path: root/files/zh-cn/web/api/canvas_api/tutorial/basic_animations/index.html

---
title: 基本的动画
slug: Web/API/Canvas_API/Tutorial/Basic_animations
tags:
  - Canvas
  - HTML5
  - 动画
  - 图像
  - 教程
  - 进阶
translation_of: Web/API/Canvas_API/Tutorial/Basic_animations
---
<div>{{CanvasSidebar}} {{PreviousNext("Web/API/Canvas_API/Tutorial/Compositing", "Web/API/Canvas_API/Tutorial/Advanced_animations")}}</div>

<div class="summary">
<p>由于我们是用 JavaScript 去操控 {{HTMLElement("canvas")}} 对象，这样要实现一些交互动画也是相当容易的。在本章中，我们将看看如何做一些基本的动画。</p>
</div>

<p>可能最大的限制就是图像一旦绘制出来，它就是一直保持那样了。如果需要移动它，我们不得不对所有东西（包括之前的）进行重绘。重绘是相当费时的，而且性能很依赖于电脑的速度。</p>

<h2 id="Basic_animation_steps" name="Basic_animation_steps">动画的基本步骤</h2>

<p>你可以通过以下的步骤来画出一帧:</p>

<ol>
 <li><strong>清空 canvas</strong><br>
  除非接下来要画的内容会完全充满 canvas （例如背景图），否则你需要清空所有。最简单的做法就是用 <code>clearRect</code> 方法。</li>
 <li><strong>保存 canvas 状态</strong><br>
  如果你要改变一些会改变 canvas 状态的设置（样式，变形之类的），又要在每画一帧之时都是原始状态的话，你需要先保存一下。</li>
 <li><strong>绘制动画图形（animated shapes）</strong><br>
  这一步才是重绘动画帧。</li>
 <li><strong>恢复 canvas 状态</strong><br>
  如果已经保存了 canvas 的状态，可以先恢复它，然后重绘下一帧。</li>
</ol>

<h2 id="Controlling_an_animation" name="Controlling_an_animation">操控动画 Controlling an animation</h2>

<p>在 canvas 上绘制内容是用 canvas 提供的或者自定义的方法，而通常，我们仅仅在脚本执行结束后才能看见结果，比如说，在 for 循环里面做完成动画是不太可能的。</p>

<p>因此， 为了实现动画，我们需要一些可以定时执行重绘的方法。有两种方法可以实现这样的动画操控。首先可以通过 <code>setInterval</code> 和 <code>setTimeout</code> 方法来控制在设定的时间点上执行重绘。</p>

<h3 id="有安排的更新画布_Scheduled_updates"><strong>有安排的更新画布 </strong>Scheduled updates</h3>

<p>首先，可以用{{domxref("window.setInterval()")}}, {{domxref("window.setTimeout()")}},和{{domxref("window.requestAnimationFrame()")}}来设定定期执行一个指定函数。</p>

<dl>
 <dt>{{domxref("WindowTimers.setInterval", "setInterval(function, delay)")}}</dt>
 <dd>当设定好间隔时间后，function会定期执行。</dd>
 <dt>{{domxref("WindowTimers.setTimeout", "setTimeout(function, delay)")}}</dt>
 <dt>在设定好的时间之后执行函数</dt>
 <dd></dd>
 <dt>{{domxref("Window.requestAnimationFrame()", "requestAnimationFrame(callback)")}}</dt>
 <dd>告诉浏览器你希望执行一个动画，并在重绘之前，请求浏览器执行一个特定的函数来更新动画。</dd>
</dl>

<p>如果你并不需要与用户互动，你可以使用setInterval()方法，它就可以定期执行指定代码。如果我们需要做一个游戏，我们可以使用键盘或者鼠标事件配合上setTimeout()方法来实现。通过设置事件监听，我们可以捕捉用户的交互，并执行相应的动作。</p>

