背景
前些日子当前端面试官，问了一个问题：“你了解过canvas吗？”
“这个我知道，我有做过DEMO，这个不难吧，看着它的api接口就能实现！”
看着他这么（蜜汁）自信，我决定深入了解（为难）一下他！
“电商中大转盘，九宫格，刮刮乐，如何使用canvas实现，讲讲你的思路？”
“二维码的生成和扫码识别如何实现？”
“图片的粒子爆炸效果呢？”
“......”
因此，打算写一系列关于 canvas 的文章，探索学习提升自己的同时顺便分享给大家。
二维码的生成
二维码的生成需借助第三方库，利用其算法对文本转化成二维码，并用 canvas 绘画出来。利用 canvas.toDataURL('image/png') 获取二维码转 base64 值，再将其赋值给 img 标签的 src 属性
这里我使用了一个库，qrcodejs.
可点击 《Demo》 查看效果
使用方法如下：
<!-- index.html -->
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Suporka Vue App</title>
<style>
.container {
padding: 60px;
margin: 0 auto;
line-height: 50px;
}
input {
display: inline-block;
width: 200px;
height: 32px;
line-height: 1.5;
padding: 4px 7px;
font-size: 12px;
border: 1px solid #dcdee2;
border-radius: 4px;
color: #515a6e;
background-color: #fff;
background-image: none;
position: relative;
cursor: text;
transition: border 0.2s ease-in-out, background 0.2s ease-in-out,
box-shadow 0.2s ease-in-out;
}
button {
color: #fff;
background-color: #19be6b;
border-color: #19be6b;
outline: 0;
vertical-align: middle;
line-height: 1.5;
display: inline-block;
font-weight: 400;
text-align: center;
-ms-touch-action: manipulation;
touch-action: manipulation;
cursor: pointer;
background-image: none;
border: 1px solid transparent;
white-space: nowrap;
-webkit-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;
padding: 5px 15px 6px;
font-size: 12px;
border-radius: 4px;
transition: color 0.2s linear, background-color 0.2s linear,
border 0.2s linear, box-shadow 0.2s linear;
}
#qrcode {
margin-top: 20px;
}
</style>
</head>
<body>
<div class="container">
<input
type="text"
placeholder="请输入您想转化成二维码的字符串"
id="input"
/>
<button onclick="creatQRcode();">一键生成</button>
<div id="qrcode"></div>
</div>
<script src="https://zxpsuper.github.io/Demo/qrcode/qrcode-dev.js"></script>
<script type="text/javascript">
var qrcode = null;
function creatQRcode() {
document.getElementById("qrcode").innerHTML = "";
qrcode = new QRCode(document.getElementById("qrcode"), {
text: document.getElementById("input").value,
width: 200,
height: 200,
colorDark: "#000000",
colorLight: "#ffffff",
correctLevel: QRCode.CorrectLevel.H
});
}
</script>
</body>
</html>options
- 首参为生成存放
img标签的父元素 - 第二个参数为
Object类型的options
| 属性 | 类型 | 说明 |
|---|---|---|
| text | String | 目标文本 |
| width | Number | 图片宽度 |
| height | Number | 图片高度 |
| colorDark | String | 二维码颜色 |
| colorLight | 默认QRCode.CorrectLevel.L | 容错级别，可设置为：QRCode.CorrectLevel.L ，QRCode.CorrectLevel.M，QRCode.CorrectLevel.Q，QRCode.CorrectLevel.H |
二维码扫码识别
这里利用了一个库 llqrcode.js, 使用 qrcode.decode() 对 id 为 qr-canvas 的 canvas 进行解码.
