uni-app 微信小程序中如何通过 canvas 画布实现电子签名？

一、实际应用场景

电子签名软件应用场景：电子签名在金融、银行、贷款行业中可以用于对内日常办公流转的文档的盖章签字，对外涉及业务合作协议，采购合同，贷款申请、信用评估、贷款合同、贷款文件表、说明函等等。

可以说，只要是涉及纸质文档签字盖章的场景，都能够使用电子印章，电子签名。

二、实现方式

利用 canvas 画布，模拟人工在纸张上手写的笔迹。原理其实很简单，就是你在画布上任意涂写即可，最后保存整个画布区域的内容为图片。

三、案例 demo 分享

这里我加了几个操作按钮，应该能满足您基本的数字签名需要了，最后我也会贴上我的源码。

 ps：微信小程序关于画布的官方文档

https://developers.weixin.qq.com/miniprogram/dev/framework/ability/canvas.html

1、先在 view 视图中添加 canvas 组件。

<view class="handCenter">
				<canvas class="handWriting" :disable-scroll="true" @touchstart="uploadScaleStart" @touchmove="uploadScaleMove"
				 @touchend="uploadScaleEnd" canvas-id="handWriting"></canvas>
</view>

canvas-id="handWriting" 是唯一标识一个 canvas，用于后续获取 Canvas 对象。

2、页面加载时，初始化 canvas

onLoad() {
			let canvasName = this.canvasName;
			let ctx = wx.createCanvasContext(canvasName);

			this.ctx = ctx;
			var query = wx.createSelectorQuery();
			query
				.select('.handCenter')
				.boundingClientRect(rect => {
					this.canvasWidth = rect.width;
					this.canvasHeight = rect.height;

					/* 将canvas背景设置为 白底，不设置  导出的canvas的背景为透明 */
					this.setCanvasBg('rgba(0,0,0,0)');
				})
				.exec();
		},

3、笔迹开始，移动，结束，还有底部预览，重写，保存，上传等附加操作按钮。

完成页面代码如下，包含我的效果图样式。

<template>
	<view>
		<view class="wrapper">
			<view class="handBtn">
				<image @click="selectColorEvent('black','#1A1A1A')" :src="selectColor === 'black' ? '/static/img/sign/color_black_selected.png' : '/static/img/sign/color_black.png'"
				 :class="[selectColor === 'black' ? 'color_select' : '', 'black-select']"></image>
				<image @click="selectColorEvent('red','#ca262a')" :src="selectColor === 'red' ? '/static/img/sign/color_red_selected.png' : '/static/img/sign/color_red.png'"
				 :class="[selectColor === 'red' ? 'color_select' : '', 'black-select']"></image>
				<button @click="retDraw" class="delBtn">重写</button>
				<button @click="saveCanvasAsImg" class="saveBtn">保存</button>
				<button @click="previewCanvasImg" class="previewBtn">预览</button>
				<button @click="uploadCanvasImg" class="uploadBtn">上传</button>
				<button @click="subCanvas" class="subBtn">完成</button>
				<button @click="back" class="subBtn">返回</button>
			</view>
			<view class="handCenter">
				<canvas class="handWriting" :disable-scroll="true" @touchstart="uploadScaleStart" @touchmove="uploadScaleMove"
				 @touchend="uploadScaleEnd" canvas-id="handWriting"></canvas>
			</view>
			<view class="handRight">
				<view class="handTitle">请签名</view>
			</view>
		</view>
	</view>
</template>