<div class="note">
<p>下面的例子，采用 {{domxref("window.requestAnimationFrame()")}}实现动画效果。这个方法提供了更加平缓并更加有效率的方式来执行动画，当系统准备好了重绘条件的时候，才调用绘制动画帧。一般每秒钟回调函数执行60次，也有可能会被降低。想要了解更多关于动画循环的信息，尤其是游戏，可以在<a href="https://developer.mozilla.org/zh-CN/docs/Games">Game development zone</a> 参考这篇文章 <a href="https://developer.mozilla.org/zh-CN/docs/Games/Anatomy">Anatomy of a video game</a>。</p>
</div>

<h2 id="太阳系的动画">太阳系的动画</h2>

<p>这个例子里面，我会做一个小型的太阳系模拟动画。</p>

<pre class="brush: js">var sun = new Image();
var moon = new Image();
var earth = new Image();
function init(){
  sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
  moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
  earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
  window.requestAnimationFrame(draw);
}

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');

  ctx.globalCompositeOperation = 'destination-over';
  ctx.clearRect(0,0,300,300); // clear canvas

  ctx.fillStyle = 'rgba(0,0,0,0.4)';
  ctx.strokeStyle = 'rgba(0,153,255,0.4)';
  ctx.save();
  ctx.translate(150,150);

  // Earth
  var time = new Date();
  ctx.rotate( ((2*Math.PI)/60)*time.getSeconds() + ((2*Math.PI)/60000)*time.getMilliseconds() );
  ctx.translate(105,0);
  ctx.fillRect(0,-12,50,24); // Shadow
  ctx.drawImage(earth,-12,-12);

  // Moon
  ctx.save();
  ctx.rotate( ((2*Math.PI)/6)*time.getSeconds() + ((2*Math.PI)/6000)*time.getMilliseconds() );
  ctx.translate(0,28.5);
  ctx.drawImage(moon,-3.5,-3.5);
  ctx.restore();

  ctx.restore();

  ctx.beginPath();
  ctx.arc(150,150,105,0,Math.PI*2,false); // Earth orbit
  ctx.stroke();

  ctx.drawImage(sun,0,0,300,300);

  window.requestAnimationFrame(draw);
}

init();
</pre>

<div class="hidden">
<pre class="brush: html">&lt;canvas id="canvas" width="300" height="300"&gt;&lt;/canvas&gt;</pre>
</div>

<p>{{EmbedLiveSample("太阳系的动画", "310", "310", "https://mdn.mozillademos.org/files/202/Canvas_animation1.png")}}</p>

<h2 id="动画时钟">动画时钟</h2>

<p>这个例子实现一个动态时钟， 可以显示当前时间。</p>

<pre class="brush: js">function clock(){
  var now = new Date();
  var ctx = document.getElementById('canvas').getContext('2d');
  ctx.save();
  ctx.clearRect(0,0,150,150);
  ctx.translate(75,75);
  ctx.scale(0.4,0.4);
  ctx.rotate(-Math.PI/2);
  ctx.strokeStyle = "black";
  ctx.fillStyle = "white";
  ctx.lineWidth = 8;
  ctx.lineCap = "round";

  // Hour marks
  ctx.save();
  for (var i=0;i&lt;12;i++){
    ctx.beginPath();
    ctx.rotate(Math.PI/6);
    ctx.moveTo(100,0);
    ctx.lineTo(120,0);
    ctx.stroke();
  }
  ctx.restore();

  // Minute marks
  ctx.save();
  ctx.lineWidth = 5;
  for (i=0;i&lt;60;i++){
    if (i%5!=0) {
      ctx.beginPath();
      ctx.moveTo(117,0);
      ctx.lineTo(120,0);
      ctx.stroke();
    }
    ctx.rotate(Math.PI/30);
  }
  ctx.restore();

  var sec = now.getSeconds();
  var min = now.getMinutes();
  var hr  = now.getHours();
  hr = hr&gt;=12 ? hr-12 : hr;

  ctx.fillStyle = "black";

