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

• 1.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>

• 2.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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