我们需要做的就是，调用设备的摄像头（后置摄像头优先），获得的画面用 video 标签实时显示出来，再定时取画面生成 canvas ，调用 qrcode.decode() 解密。
// variable.js
var gCtx = null; //canvas.ctx
var gCanvas = null; // qr-canvas
// var c = 0;
var stype = 0; // 识别流程 0未开始 1进行中 2已结束
var gUM = false;
var webkit = false;
var moz = false;
var v = null; // 存放视频的变量
var scanCodeStart = false; // 开始扫码
var mediaStreamTrack = null; // mediaStreamTrack 实现关闭摄像头功能 mediaStreamTrack.stop()
var imghtml =
'<div id="qrfile"><canvas id="out-canvas" width="320" height="240"></canvas>' +
'<div id="imghelp">drag and drop a QRCode here' +
"<br>or select a file" +
'<input type="file" onchange="handleFiles(this.files)" id="upload-img"/>' +
"</div>" +
"</div>";
var vidhtml = '<video id="v" autoplay muted></video>';// methods.js
function qrcodeScanLoad(width, height) {
if (isCanvasSupported() && window.File && window.FileReader) {
initCanvas(width, height);
qrcode.callback = scanCodeCallback;
document.getElementById("mainbody").style.display = "inline";
setwebcam();
} else {
document.getElementById("mainbody").style.display = "inline";
document.getElementById("mainbody").innerHTML =
'<p id="mp1">QR code scanner for HTML5 capable browsers</p><br>' +
'<br><p id="mp2">sorry your browser is not supported</p><br><br>';
}
}
// 绘制遮罩层canvas
function setMask() {
var canvas = document.querySelector("#scancode-mask");
canvas.width =
document.body.clientWidth > 1024 ? 1024 : document.body.clientWidth;
canvas.height =
document.body.clientWidth > 1024 ? 1136 : document.body.clientHeight;
var ctx = canvas.getContext("2d");
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.globalCompositeOperation = "destination-out";
ctx.beginPath();
let x1,
y1,
width = canvas.width * 0.6;
x1 = (canvas.width - width) / 2;
y1 = (canvas.height - width) / 2;
ctx.fillRect(x1, y1, width, width);
ctx.fill();
ctx.save();
ctx.globalCompositeOperation = "source-over";
// 第二象限点
ctx.moveTo(x1, y1 + 2);
ctx.lineTo(x1 + 20, y1 + 2);
ctx.moveTo(x1 + 2, y1);
ctx.lineTo(x1 + 2, y1 + 20);
// 第一象限点
ctx.moveTo(x1 + width - 20, y1 + 2);
ctx.lineTo(x1 + width, y1 + 2);
ctx.moveTo(x1 + width - 2, y1 + 1);
ctx.lineTo(x1 + width - 2, y1 + 20);
// 第四象限点
ctx.moveTo(x1 + width - 20, y1 + width - 2);
ctx.lineTo(x1 + width, y1 + width - 2);
ctx.moveTo(x1 + width - 2, y1 + width - 1);
ctx.lineTo(x1 + width - 2, y1 + width - 20);
// 第三象限点
ctx.moveTo(x1 + 20, y1 + width - 2);
ctx.lineTo(x1, y1 + width - 2);
ctx.moveTo(x1 + 2, y1 + width - 2);
ctx.lineTo(x1 + 2, y1 + width - 20);
ctx.lineWidth = 4;
ctx.strokeStyle = "green";
ctx.stroke();
}
function setwebcam() {
var options = true;
if (navigator.mediaDevices && navigator.mediaDevices.enumerateDevices) {
try {
navigator.mediaDevices.enumerateDevices().then(function(devices) {
let video = [];
devices.forEach(function(device) {
if (device.kind === "videoinput") {
video.push(device);
}
});
// 调用设备的摄像头，video[1] 为后置摄像头，或者label含有‘back’为后置摄像头
if (video.length >= 2) {
options = {
deviceId: { exact: video[1].deviceId },
facingMode: { exact: "environment" }
};
}
video.forEach(item => {
if (item.label.toLowerCase().search("back") > -1)
options = {
deviceId: { exact: device.deviceId },
facingMode: { exact: "environment" }
};
});
scanCodeStart = true;
setwebcam2(options);
});
} catch (e) {
console.error(e);
}
} else {
console.log("no navigator.mediaDevices.enumerateDevices");
setwebcam2(options);
}
}
function setwebcam2(options) {
if (stype == 1) {
setTimeout(captureToCanvas, 500);
return;
}
var n = navigator;
document.getElementById("outdiv").innerHTML = vidhtml;
v = document.getElementById("v");
try {
if (n.mediaDevices && n.mediaDevices.getUserMedia) {
n.mediaDevices
.getUserMedia({ video: options, audio: false })
.then(function(stream) {
success(stream);
})
.catch(function(error) {
error(error);
});
} else if (n.getUserMedia) {
webkit = true;
n.getUserMedia({ video: options, audio: false }, success, error);