  // write Hours
  ctx.save();
  ctx.rotate( hr*(Math.PI/6) + (Math.PI/360)*min + (Math.PI/21600)*sec )
  ctx.lineWidth = 14;
  ctx.beginPath();
  ctx.moveTo(-20,0);
  ctx.lineTo(80,0);
  ctx.stroke();
  ctx.restore();

  // write Minutes
  ctx.save();
  ctx.rotate( (Math.PI/30)*min + (Math.PI/1800)*sec )
  ctx.lineWidth = 10;
  ctx.beginPath();
  ctx.moveTo(-28,0);
  ctx.lineTo(112,0);
  ctx.stroke();
  ctx.restore();

  // Write seconds
  ctx.save();
  ctx.rotate(sec * Math.PI/30);
  ctx.strokeStyle = "#D40000";
  ctx.fillStyle = "#D40000";
  ctx.lineWidth = 6;
  ctx.beginPath();
  ctx.moveTo(-30,0);
  ctx.lineTo(83,0);
  ctx.stroke();
  ctx.beginPath();
  ctx.arc(0,0,10,0,Math.PI*2,true);
  ctx.fill();
  ctx.beginPath();
  ctx.arc(95,0,10,0,Math.PI*2,true);
  ctx.stroke();
  ctx.fillStyle = "rgba(0,0,0,0)";
  ctx.arc(0,0,3,0,Math.PI*2,true);
  ctx.fill();
  ctx.restore();

  ctx.beginPath();
  ctx.lineWidth = 14;
  ctx.strokeStyle = '#325FA2';
  ctx.arc(0,0,142,0,Math.PI*2,true);
  ctx.stroke();

  ctx.restore();

  window.requestAnimationFrame(clock);
}

window.requestAnimationFrame(clock);
</pre>

<div class="hidden">
<pre class="brush: html">&lt;canvas id="canvas" width="150" height="150"&gt;&lt;/canvas&gt;</pre>
</div>

<p>{{EmbedLiveSample("动画时钟", "180", "180", "https://mdn.mozillademos.org/files/203/Canvas_animation2.png")}}</p>

<h2 id="循环全景照片">循环全景照片</h2>

<p>在这个例子中，会有一个自左向右滑动的全景图。我们使用了在维基百科中找到的<a href="https://mdn.mozillademos.org/files/4553/Capitan_Meadows,_Yosemite_National_Park.jpg">尤塞米提国家公园的图片</a>，当然你可以随意找一张任何尺寸大于canvas的图片。</p>

<pre class="brush: js">var img = new Image();

// User Variables - customize these to change the image being scrolled, its
// direction, and the speed.

img.src = 'https://mdn.mozillademos.org/files/4553/Capitan_Meadows,_Yosemite_National_Park.jpg';
var CanvasXSize = 800;
var CanvasYSize = 200;
var speed = 30; // lower is faster
var scale = 1.05;
var y = -4.5; // vertical offset

// Main program

var dx = 0.75;
var imgW;
var imgH;
var x = 0;
var clearX;
var clearY;
var ctx;

img.onload = function() {
    imgW = img.width * scale;
    imgH = img.height * scale;

    if (imgW &gt; CanvasXSize) {
        // image larger than canvas
        x = CanvasXSize - imgW;
    }
    if (imgW &gt; CanvasXSize) {
        // image width larger than canvas
        clearX = imgW;
    } else {
        clearX = CanvasXSize;
    }
    if (imgH &gt; CanvasYSize) {
        // image height larger than canvas
        clearY = imgH;
    } else {
        clearY = CanvasYSize;
    }

    // get canvas context
    ctx = document.getElementById('canvas').getContext('2d');

    // set refresh rate
    return setInterval(draw, speed);
}

function draw() {
    ctx.clearRect(0, 0, clearX, clearY); // clear the canvas

    // if image is &lt;= Canvas Size
    if (imgW &lt;= CanvasXSize) {
        // reset, start from beginning
        if (x &gt; CanvasXSize) {
            x = -imgW + x;
        }
        // draw additional image1
        if (x &gt; 0) {
            ctx.drawImage(img, -imgW + x, y, imgW, imgH);
        }
        // draw additional image2
        if (x - imgW &gt; 0) {
            ctx.drawImage(img, -imgW * 2 + x, y, imgW, imgH);
        }
    }