<script>
	export default {
		data() {
			return {
				canvasName: 'handWriting',
				ctx: '',
				canvasWidth: 0,
				canvasHeight: 0,
				transparent: 1, // 透明度
				selectColor: 'black',
				lineColor: '#1A1A1A', // 颜色
				lineSize: 1.5, // 笔记倍数
				lineMin: 0.5, // 最小笔画半径
				lineMax: 4, // 最大笔画半径
				pressure: 1, // 默认压力
				smoothness: 60, //顺滑度，用60的距离来计算速度
				currentPoint: {},
				currentLine: [], // 当前线条
				firstTouch: true, // 第一次触发
				radius: 1, //画圆的半径
				cutArea: {
					top: 0,
					right: 0,
					bottom: 0,
					left: 0
				}, //裁剪区域
				bethelPoint: [], //保存所有线条 生成的贝塞尔点；
				lastPoint: 0,
				chirography: [], //笔迹
				currentChirography: {}, //当前笔迹
				linePrack: [] //划线轨迹 , 生成线条的实际点
			};
		},
		onLoad() {
			let canvasName = this.canvasName;
			let ctx = wx.createCanvasContext(canvasName);

			this.ctx = ctx;
			var query = wx.createSelectorQuery();
			query
				.select('.handCenter')
				.boundingClientRect(rect => {
					this.canvasWidth = rect.width;
					this.canvasHeight = rect.height;

					/* 将canvas背景设置为 白底，不设置  导出的canvas的背景为透明 */
					this.setCanvasBg('rgba(0,0,0,0)');
				})
				.exec();
		},
		methods: {
			// 笔迹开始
			uploadScaleStart(e) {
				if (e.type != 'touchstart') return false;
				let ctx = this.ctx;
				ctx.setFillStyle(this.lineColor); // 初始线条设置颜色
				ctx.setGlobalAlpha(this.transparent); // 设置半透明
				let currentPoint = {
					x: e.touches[0].x,
					y: e.touches[0].y
				};
				let currentLine = this.currentLine;
				currentLine.unshift({
					time: new Date().getTime(),
					dis: 0,
					x: currentPoint.x,
					y: currentPoint.y
				});
				this.currentPoint = currentPoint;
				// currentLine
				if (this.firstTouch) {
					this.cutArea = {
						top: currentPoint.y,
						right: currentPoint.x,
						bottom: currentPoint.y,
						left: currentPoint.x
					};
					this.firstTouch = false;
				}
				this.pointToLine(currentLine);
			},
			// 笔迹移动
			uploadScaleMove(e) {
				if (e.type != 'touchmove') return false;
				if (e.cancelable) {
					// 判断默认行为是否已经被禁用
					if (!e.defaultPrevented) {
						e.preventDefault();
					}
				}
				let point = {
					x: e.touches[0].x,
					y: e.touches[0].y
				};

				//测试裁剪
				if (point.y < this.cutArea.top) {
					this.cutArea.top = point.y;
				}
				if (point.y < 0) this.cutArea.top = 0;

				if (point.x > this.cutArea.right) {
					this.cutArea.right = point.x;
				}
				if (this.canvasWidth - point.x <= 0) {
					this.cutArea.right = this.canvasWidth;
				}
				if (point.y > this.cutArea.bottom) {
					this.cutArea.bottom = point.y;
				}
				if (this.canvasHeight - point.y <= 0) {
					this.cutArea.bottom = this.canvasHeight;
				}
				if (point.x < this.cutArea.left) {
					this.cutArea.left = point.x;
				}
				if (point.x < 0) this.cutArea.left = 0;

				this.lastPoint = this.currentPoint;
				this.currentPoint = point;

				let currentLine = this.currentLine;
				currentLine.unshift({
					time: new Date().getTime(),
					dis: this.distance(this.currentPoint, this.lastPoint),
					x: point.x,
					y: point.y
				});

				this.pointToLine(currentLine);
			},
			// 笔迹结束
			uploadScaleEnd(e) {
				if (e.type != 'touchend') return 0;
				let point = {
					x: e.changedTouches[0].x,
					y: e.changedTouches[0].y
				};
				this.lastPoint = this.currentPoint;
				this.currentPoint = point;

				let currentLine = this.currentLine;
				currentLine.unshift({
					time: new Date().getTime(),
					dis: this.distance(this.currentPoint, this.lastPoint),
					x: point.x,
					y: point.y
				});

