canvas实现签名（保存图片、改变字体颜色、线条粗细）

一、 前言

• 很久以前公司有实现过类似功能，最近又有一个场景使用到，特写此篇，以做记录；
• 文章使用纯原生实现，我在公司vue项目中替换为了vue相关语法，其他语法也可自行扩展；
• 其中包含改变颜色和线条粗细相关，需要其他功能或者剔除相关功能，直接注释即可；
• 各功能间关联性不强，如有使用，可根据业务场景自行调整。

二、代码注释详解

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

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    * {
        margin: 0;
        padding: 0;
    }

    #canvas {
        border: 1px solid black;
    }
</style>

<body>
    <div class="container">
        <canvas id="canvas" width="500" height="500"></canvas>
        <div>
            <button id="clear">清空画布</button>
            线条粗细
            <select id="selWidth">
                <option value="2">2</option>
                <option value="4">4</option>
                <option value="6">6</option>
                <option value="9">9</option>
            </select>
            线条颜色
            <select id="selColor">
                <option value="red">red</option>
                <option value="blue">blue</option>
                <option value="pink">pink</option>
                <option value="orange">orange</option>
            </select>
            <button id="imgInfo">保存签名</button>
        </div>

        <div class="imgs" id="imgs"></div>
    </div>
</body>
<script>
    //1.获取canvas
    var myCanvas = document.getElementById("canvas");
    //获取2d对象
    var ctx = myCanvas.getContext("2d");
    //清空画布
    var clear = document.getElementById("clear");
    //线条
    var selWidth = document.getElementById("selWidth");
    // 颜色
    var selColor = document.getElementById("selColor");
    // 保存签名
    var imgInfo = document.getElementById("imgInfo");
    // 保存的盒子
    var imgs = document.getElementById("imgs");
    //控制线条是否画
    var isMouseMove = false;
    //线条位置
    var lastX, lastY;
    //线条样式
    var widthVal = selWidth[0].value, colorVal = selColor[0].value;
    //监听鼠标按下事件
    window.onload = function () {
        initCanvas();
    };
    //初始化
    function initCanvas() {
        //按下
        var down = (e) => {
            isMouseMove = true;
            drawLine(
                event.pageX - myCanvas.offsetLeft,
                event.pageY - myCanvas.offsetTop,
                false
            );
        };
        //移动
        let move = (e) => {
            if (isMouseMove) {
                drawLine(
                    event.pageX - myCanvas.offsetLeft,
                    event.pageY - myCanvas.offsetTop,
                    true
                );
            }
        };
        //松开
        let up = (e) => {
            isMouseMove = false;
        };
        //离开
        let leave = (e) => {
            isMouseMove = false;
        };
        //监听事件根据动作画线
        myCanvas.addEventListener("mousedown", down);
        myCanvas.addEventListener("mousemove", move);
        myCanvas.addEventListener("mouseup", up);
        myCanvas.addEventListener("mouseleave", leave);
    }
    //画线
    function drawLine(x, y, isT) {
        if (isT) {
            ctx.beginPath();//开始绘制
            ctx.lineWidth = widthVal; //设置线宽状态
            ctx.strokeStyle = colorVal; //设置线的颜色状态
            ctx.lineCap = 'round'
            ctx.lineJoin = "round";
            ctx.moveTo(lastX, lastY);
            ctx.lineTo(x, y);
            ctx.stroke();
            ctx.closePath();
        }
        // 每次移动都要更新坐标位置
        lastX = x;
        lastY = y;
    }

    //清空画图
    function clearCanvas() {
        imgs.innerHTML = ""
        ctx.beginPath();
        ctx.clearRect(0, 0, myCanvas.width, myCanvas.height);
        ctx.closePath(); //可加入，可不加入
    }
    //线条粗细
    function lineCrude() {
        let activeIndex = selWidth.selectedIndex;
        widthVal = selWidth[activeIndex].value;
    }
    //改变颜色
    function setColor() {
        let activeIndex = selColor.selectedIndex;
        colorVal = selColor[activeIndex].value;
    }
    //保存图片
    function saveImgInfo() {
        var images = myCanvas.toDataURL('image/png');
        console.log(images, '图片base64');
        imgs.innerHTML = `<img src='${images}'>`
    }
    // 清除
    clear.addEventListener("click", clearCanvas);
    // 线条粗细
    selWidth.addEventListener("change", lineCrude);
    // 线条颜色
    selColor.addEventListener("change", setColor);
    // 保存图片
    imgInfo.addEventListener("click", saveImgInfo);
</script>

</html>