    // image is &gt; Canvas Size
    else {
        // reset, start from beginning
        if (x &gt; (CanvasXSize)) {
            x = CanvasXSize - imgW;
        }
        // draw aditional image
        if (x &gt; (CanvasXSize-imgW)) {
            ctx.drawImage(img, x - imgW + 1, y, imgW, imgH);
        }
    }
    // draw image
    ctx.drawImage(img, x, y,imgW, imgH);
    // amount to move
    x += dx;
}</pre>

<div class="hidden">
<pre class="brush: html">&lt;canvas id="canvas" width="800" height="200"&gt;&lt;/canvas&gt;</pre>
</div>

<p>下方就是是图片在其中滑动的 {{HTMLElement("canvas")}}。需要注意的是这里定义的width和height必须与JavaScript代码中的变量值<code>CanvasXZSize</code>和<code>CanvasYSize</code>保持一致。 </p>

<pre>&lt;canvas id="canvas" width="800" height="200"&gt;&lt;/canvas&gt;</pre>

<p>{{EmbedLiveSample("循环全景照片", "830", "230")}}</p>

<h2 id="鼠标追踪动画">鼠标追踪动画</h2>

<pre class="brush: html">&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
    &lt;head&gt;
        &lt;meta charset="UTF-8"&gt;
        &lt;meta name="viewport" content="width=device-width, initial-scale=1.0"&gt;
        &lt;meta http-equiv="X-UA-Compatible" content="ie=edge"&gt;
        &lt;title&gt;Document&lt;/title&gt;
        &lt;script&gt;
            var cn;
            //= document.getElementById('cw');
            var c;
            var u = 10;
            const m = {
                x: innerWidth / 2,
                y: innerHeight / 2
            };
            window.onmousemove = function(e) {
                m.x = e.clientX;
                m.y = e.clientY;

            }
            function gc() {
                var s = "0123456789ABCDEF";
                var c = "#";
                for (var i = 0; i &lt; 6; i++) {
                    c += s[Math.ceil(Math.random() * 15)]
                }
                return c
            }
            var a = [];
            window.onload = function myfunction() {
                cn = document.getElementById('cw');
                c = cn.getContext('2d');

                for (var i = 0; i &lt; 10; i++) {
                    var r = 30;
                    var x = Math.random() * (innerWidth - 2 * r) + r;
                    var y = Math.random() * (innerHeight - 2 * r) + r;
                    var t = new ob(innerWidth / 2,innerHeight / 2,5,"red",Math.random() * 200 + 20,2);
                    a.push(t);
                }
                //cn.style.backgroundColor = "#700bc8";

                c.lineWidth = "2";
                c.globalAlpha = 0.5;
                resize();
                anim()
            }
            window.onresize = function() {

                resize();

            }
            function resize() {
                cn.height = innerHeight;
                cn.width = innerWidth;
                for (var i = 0; i &lt; 101; i++) {
                    var r = 30;
                    var x = Math.random() * (innerWidth - 2 * r) + r;
                    var y = Math.random() * (innerHeight - 2 * r) + r;
                    a[i] = new ob(innerWidth / 2,innerHeight / 2,4,gc(),Math.random() * 200 + 20,0.02);

                }
                //  a[0] = new ob(innerWidth / 2, innerHeight / 2, 40, "red", 0.05, 0.05);
                //a[0].dr();
            }
            function ob(x, y, r, cc, o, s) {
                this.x = x;
                this.y = y;
                this.r = r;
                this.cc = cc;
                this.theta = Math.random() * Math.PI * 2;
                this.s = s;
                this.o = o;
                this.t = Math.random() * 150;

                this.o = o;
                this.dr = function() {
                    const ls = {
                        x: this.x,
                        y: this.y
                    };
                    this.theta += this.s;
                    this.x = m.x + Math.cos(this.theta) * this.t;
                    this.y = m.y + Math.sin(this.theta) * this.t;
                    c.beginPath();
                    c.lineWidth = this.r;
                    c.strokeStyle = this.cc;
                    c.moveTo(ls.x, ls.y);
                    c.lineTo(this.x, this.y);
                    c.stroke();
                    c.closePath();

