            yDirection = 1;
        }
    }
});

$('#beachBallButton').click(function() {
    x = 0;
    y = 0;
    FPS = 60;
    image.src = 'beachball.png';

    setInterval(draw, 1000 / FPS);

    function draw() {
        ctx.clearRect(0,0,canvas.width,canvas.height);
        ctx.fillRect(0,0,canvas.width,canvas.height);
        ctx.drawImage(image, x, y);

        x += 5 * xDirection;
        y += 5 * yDirection;

        if (x >= 450)
        {
            x = 450;
            xDirection = -1;
        }
        else if (x <= 0)
        {
            x = 0;
            xDirection = 1;
        }

        if (y >= 250)
        {
            y = 250;
            yDirection = -1;
        }
        else if (y <= 0)
        {
            y = 0;
            yDirection = 1;
        }
    }
});

});

现在,假设您点击Avatar按钮,然后点击Beach Ball,FPS加速.但是,我在两个函数的单击函数中重置了FPS变量.我也重置了x和y变量.

另外,我可以继续点击相同的按钮,速度也会急剧增加.

有人可以帮助解释为什么会这样吗？

谢谢！

解决方法:

MMM ….. const FPS; < ---这是什么？据我记得,javascript中没有常量.无论如何,如果它是一个常量,为什么你试着稍后设置它的值几次？我认为这个语句失败了,第一次设置FPS时,你创建了一个全局变量,后来这个全局变量被所有函数共享. 此外,您不使用clearInterval,并且每次单击时都调用一个新的setInterval,因此如果在“beachBallButton”中单击3次,则将运行3个setIntervals函数,每个函数都执行代码.这很可能导致“加速”.

标签：frame-rate,javascript,canvas
来源： https://codeday.me/bug/20190827/1737268.html