} else if (n.webkitGetUserMedia) {
webkit = true;
n.webkitGetUserMedia({ video: options, audio: false }, success, error);
}
} catch (err) {
console.log(err);
}
stype = 1;
if (getSystem() === "ios") {
alert("您的設備暫不支持實時掃碼，請上傳圖片識別！");
return;
}
if (
(n.mediaDevices && n.mediaDevices.getUserMedia) ||
n.getUserMedia ||
n.webkitGetUserMedia
)
setTimeout(captureToCanvas, 500);
else {
alert("您的設備暫不支持實時掃碼，請上傳圖片識別！");
}
}
// 获取操作系统
function getSystem() {
var u = navigator.userAgent;
var isAndroid = u.indexOf("Android") > -1 || u.indexOf("Linux") > -1; //g
var isIOS = !!u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/); //ios终端
if (isAndroid) {
//这个是安卓操作系统
return "android";
} else if (isIOS) {
//这个是ios操作系统
return "ios";
} else {
return "other";
}
}
// 选择图片上传
function setimg($event) {
qrcode.callback = scanCodeCallback;
$event && $event.preventDefault();
stype = 2;
let file = document.getElementById("upload-img");
file.click();
}
// 上传成功的回调
function scanCodeCallback(a) {
var html = htmlEntities(a);
stype = 0;
alert(html);
}
// 处理上传文件识别
function handleFiles(f) {
var o = [];
for (var i = 0; i < f.length; i++) {
var reader = new FileReader();
reader.onload = (function(theFile) {
return function(e) {
gCtx.clearRect(0, 0, gCanvas.width, gCanvas.height);
qrcode.decode(e.target.result);
};
})(f[i]);
reader.readAsDataURL(f[i]);
}
}
function initCanvas(w, h) {
gCanvas = document.getElementById("qr-canvas");
gCanvas.style.width = w + "px";
gCanvas.style.height = h + "px";
gCanvas.width = w;
gCanvas.height = h;
gCtx = gCanvas.getContext("2d");
gCtx.clearRect(0, 0, w, h);
}
// 画面转 canvas
function captureToCanvas() {
if (stype != 1) return;
if (gUM && scanCodeStart) {
try {
gCtx.drawImage(v, 0, 0);
try {
qrcode.decode(); // 默认为id=qr-canvas的canvas转成图片的base64
} catch (e) {
console.log(e);
setTimeout(captureToCanvas, 500);
}
} catch (e) {
console.log(e);
setTimeout(captureToCanvas, 500);
}
}
}
// 对特殊符号进行处理
function htmlEntities(str) {
return String(str)
.replace(/&/g, "&")
.replace(/</g, "<")
.replace(/>/g, ">")
.replace(/"/g, """);
}
// 判断是否支持canvas
function isCanvasSupported() {
var elem = document.createElement("canvas");
return !!(elem.getContext && elem.getContext("2d"));
}
function success(stream) {
// mediaStreamTrack 实现关闭摄像头功能
if (stream)
mediaStreamTrack =
typeof stream.stop === "function" ? stream : stream.getTracks()[0];
v.srcObject = stream;
if (scanCodeStart) {
v.play();
gUM = true;
setTimeout(captureToCanvas, 500);
} else {
}
}
function error(error) {
gUM = false;
return;
}其中 setMask 为绘制遮罩层方法，qrcodeScanLoad 为初始化加载方法。
<body>
<div class="body">
<div id="mainbody" style="display: inline;">
<div id="outdiv" autoplay muted></div>
</div>
<canvas id="qr-canvas" width="800" height="600"></canvas>
<canvas id="scancode-mask"></canvas>
<div class="scancode-tips-group" id="scancode-tips-group">
<span class="tips">將QR Code 放入框内，即可自動掃描</span>
<div class="upload-my-code" onClick="setimg()">我的QR Code</div>
</div>
<div id="img-upload-container" style="display: none">
<div id="qrfile">
<canvas id="out-canvas" width="320" height="240"></canvas>
<div id="imghelp">
drag and drop a QRCode here
<br />or select a file
<input
type="file"
onchange="handleFiles(this.files)"
id="upload-img"
/>
</div>
</div>
</div>
</div>
<script src="./llqrcode.js"></script>
<script src="./variable.js"></script>
<script src="./methods.js"></script>
<script>
qrcodeScanLoad(320, 400);
setMask();
// 对我的 QR Code 进行定位显示
if (document.body.clientWidth < 1025) {
document.getElementById("scancode-tips-group").style.top =
(document.body.clientHeight - document.body.clientWidth) / 2 +
document.body.clientWidth * 0.9 -
10 +
"px";
} else {
document.getElementById("scancode-tips-group").style.top = "720px";
}
</script>
</body>这里做了适配，当超过 1024 时，canvas 宽度就设为 1024。详细在代码中已经做了注释。
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