                }
            }
            function anim() {
                requestAnimationFrame(anim);
                c.fillStyle = "rgba(0,0,0,0.05)";
                c.fillRect(0, 0, cn.width, cn.height);
                a.forEach(function(e, i) {
                    e.dr();
                });

            }
        &lt;/script&gt;
        &lt;style&gt;
            #cw {
                position: fixed;
                z-index: -1;
            }

            body {
                margin: 0;
                padding: 0;
                background-color: rgba(0,0,0,0.05);
            }
        &lt;/style&gt;
    &lt;/head&gt;
    &lt;body&gt;
        &lt;canvas id="cw"&gt;&lt;/canvas&gt;
        qwerewr

    &lt;/body&gt;
&lt;/html&gt;
</pre>

<h5 id="OutPut">OutPut</h5>

<table class="standard-table">
 <tbody>
  <tr>
   <td>
    <p><a href="https://kunalverma94.github.io/gallery/gags/beyblade.html"><img alt="beyblade" src="https://kunalverma94.github.io/gallery/beyblade.jpg" style="height: 298px; width: 399px;"></a></p>
   </td>
  </tr>
 </tbody>
</table>

<h2 id="Snake_Game">Snake Game</h2>

<pre class="brush: html">&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;

&lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;meta name="viewport" content="width=device-width,initial-scale=1"&gt;
    &lt;meta http-equiv="X-UA-Compatible" content="ie=edge"&gt;
    &lt;title&gt;Nokia 1100:snake..Member berries&lt;/title&gt;
&lt;/head&gt;

&lt;body&gt;
    &lt;div class="keypress hide"&gt;
        &lt;div class="up" onclick="emit(38)"&gt;&amp;#8593;&lt;/div&gt;
        &lt;div class="right" onclick="emit(39)"&gt;&amp;#8594;&lt;/div&gt;
        &lt;div class="left" onclick="emit(37)"&gt;&amp;#8592;&lt;/div&gt;
        &lt;div class="down" onclick="emit(40)"&gt;&amp;#8595;&lt;/div&gt;
    &lt;/div&gt;
    &lt;div class="banner" id="selector"&gt;
        &lt;div&gt;
            Time :&lt;span id="time"&gt;0&lt;/span&gt;
        &lt;/div&gt;
        &lt;div&gt;LousyGames ©&lt;/div&gt;
        &lt;div&gt;
            Score :&lt;span id="score"&gt;0&lt;/span&gt;
        &lt;/div&gt;
        &lt;div class="touch off" onclick="touch(this)"&gt;touch&lt;/div&gt;
    &lt;/div&gt;
    &lt;canvas id="main"&gt;&lt;/canvas&gt;
&lt;/body&gt;
&lt;style&gt;
    body {
        margin: 0;
        overflow: hidden;
        background: #000
    }

    .banner {
        text-align: center;
        color: #fff;
        background: #3f51b5;
        line-height: 29px;
        position: fixed;
        left: 0;
        top: 0;
        right: 0;
        font-family: monospace;
        height: 30px;
        opacity: .4;
        display: flex;
        transition: .5s
    }

    .banner:hover {
        opacity: 1
    }

    div#selector&gt;div {
        flex-basis: 30%
    }

    @keyframes diss {
        from {
            opacity: 1
        }

        to {
            opacity: 0
        }
    }

    .keypress&gt;div {
        border: dashed 3px #fff;
        height: 48%;
        width: 48%;
        display: flex;
        align-content: center;
        justify-content: center;
        align-self: center;
        align-items: center;
        font-size: -webkit-xxx-large;
        font-weight: 900;
        color: #fff;
        transition: .5s;
        opacity: .1;
        border-radius: 7px
    }

