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基本的动画 - Web API 接口参考编辑

由于我们是用 JavaScript 去操控 <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。

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。

动画的基本步骤

你可以通过以下的步骤来画出一帧:

清空 canvas
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 clearRect 方法。
保存 canvas 状态
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。
绘制动画图形（animated shapes）
这一步才是重绘动画帧。
恢复 canvas 状态
如果已经保存了 canvas 的状态，可以先恢复它，然后重绘下一帧。

操控动画 Controlling an animation

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。

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 setInterval 和 setTimeout 方法来控制在设定的时间点上执行重绘。

有安排的更新画布 Scheduled updates

首先，可以用window.setInterval(), window.setTimeout(),和window.requestAnimationFrame()来设定定期执行一个指定函数。

setInterval(function, delay): 当设定好间隔时间后，function会定期执行。
setTimeout(function, delay)
在设定好的时间之后执行函数
requestAnimationFrame(callback): 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。

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。

下面的例子，采用 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Game development zone 参考这篇文章 Anatomy of a video game。

太阳系的动画

这个例子里面，我会做一个小型的太阳系模拟动画。

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();

<canvas id="canvas" width="300" height="300"></canvas>

Screenshot	Live sample

动画时钟

这个例子实现一个动态时钟，可以显示当前时间。

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<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<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>=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);

<canvas id="canvas" width="150" height="150"></canvas>

Screenshot	Live sample

循环全景照片

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尤塞米提国家公园的图片，当然你可以随意找一张任何尺寸大于canvas的图片。

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 > CanvasXSize) {
        // image larger than canvas
        x = CanvasXSize - imgW;
    }
    if (imgW > CanvasXSize) {
        // image width larger than canvas
        clearX = imgW;
    } else {
        clearX = CanvasXSize;
    }
    if (imgH > 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 <= Canvas Size
    if (imgW <= CanvasXSize) {
        // reset, start from beginning
        if (x > CanvasXSize) {
            x = -imgW + x;
        }
        // draw additional image1
        if (x > 0) {
            ctx.drawImage(img, -imgW + x, y, imgW, imgH);
        }
        // draw additional image2
        if (x - imgW > 0) {
            ctx.drawImage(img, -imgW * 2 + x, y, imgW, imgH);
        }
    }

    // image is > Canvas Size
    else {
        // reset, start from beginning
        if (x > (CanvasXSize)) {
            x = CanvasXSize - imgW;
        }
        // draw aditional image
        if (x > (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;
}

<canvas id="canvas" width="800" height="200"></canvas>

下方就是是图片在其中滑动的 <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CanvasXZSize和CanvasYSize保持一致。

<canvas id="canvas" width="800" height="200"></canvas>

鼠标追踪动画

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta http-equiv="X-UA-Compatible" content="ie=edge">
        <title>Document</title>
        <script>
            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 < 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 < 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 < 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();
                });

            }
        </script>
        <style>
            #cw {
                position: fixed;
                z-index: -1;
            }

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

    </body>
</html>

OutPut

Snake Game

<!DOCTYPE html>
<html lang="en">

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

<body>
    <div class="keypress hide">
        <div class="up" onclick="emit(38)">&#8593;</div>
        <div class="right" onclick="emit(39)">&#8594;</div>
        <div class="left" onclick="emit(37)">&#8592;</div>
        <div class="down" onclick="emit(40)">&#8595;</div>
    </div>
    <div class="banner" id="selector">
        <div>
            Time :<span id="time">0</span>
        </div>
        <div>LousyGames ©</div>
        <div>
            Score :<span id="score">0</span>
        </div>
        <div class="touch off" onclick="touch(this)">touch</div>
    </div>
    <canvas id="main"></canvas>
</body>
<style>
    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>div {
        flex-basis: 30%
    }

    @keyframes diss {
        from {
            opacity: 1
        }

        to {
            opacity: 0
        }
    }

    .keypress>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>div:hover {
        opacity: 1
    }

    .touch {
        background: #8bc34a
    }

    .off {
        background: #f44336
    }

    .hide {
        opacity: 0
    }
</style>
</html>

Javascript

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 < e.x + e.w && t.x + t.w > e.x && t.y < e.y + e.h && t.h + t.y > 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 < 5; e++) this.sn.push(Object.assign({}, t)), t.x += this.w;
        this.draw = function () {
            var t = d && d.search("Arrow") > -1,
                e = -1;
            if (t) {
                var i = {
                    ...this.sn[0]
                };
                if ("ArrowUp" == d && (i.y -= this.h), "ArrowDown" == d && (i.y += this.h), "ArrowLeft" == d && (i.x -= this.w), "ArrowRight" == d && (i.x += this.w), i.x >= w ? i.x = 0 : i.x < 0 && (i.x = w - this.w), i.y > h ? i.y = 0 : i.y < 0 && (i.y = h), e = fa.findIndex(t => 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) => {
                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 && 0 != e && c.strokeRect(t.x, t.y, this.w, this.h), 0 == e && (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 < 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 = (() => {
            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 < 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 => t.put()), s.draw(), document.getElementById("time").innerText = tmz(), setTimeout(() => {
            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 && 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") > -1 || "1" == e) && (d = t.key), "i" != e && "I" != e || (console.log("inc"), fw -= 10), "d" != e && "D" != e || (console.log("dec"), fw += 10)
    }, init();