				if (currentLine.length > 2) {
					var info = (currentLine[0].time - currentLine[currentLine.length - 1].time) / currentLine.length;
					//$("#info").text(info.toFixed(2));
				}
				//一笔结束，保存笔迹的坐标点，清空，当前笔迹
				//增加判断是否在手写区域；
				this.pointToLine(currentLine);
				var currentChirography = {
					lineSize: this.lineSize,
					lineColor: this.lineColor
				};
				var chirography = this.chirography;
				chirography.unshift(currentChirography);
				this.chirography = chirography;

				var linePrack = this.linePrack;
				linePrack.unshift(this.currentLine);
				this.linePrack = linePrack;
				this.currentLine = [];
			},
			retDraw() {
				this.ctx.clearRect(0, 0, 700, 730);
				this.ctx.draw();

				//设置canvas背景
				this.setCanvasBg('rgba(0,0,0,0)');
			},
			//画两点之间的线条；参数为:line，会绘制最近的开始的两个点；
			pointToLine(line) {
				this.calcBethelLine(line);
				return;
			},
			//计算插值的方式；
			calcBethelLine(line) {
				if (line.length <= 1) {
					line[0].r = this.radius;
					return;
				}
				let x0,
					x1,
					x2,
					y0,
					y1,
					y2,
					r0,
					r1,
					r2,
					len,
					lastRadius,
					dis = 0,
					time = 0,
					curveValue = 0.5;
				if (line.length <= 2) {
					x0 = line[1].x;
					y0 = line[1].y;
					x2 = line[1].x + (line[0].x - line[1].x) * curveValue;
					y2 = line[1].y + (line[0].y - line[1].y) * curveValue;
					//x2 = line[1].x;
					//y2 = line[1].y;
					x1 = x0 + (x2 - x0) * curveValue;
					y1 = y0 + (y2 - y0) * curveValue;
				} else {
					x0 = line[2].x + (line[1].x - line[2].x) * curveValue;
					y0 = line[2].y + (line[1].y - line[2].y) * curveValue;
					x1 = line[1].x;
					y1 = line[1].y;
					x2 = x1 + (line[0].x - x1) * curveValue;
					y2 = y1 + (line[0].y - y1) * curveValue;
				}
				//从计算公式看，三个点分别是(x0,y0),(x1,y1),(x2,y2) ；(x1,y1)这个是控制点，控制点不会落在曲线上；实际上，这个点还会手写获取的实际点，却落在曲线上
				len = this.distance({
					x: x2,
					y: y2
				}, {
					x: x0,
					y: y0
				});
				lastRadius = this.radius;
				for (let n = 0; n < line.length - 1; n++) {
					dis += line[n].dis;
					time += line[n].time - line[n + 1].time;
					if (dis > this.smoothness) break;
				}