    .keypress {
        position: fixed;
        width: 100vw;
        height: 100vh;
        top: 0;
        left: 0;
        display: flex;
        flex-wrap: wrap;
        justify-content: space-around;
        opacity: 1;
        user-select: none
    }

    .keypress&gt;div:hover {
        opacity: 1
    }

    .touch {
        background: #8bc34a
    }

    .off {
        background: #f44336
    }

    .hide {
        opacity: 0
    }
&lt;/style&gt;
&lt;/html&gt;</pre>

<p>Javascript</p>

<pre class="brush: js">function tmz() {
        var e = new Date(t),
            i = new Date,
            n = Math.abs(i.getMinutes() - e.getMinutes()),
            o = Math.abs(i.getSeconds() - e.getSeconds());
        return n + " : " + o
    }

    function coll(t, e) {
        return t.x &lt; e.x + e.w &amp;&amp; t.x + t.w &gt; e.x &amp;&amp; t.y &lt; e.y + e.h &amp;&amp; t.h + t.y &gt; e.y
    }

    function snake() {
        this.w = 15, this.h = 15, this.dx = 1, this.dy = 1, this.xf = 1, this.yf = 1, this.sn = [];
        for (var t = {
            x: w / 2,
            y: h / 2
        }, e = 0; e &lt; 5; e++) this.sn.push(Object.assign({}, t)), t.x += this.w;
        this.draw = function () {
            var t = d &amp;&amp; d.search("Arrow") &gt; -1,
                e = -1;
            if (t) {
                var i = {
                    ...this.sn[0]
                };
                if ("ArrowUp" == d &amp;&amp; (i.y -= this.h), "ArrowDown" == d &amp;&amp; (i.y += this.h), "ArrowLeft" == d &amp;&amp; (i.x -= this.w), "ArrowRight" == d &amp;&amp; (i.x += this.w), i.x &gt;= w ? i.x = 0 : i.x &lt; 0 &amp;&amp; (i.x = w - this.w), i.y &gt; h ? i.y = 0 : i.y &lt; 0 &amp;&amp; (i.y = h), e = fa.findIndex(t =&gt; coll({
                    ...this.sn[0],
                    h: this.h,
                    w: this.w
                }, t)), this.sn.unshift(i), -1 != e) return console.log(e), fa[e].renew(), void (document.getElementById("score").innerText = Number(document.getElementById("score").innerText) + 1);
                this.sn.pop(), console.log(6)
            }
            this.sn.forEach((t, e, i) =&gt; {
                if (0 == e || i.length - 1 == e) {
                    var n = c.createLinearGradient(t.x, t.y, t.x + this.w, t.y + this.h);
                    i.length - 1 == e ? (n.addColorStop(0, "black"), n.addColorStop(1, "#8BC34A")) : (n.addColorStop(0, "#8BC34A"), n.addColorStop(1, "white")), c.fillStyle = n
                } else c.fillStyle = "#8BC34A";
                c.fillRect(t.x, t.y, this.w, this.h), c.strokeStyle = "#E91E63", c.font = "30px serif", c.strokeStyle = "#9E9E9E", i.length - 1 != e &amp;&amp; 0 != e &amp;&amp; c.strokeRect(t.x, t.y, this.w, this.h), 0 == e &amp;&amp; (c.beginPath(), c.fillStyle = "#F44336", c.arc(t.x + 10, t.y + 2, 5, 360, 0), c.fill()), c.arc(t.x + 10, t.y + 2, 5, 360, 0), c.fill(), c.beginPath()
            })
        }
    }

    function gc() {
        for (var t = "0123456789ABCDEF", e = "#", i = 0; i &lt; 6; i++) e += t[Math.ceil(15 * Math.random())];
        return e
    }

