微信小程序实现手写签名的示例代码

在微信小程序上实现手写签名，获取canvascontext新版本和旧版本有点坑，新版本在获取canvas后如果页面有滑动，则签名坐标出现异常(在微信开发者工具上会出现2022-2-17)，但是在真机上即使滑动也不会出现异常，为了防止出现问题，暂时使用旧版本获取canvascontext

1.效果图

2.相关代码

canvas代码

新版2d canvas

<canvas
  id="canvas"
  class="canvas"
  canvas-id="canvas"
  type="2d"
  :disable-scroll="true"
  @touchstart="handleTouchStart"
  @touchmove="handleTouchMove"
  @touchend="handleTouchEnd"
  @touchcancel="handleTouchCancel"
></canvas>

旧版canvas

<canvas
  class="canvas"
  canvas-id="canvas"
  :disable-scroll="true"
  @touchstart="handleTouchStart"
  @touchmove="handleTouchMove"
  @touchend="handleTouchEnd"
  @touchcancel="handleTouchCancel"
></canvas>

js相关

获取新版2d canvas对象

const query = uni.createSelectorQuery().in(this);
query.select('.canvas').node(res => {
  const {
      _width,
      _height
  } = res.node;
  
  /* 获取canvas wxml节点 */
  this.canvas = res.node;
  this.canvasWidth = _width;
  this.canvasHeight = _height;
  /* 获取canvas 2dcontext */
  this.canvasContext= this.canvas.getContext('2d');
  
  /* 缩放设置canvas画布大小，防止笔迹错位 */
  const ratio = wx.getSystemInfoSync().pixelRatio;
  this.canvas.width = this.canvasWidth * ratio;
  this.canvas.height = this.canvasHeight * ratio;
  this.canvasContext.scale(ratio, ratio);
  
  /* 设置线条颜色 */
  this.canvasContext.strokeStyle = '#2A2A2A';
  /* 设置线条粗细 */
  this.canvasContext.lineWidth = 4;
  /* 设置线条的结束端点样式 */
  this.canvasContext.lineCap = 'round';
}).exec()

缩放设置canvas画布大小，防止笔迹错位，这点和页面滑动没有关系，不设置也会导致坐标错位

const ratio = wx.getSystemInfoSync().pixelRatio;
this.canvas.width = this.canvasWidth * ratio;
this.canvas.height = this.canvasHeight * ratio;
this.canvasContext.scale(ratio, ratio);

旧版本获取canvas

this.canvasContext = uni.createCanvasContext('canvas', this);
/* 设置线条颜色 */
this.canvasContext.setStrokeStyle('#2A2A2A');
/* 设置线条粗细 */
this.canvasContext.setLineWidth(4);
/* 设置线条的结束端点样式 */
this.canvasContext.setLineCap('round');

签名js方法，新版本和旧版本只有一个draw的区别，新版本不需要使用draw方法

/* 触摸开始 */
handleTouchStart(e) {
  this.drawStartX = e.changedTouches[0].x;
  this.drawStartY = e.changedTouches[0].y;
    this.canvasContext.beginPath();
},
/* 触摸移动 */
handleTouchMove(e) {
    /* 记录当前位置 */
    const tempX = e.changedTouches[0].x;
    const tempY = e.changedTouches[0].y;

    /* 画线 */
    this.canvasContext.moveTo(this.drawStartX, this.drawStartY);
    this.canvasContext.lineTo(tempX, tempY);
    this.canvasContext.stroke();

    /* 旧版draw方法，新版本不需要draw */
    this.canvasContext.draw(true);

    /* 重新记录起始位置 */
    this.drawStartX = tempX;
    this.drawStartY = tempY;
},
/* 触摸结束 */
handleTouchEnd(e) {
    this.canvasContext.save();
},
/* 触摸取消 */
handleTouchCancel(e) {
    this.canvasContext.save();
},
/* 清空画布 */
clearCanvas() {
    this.canvasContext.clearRect(0, 0, this.canvasWidth, this.canvasHeight);
},

canvas生成本地图片(我这里封装了组件，需要传入this防止this指向异常)

/* 生成签名图片 */
generateSignImage() {
    return new Promise((resolve, reject) => {
        uni.canvasToTempFilePath({
          x: 0,
          y: 0,
          // canvas: this.canvas, // 新版
          canvasId: 'canvas', // 旧版使用id
          width: this.canvasWidth,
          height: this.canvasHeight,
          destWidth: this.canvasWidth,
          destHeight: this.canvasHeight,
          fileType: 'png',
          quality: 1,
          success: res => {
              resolve(res.tempFilePath)
          },
          fail: err => {
              reject(err);
          }
        }, this)
    })
},

新版本的canvas主要是canvas wxml节点和canvas context中做了区分，旧版则只有一个canvas context就可以做全部的操作，在生成图片时，新版本是传入wxml对象，旧版本则是传入唯一canvasId，新版本canvas取消了draw方法

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