				this.radius = Math.min((time / len) * this.pressure + this.lineMin, this.lineMax) * this.lineSize;
				line[0].r = this.radius;
				//计算笔迹半径；
				if (line.length <= 2) {
					r0 = (lastRadius + this.radius) / 2;
					r1 = r0;
					r2 = r1;
					//return;
				} else {
					r0 = (line[2].r + line[1].r) / 2;
					r1 = line[1].r;
					r2 = (line[1].r + line[0].r) / 2;
				}
				let n = 5;
				let point = [];
				for (let i = 0; i < n; i++) {
					let t = i / (n - 1);
					let x = (1 - t) * (1 - t) * x0 + 2 * t * (1 - t) * x1 + t * t * x2;
					let y = (1 - t) * (1 - t) * y0 + 2 * t * (1 - t) * y1 + t * t * y2;
					let r = lastRadius + ((this.radius - lastRadius) / n) * i;
					point.push({
						x: x,
						y: y,
						r: r
					});
					if (point.length == 3) {
						let a = this.ctaCalc(point[0].x, point[0].y, point[0].r, point[1].x, point[1].y, point[1].r, point[2].x, point[2]
							.y, point[2].r);
						a[0].color = this.lineColor;
						// let bethelPoint = this.bethelPoint;
						// bethelPoint = bethelPoint.push(a);
						this.bethelDraw(a, 1);
						point = [{
							x: x,
							y: y,
							r: r
						}];
					}
				}
				this.currentLine = line;
			},
			//求两点之间距离
			distance(a, b) {
				let x = b.x - a.x;
				let y = b.y - a.y;
				return Math.sqrt(x * x + y * y);
			},
			ctaCalc(x0, y0, r0, x1, y1, r1, x2, y2, r2) {
				let a = [],
					vx01,
					vy01,
					norm,
					n_x0,
					n_y0,
					vx21,
					vy21,
					n_x2,
					n_y2;
				vx01 = x1 - x0;
				vy01 = y1 - y0;
				norm = Math.sqrt(vx01 * vx01 + vy01 * vy01 + 0.0001) * 2;
				vx01 = (vx01 / norm) * r0;
				vy01 = (vy01 / norm) * r0;
				n_x0 = vy01;
				n_y0 = -vx01;
				vx21 = x1 - x2;
				vy21 = y1 - y2;
				norm = Math.sqrt(vx21 * vx21 + vy21 * vy21 + 0.0001) * 2;
				vx21 = (vx21 / norm) * r2;
				vy21 = (vy21 / norm) * r2;
				n_x2 = -vy21;
				n_y2 = vx21;
				a.push({
					mx: x0 + n_x0,
					my: y0 + n_y0,
					color: '#1A1A1A'
				});
				a.push({
					c1x: x1 + n_x0,
					c1y: y1 + n_y0,
					c2x: x1 + n_x2,
					c2y: y1 + n_y2,
					ex: x2 + n_x2,
					ey: y2 + n_y2
				});
				a.push({
					c1x: x2 + n_x2 - vx21,
					c1y: y2 + n_y2 - vy21,
					c2x: x2 - n_x2 - vx21,
					c2y: y2 - n_y2 - vy21,
					ex: x2 - n_x2,
					ey: y2 - n_y2
				});
				a.push({
					c1x: x1 - n_x2,
					c1y: y1 - n_y2,
					c2x: x1 - n_x0,
					c2y: y1 - n_y0,
					ex: x0 - n_x0,
					ey: y0 - n_y0
				});
				a.push({
					c1x: x0 - n_x0 - vx01,
					c1y: y0 - n_y0 - vy01,
					c2x: x0 + n_x0 - vx01,
					c2y: y0 + n_y0 - vy01,
					ex: x0 + n_x0,
					ey: y0 + n_y0
				});
				a[0].mx = a[0].mx.toFixed(1);
				a[0].mx = parseFloat(a[0].mx);
				a[0].my = a[0].my.toFixed(1);
				a[0].my = parseFloat(a[0].my);
				for (let i = 1; i < a.length; i++) {
					a[i].c1x = a[i].c1x.toFixed(1);
					a[i].c1x = parseFloat(a[i].c1x);
					a[i].c1y = a[i].c1y.toFixed(1);
					a[i].c1y = parseFloat(a[i].c1y);
					a[i].c2x = a[i].c2x.toFixed(1);
					a[i].c2x = parseFloat(a[i].c2x);
					a[i].c2y = a[i].c2y.toFixed(1);
					a[i].c2y = parseFloat(a[i].c2y);
					a[i].ex = a[i].ex.toFixed(1);
					a[i].ex = parseFloat(a[i].ex);
					a[i].ey = a[i].ey.toFixed(1);
					a[i].ey = parseFloat(a[i].ey);
				}
				return a;
			},
			bethelDraw(point, is_fill, color) {
				let ctx = this.ctx;
				ctx.beginPath();
				ctx.moveTo(point[0].mx, point[0].my);
				if (undefined != color) {
					ctx.setFillStyle(color);
					ctx.setStrokeStyle(color);
				} else {
					ctx.setFillStyle(point[0].color);
					ctx.setStrokeStyle(point[0].color);
				}
				for (let i = 1; i < point.length; i++) {
					ctx.bezierCurveTo(point[i].c1x, point[i].c1y, point[i].c2x, point[i].c2y, point[i].ex, point[i].ey);
				}
				ctx.stroke();
				if (undefined != is_fill) {
					ctx.fill(); //填充图形 ( 后绘制的图形会覆盖前面的图形, 绘制时注意先后顺序 )
				}
				ctx.draw(true);
			},
			selectColorEvent(str, color) {
				this.selectColor = str;
				this.lineColor = color;
			},
			//将Canvas内容转成 临时图片 --> cb 为回调函数 形参 tempImgPath 为 生成的图片临时路径
			canvasToImg(cb) {
				//这种写法移动端 出不来
				this.ctx.draw(true, () => {
					wx.canvasToTempFilePath({
						canvasId: 'handWriting',
						fileType: 'png',
						quality: 1, //图片质量
						success(res) {
							// console.log(res.tempFilePath, 'canvas生成图片地址');