    function food() {
        this.x = 0, this.y = 0, this.b = 10, this.w = this.b, this.h = this.b, this.color = gc(), this.renew = function () {
            this.x = Math.floor(Math.random() * (w - 200) + 10), this.y = Math.floor(Math.random() * (h - 200) + 30), this.color = gc()
        }, this.renew(), this.put = (() =&gt; {
            c.fillStyle = this.color, c.arc(this.x, this.y, this.b - 5, 0, 2 * Math.PI), c.fill(), c.beginPath(), c.arc(this.x, this.y, this.b - 5, 0, Math.PI), c.strokeStyle = "green", c.lineWidth = 10, c.stroke(), c.beginPath(), c.lineWidth = 1
        })
    }

    function init() {
        cc.height = h, cc.width = w, c.fillRect(0, 0, w, innerHeight);
        for (var t = 0; t &lt; 10; t++) fa.push(new food);
        s = new snake(w / 2, h / 2, 400, 4, 4), anima()
    }

    function anima() {
        c.fillStyle = "rgba(0,0,0,0.11)", c.fillRect(0, 0, cc.width, cc.height), fa.forEach(t =&gt; t.put()), s.draw(), document.getElementById("time").innerText = tmz(), setTimeout(() =&gt; {
            requestAnimationFrame(anima)
        }, fw)
    }

    function emit(t) {
        key.keydown(t)
    }

    function touch(t) {
        t.classList.toggle("off"), document.getElementsByClassName("keypress")[0].classList.toggle("hide")
    }
    var t = new Date + "",
        d = void 0,
        cc = document.getElementsByTagName("canvas")[0],
        c = cc.getContext("2d");
    key = {}, key.keydown = function (t) {
        var e = document.createEvent("KeyboardEvent");
        Object.defineProperty(e, "keyCode", {
            get: function () {
                return this.keyCodeVal
            }
        }), Object.defineProperty(e, "key", {
            get: function () {
                return 37 == this.keyCodeVal ? "ArrowLeft" : 38 == this.keyCodeVal ? "ArrowUp" : 39 == this.keyCodeVal ? "ArrowRight" : "ArrowDown"
            }
        }), Object.defineProperty(e, "which", {
            get: function () {
                return this.keyCodeVal
            }
        }), e.initKeyboardEvent ? e.initKeyboardEvent("keydown", !0, !0, document.defaultView, !1, !1, !1, !1, t, t) : e.initKeyEvent("keydown", !0, !0, document.defaultView, !1, !1, !1, !1, t, 0), e.keyCodeVal = t, e.keyCode !== t &amp;&amp; alert("keyCode mismatch " + e.keyCode + "(" + e.which + ")"), document.dispatchEvent(e)
    };
    var o, s, h = innerHeight,
        w = innerWidth,
        fw = 60,
        fa = [];
    window.onkeydown = function (t) {
        var e = t.key;
        (e.search("Arrow") &gt; -1 || "1" == e) &amp;&amp; (d = t.key), "i" != e &amp;&amp; "I" != e || (console.log("inc"), fw -= 10), "d" != e &amp;&amp; "D" != e || (console.log("dec"), fw += 10)
    }, init();
</pre>

<h5 id="Output">Output</h5>

<table class="standard-table">
 <tbody>
  <tr>
   <td>
    <h2 id="sect1"><a href="https://kunalverma94.github.io/pokemon/snake.html"><img alt="Snake game" src="https://kunalverma94.github.io/view/images/snake.jpg" style="height: 400px; width: 600px;"></a></h2>
   </td>
  </tr>
 </tbody>
</table>

<h2 id="Other_examples" name="Other_examples">其它例子</h2>

<dl>
 <dt><a href="https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API/A_basic_ray-caster" title="/zh-CN/docs/Web/Guide/HTML/A_basic_ray-caster">A basic ray-caster</a></dt>
 <dd>一个关于如何使用键盘关联控制动画的优秀的案例。</dd>
 <dt><a href="https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API/Tutorial/Advanced_animations">Advanced animations</a></dt>
 <dd>我们将在下一章看到一些先进的动画技术和物理现象。</dd>
</dl>

<p>{{PreviousNext("Web/API/Canvas_API/Tutorial/Compositing", "Web/API/Canvas_API/Tutorial/Advanced_animations")}}</p>