关于javascript:小程序实现手写签名

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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呈现 2022-2-17)，然而在真机上即便滑动也不会出现异常，为了防止出现问题，临时应用旧版本获取 canvascontext

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 相干

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，避免笔迹错位 */
  const ratio = wx.getSystemInfoSync().pixelRatio;
  this.canvas.width = this.canvasWidth * ratio;
  this.canvas.height = this.canvasHeight * ratio;
  this.canvasContext.scale(ratio, ratio);
  
  /* 设置线条色彩 */
  this.canvasContext.strokeStyle = '#2A2A2A';
  /* 设置线条粗细 */
  this.canvasContext.lineWidth = 4;
  /* 设置线条的完结端点款式 */
  this.canvasContext.lineCap = 'round';
}).exec()

缩放设置 canvas 画布大小，避免笔迹错位，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);
/* 设置线条色彩 */
this.canvasContext.setStrokeStyle('#2A2A2A');
/* 设置线条粗细 */
this.canvasContext.setLineWidth(4);
/* 设置线条的完结端点款式 */
this.canvasContext.setLineCap('round');

签名 js 办法，新版本和旧版本只有一个 draw 的区别，新版本不须要应用 draw 办法

/* 触摸开始 */
handleTouchStart(e) {this.drawStartX = e.changedTouches[0].x;
  this.drawStartY = e.changedTouches[0].y;
    this.canvasContext.beginPath();},
/* 触摸挪动 */
handleTouchMove(e) {
    /* 记录以后地位 */
    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 办法，新版本不须要 draw */
    this.canvasContext.draw(true);

    /* 从新记录起始地位 */
    this.drawStartX = tempX;
    this.drawStartY = tempY;
},
/* 触摸完结 */
handleTouchEnd(e) {this.canvasContext.save();
},
/* 触摸勾销 */
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，须要传入 this 避免 this 指向异样)

/* 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', // 旧版应用 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)
    })
},