							wx.showToast({
								title: '执行了吗？'
							});

							cb(res.tempFilePath);
						}
					});
				});
			},
			//完成
			subCanvas() {
				this.ctx.draw(true, () => {
					wx.canvasToTempFilePath({
						canvasId: 'handWriting',
						fileType: 'png',
						quality: 1, //图片质量
						success(res) {
							// console.log(res.tempFilePath, '已完成，canvas生成图片地址');
							// wx.showToast({
							// 	title: '已保存'
							// });
							//保存到系统相册
							wx.saveImageToPhotosAlbum({
								filePath: res.tempFilePath,
								success(res) {
									console.log("已完成并保存到相册");
									wx.showToast({
										title: '已成功保存到相册',
										duration: 2000
									});
								}
							});
						}
					});
				});
			},
			//保存到相册
			saveCanvasAsImg() {
				/*
				this.canvasToImg( tempImgPath=>{
					// console.log(tempImgPath, '临时路径');
					wx.saveImageToPhotosAlbum({
						filePath: tempImgPath,
						success(res) {
							wx.showToast({
								title: '已保存到相册',
								duration: 2000
							});
						}
					})
				} );
		*/

				wx.canvasToTempFilePath({
					canvasId: 'handWriting',
					fileType: 'png',
					quality: 1, //图片质量
					success(res) {
						// console.log(res.tempFilePath, 'canvas生成图片地址');
						wx.saveImageToPhotosAlbum({
							filePath: res.tempFilePath,
							success(res) {
								wx.showToast({
									title: '已保存到相册',
									duration: 2000
								});
							}
						});
					}
				});
			},
			//预览
			previewCanvasImg() {
				wx.canvasToTempFilePath({
					canvasId: 'handWriting',
					fileType: 'jpg',
					quality: 1, //图片质量
					success(res) {
						// console.log(res.tempFilePath, 'canvas生成图片地址');
						wx.previewImage({
							urls: [res.tempFilePath] //预览图片 数组
						});
					}
				});

