JS实现移动端在线签协议功能

在一个风和日丽的下午，刚准备下班，突然接到需求说要做一个在线签协议功能，当时心里想着不就百度一顿拷贝就完事了吗(因为我没用过canvas，所谓初生牛犊不怕虎 )，谁知做起来如此吃力，下面就来记录下历程。

协议模板

分析

如上图，需要做的就是做一个签字板可以在上面写字，写完后点击完成可以生成如上图的图片所示，把签好的字放到指定的位置。

做这个第一反应肯定就是使用canvas绘制路径

我的思路是:

一个字一个字写，每写一个字点一下记录，最后拼接，但想到用户体验问题就pass了这个思路。

最后的思路：一行可以写很多个字，可以让用户滑动canvas，一直写下去(因为协议模板最后还要抄写一段话)

canvas绘制路径--实现签名功能

<canvas id="canvas" style="top:0">您的手机不支持在线签署</canvas>

const canvasPaint = {};//定义一个全局对象，把canvas的各种状态存进去
canvasPaint.canvas = document.getElementById("canvas");
canvasPaint.ctx = document.getElementById("canvas").getContext("2d");
canvasPaint.ctx.lineCap = 'round';//让结束线帽呈现圆滑状
canvasPaint.ctx.lineJoin = 'round';//交汇时呈现圆滑状
canvasPaint.ctx.strokeWidth = 5;//描边宽度
canvasPaint.ctx.lineWidth = 5;//线条宽度

初始化好画布后，我们需要监听画布上的滑动事件

canvasPaint.canvas.addEventListener('touchstart', startEventHandler, {passive: false});
function startEventHandler(event) {
 event.preventDefault();
 canvasPaint.ctx.beginPath();//每次都是一个新路径,不写会和上个字的最后一笔连起来
 canvasPaint.canvas.addEventListener('touchmove', moveEventHandler, {passive: false});
 canvasPaint.canvas.addEventListener('touchend', endEventHandler, {passive: false});
}

passive: false 和 event.preventDefault() 这两个是绝配哦， event.preventDefault() 阻止默认行为，防止在画布上写字时触发了浏览器自带的下拉动作之类的。那 passive: false 是谷歌56版本后提出的新属性，设置为 false 就是告诉浏览器我有阻止默认行为的代码，刚开始不要给我滑动，你需要执行我的 event.preventDefault() 这句代码，如果设置为了 true ,浏览器会自动忽略这句代码，从而不能阻止成功，默认是 true ，所以这里就是坑之一了。

我们继续编写移动划线逻辑

function moveEventHandler(event) {
 event.preventDefault();
 var coverPos = canvasPaint.canvas.getBoundingClientRect();
 canvasPaint.mouseX = event.clientX - coverPos.left;
 canvasPaint.mouseY = event.clientY - coverPos.top;
 if (canvasPaint.canPaint) {//后续为拖动画布功能设置的状态
 canvasPaint.ctx.lineTo(//使用lineTo将移动过的坐标绘制成线
 canvasPaint.mouseX,
 canvasPaint.mouseY
 );
 canvasPaint.ctx.stroke();//绘制
 }
}
function endEventHandler(event) {
 event.preventDefault();
 //抬起手指时取消move和end事件的监听
 canvasPaint.canvas.removeEventListener('touchmove', moveEventHandler, false);
 canvasPaint.canvas.removeEventListener('touchend', endEventHandler, false);
}

canvas--清除屏幕功能

这个功能比较简单就一句话

function clearCanvas() {
 canvasPaint.ctx.clearRect(0, 0, canvasPaint.canvas.width, canvasPaint.canvas.height);
}

提交签名功能

首先需要将画布上的文字转换为img对象，然后使用drawImage绘制到协议上去

preLoadImg(['/assets/index/images/agree.jpg', canvasPaint.canvas.toDataURL()], result);
//agree.jpg为协议名，canvasPaint.canvas.toDataURL()就是签好的字转换为base64的结果
function preLoadImg(source, callBack, args) {
 var pr = [];
 source.forEach(url => {
 var p = loadImage(url)
 .then(function (img) {
 return img;
 })
 .catch(function (err) {
 console.log(err);
 });
 pr.push(p);
 });

 Promise.all(pr)
 .then(function (imgArray) {
 callBack(imgArray, args);
 });

}
function loadImage(url) {
 return new Promise((resolve, reject) => {
 var img = new Image();
 img.onload = function () {
 resolve(img);
 };
 img.onerror = reject;
 img.src = url;
 });
}