				/*	//移动端出不来  ^~^！！
						this.canvasToImg( tempImgPath=>{
							wx.previewImage({
								urls: [tempImgPath], //预览图片 数组
							})
						} );
				*/
			},
			//上传
			uploadCanvasImg() {
				wx.canvasToTempFilePath({
					canvasId: 'handWriting',
					fileType: 'png',
					quality: 1, //图片质量
					success(res) {
						console.log(res.tempFilePath, '待上传服务器canvas生成图片地址');
						//上传
						wx.uploadFile({
							url: 'https://xxx/uploadSignFile', // 仅为示例，非真实的接口地址
							filePath: res.tempFilePath,
							name: 'file_signature',
							formData: {
								user: 'test'
							},
							success(res) {
								if (res.code == 1 ) {
									wx.showToast({
										title: '上传成功',
										duration: 2000
									});
								}else{
									uni.showToast({
										title: res.message,
										icon: "none",
										position: 'bottom',
										duration: 3000
									});
								}
							}
						});
					}
				});
			},
			//设置canvas背景色  不设置  导出的canvas的背景为黑色
			//@params：字符串  color
			setCanvasBg(color) {
				/* 将canvas背景设置为 白底，不设置  导出的canvas的背景默认为黑色 */
				//rect() 参数说明  矩形路径左上角的横坐标，左上角的纵坐标, 矩形路径的宽度, 矩形路径的高度
				//这里是 canvasHeight - 4 是因为下边盖住边框了，所以手动减了写
				this.ctx.rect(0, 0, this.canvasWidth, this.canvasHeight - 4);
				this.ctx.setFillStyle(color);
				this.ctx.fill(); //设置填充
				this.ctx.draw(); //开画
			},
			//返回
			back(){
				uni.reLaunch({
					url:'/pages/ucenter/ucenter'
				})
			}
		}
	};
</script>

<style>
	page {
		background: #fbfbfb;
		height: auto;
		overflow: hidden;
	}

	.wrapper {
		width: 100%;
		height: 95vh;
		margin: 30rpx 0;
		overflow: hidden;
		display: flex;
		align-content: center;
		flex-direction: row;
		justify-content: center;
		font-size: 28rpx;
	}

	.handWriting {
		background: #fff;
		width: 100%;
		height: 95vh;
	}

	.handRight {
		display: inline-flex;
		align-items: center;
	}

	.handCenter {
		border: 4rpx dashed #e9e9e9;
		flex: 5;
		overflow: hidden;
		box-sizing: border-box;
	}

	.handTitle {
		transform: rotate(90deg);
		flex: 1;
		color: #666;
	}

	.handBtn button {
		font-size: 28rpx;
	}

	.handBtn {
		height: 95vh;
		display: inline-flex;
		flex-direction: column;
		justify-content: space-between;
		align-content: space-between;
		flex: 1;
	}

	.delBtn {
		position: absolute;
		top: 250rpx;
		left: 0rpx;
		transform: rotate(90deg);
		color: #666;
	}

	.delBtn image {
		position: absolute;
		top: 13rpx;
		left: 25rpx;
	}

	.subBtn {
		position: absolute;
		bottom: 52rpx;
		left: -3rpx;
		display: inline-flex;
		transform: rotate(90deg);
		background: #008ef6;
		color: #fff;
		margin-bottom: 30rpx;
		text-align: center;
		justify-content: center;
	}

	/*Peach - 新增 - 保存*/

	.saveBtn {
		position: absolute;
		top: 375rpx;
		left: 0rpx;
		transform: rotate(90deg);
		color: #666;
	}

	.previewBtn {
		position: absolute;
		top: 500rpx;
		left: 0rpx;
		transform: rotate(90deg);
		color: #666;
	}

	.uploadBtn {
		position: absolute;
		top: 625rpx;
		left: 0rpx;
		transform: rotate(90deg);
		color: #666;
	}

	/*Peach - 新增 - 保存*/

	.black-select {
		width: 60rpx;
		height: 60rpx;
		position: absolute;
		top: 30rpx;
		left: 25rpx;
	}

	.black-select.color_select {
		width: 90rpx;
		height: 90rpx;
		top: 100rpx;
		left: 10rpx;
	}

	.red-select {
		width: 60rpx;
		height: 60rpx;
		position: absolute;
		top: 140rpx;
		left: 25rpx;
	}

	.red-select.color_select {
		width: 90rpx;
		height: 90rpx;
		top: 120rpx;
		left: 10rpx;
	}
</style>

代码中都有注释说明，希望能帮助到需要的小伙伴。具体api细节 不懂的还请自行查阅官方文档说明。

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