由于img赋值src是异步的，我们必须要一个完整的image对象，所以我们使用promise包装，使得我们所有图片都转换完之后再将结果传入回调函数(result)中

function result(imgArr) {
 drawName(imgArr);
}
function drawName(imgArr) {
 //绘制名字和底部的名字和日期
 canvasPaint.canvas2 = document.getElementById('canvas2');
 canvasPaint.context2 = canvasPaint.canvas2.getContext('2d');
 canvasPaint.ratio = canvasPaint.canvas.height / canvasPaint.canvas.width; //计算画布比例
 canvasPaint.context2.drawImage(imgArr[0], 0, 0, 500, 707);//img0是底图原协议
 canvasPaint.context2.save();
 canvasPaint.context2.translate(50, 190);
 canvasPaint.context2.rotate(270 * Math.PI / 180);
 canvasPaint.context2.drawImage(imgArr[1], 80, 50, 33, 33 * canvasPaint.ratio);//画反转后的名字
 canvasPaint.context2.restore();
 canvasPaint.context2.save();
 canvasPaint.context2.translate(67, 723);//下方的字
 canvasPaint.context2.rotate(270 * Math.PI / 180);
 canvasPaint.context2.drawImage(imgArr[1], 80, 50, 33, 33 * canvasPaint.ratio);//画反转后的名字
 canvasPaint.context2.restore();
 canvasPaint.context2.save();
 canvasPaint.context2.translate(400, 625);//下方的字
 canvasPaint.context2.font = "11px 微软雅黑";
 canvasPaint.context2.fillStyle = "#000";
 canvasPaint.context2.textAlign = "center";
 canvasPaint.context2.textBaseline = "middle";
 var time = new Date().toLocaleString().split(' ')[0];
 canvasPaint.context2.fillText(time, 0, 0);
 canvasPaint.context2.restore();
 prevDrawStatement();
}

这里最主要的还是要理解下画布的rotate和translate方法，就可以把文字旋转任意角度和放到任意位置了

长字手写--画布拖动

上面签字完成后，我们其实已经用了另一个canvas合成了文字和原协议，现在我们要做无限拖动功能，其实也很简单。

在此之前我们需要清空之前的画布

function prevDrawStatement() {
 clearCanvas();//清除画布
 canvasPaint.finish.innerHTML = "提交抄写";
 canvasPaint.pencilBtn.style.display = 'block';
 canvasPaint.secondState.style.display = 'block';
 canvasPaint.tips.innerHTML = "(最后一步)请抄写屏幕上方引号内的确认语句";
 canvasPaint.tips.style.color = 'red';
 setTimeout(function () {
 canvasPaint.tips.style.color = '#666';
 }, 2000);
 state = STATEMENT;//开始写句子
}

右上角有个移动签字板功能，这里实现的是左右移动，相关代码如下

function togglePencil() {
 if (canvasPaint.canPaint) {
 canvasPaint.canPaint = false;
 canvasPaint.pencilBtn.innerText = "使用签字笔";
 //不能签字时应该把开始写字事件去掉，同时加上document事件
 canvasPaint.canvas.removeEventListener('touchstart', startEventHandler, false);
 document.addEventListener('touchstart', documentStartEventHandler, {passive: false});
 } else {
 canvasPaint.canPaint = true;
 canvasPaint.pencilBtn.innerText = "移动签字板";
 //能签字时应该把开始写字事件绑定上去，同时去掉document事件
 canvasPaint.canvas.addEventListener('touchstart', startEventHandler, {passive: false});
 document.removeEventListener('touchstart', documentStartEventHandler, false);
 }
}
function documentStartEventHandler(event) {
 event.preventDefault();
 canvasPaint.y = event.clientY;
 canvasPaint.top = parseFloat(canvasPaint.canvas.style.top);//画板距离顶部的值
 document.addEventListener('touchmove', documentMoveEventHandler, {passive: false});
 document.addEventListener('touchend', documentEndEventHandler, {passive: false});
}
function documentMoveEventHandler(event) {
 event.preventDefault();
 canvasPaint.newY = event.clientY - canvasPaint.y;
 if (!canvasPaint.canPaint) {
 canvasPaint.canvas.style.top = canvasPaint.newY + canvasPaint.top + 'px';
 if (parseFloat(canvasPaint.canvas.style.top) > 0) {//限制边界
  canvasPaint.canvas.style.top = 0 + 'px';
 }
 }
}

function documentEndEventHandler(event) {
 event.preventDefault();
}

合成长句到协议中并显示最终图片

提交抄写按钮点击后执行下面的函数

function statementDraw(imgArr) {
 canvasPaint.context2.save();
 canvasPaint.context2.translate(52, 690);
 canvasPaint.context2.rotate(270 * Math.PI / 180);
 canvasPaint.context2.drawImage(imgArr[0], 80, 50, 33, 33 * canvasPaint.ratio);//画反转后的名字
 canvasPaint.context2.restore();
 console.log(canvasPaint.canvas2.toDataURL());
 document.getElementById('resultImg').setAttribute('src', canvasPaint.canvas2.toDataURL());
 document.getElementById('resultImg').style.position = 'absolute';
 document.getElementById('resultImg').style.left = 0;
 document.getElementById('resultImg').style.top = 0;
 document.getElementById('resultImg').style.zIndex = 50;
}