//因为项目是dialog展示自定义画板，所以图片展示就写在了dialog打开的钩子里，如果需要直接复制
vue.nextTick里面的代码就行
show () {
   vue.nextTick(_ => {
     let customCanvas =this.$refs.table;// canvas显示层
     this.customstyle ='';
     customCanvas.height = 740;
     customCanvas.width = 1460;
     this.customcxt = customCanvas.getContext("2d");
     let img = new Image();
     img.src = this.imgSrc;
     let that = this;
     img.onload = function () {
       that.customRwidth = customCanvas.width / img.width; //原图与展示图片的宽高比
       that.customRheight = customCanvas.height / img.height;
       that.customcxt.drawImage(img, 0, 0, customCanvas.width, customCanvas.height); 
     };

   })

},

//鼠标按下时执行
mousedown(e){
  this.isMouseDownInCanvas = true;
  // 鼠标按下时开始位置与结束位置相同 防止鼠标在画完矩形后 点击图画形成第二个图形
  this.endX = e.offsetX;
  this.endY = e.offsetY;
  this.startX = e.offsetX;
  this.startY = e.offsetY;

},
//鼠标移动式时执行
mousemove(e){
  if (this.isMouseDownInCanvas){ // 当鼠标有按下操作时执行
    console.log( e.offsetX,e.offsetY);
    if((this.endX != e.offsetX)||( this.endY != e.offsetY)){
      this.endX = e.offsetX;
      this.endY = e.offsetY;
      let wwidth = this.endX - this.startX;
      let wheigth = this.endY - this.startY;
      let tempCanvas = this.$refs.table2; // canvas临时层
      let tempCtx = tempCanvas.getContext('2d');
      tempCanvas.width = 1460; tempCanvas.height = 740; // 设置宽高
      // 清除临时层指定区域的所有像素
      tempCtx.clearRect(0, 0, 1460, 740);
      // 重新展示图片
      let img = new Image();
      img.src = this.imgSrc;
      let that = this;
      img.onload = function () {
        that.customcxt.drawImage(img, 0, 0,1460, 740);
      };
      this.customcxt.strokeStyle=" #00ff00"; //矩形框颜色
      this.customcxt.lineWidth="2"; //矩形框宽度
      this.customcxt.strokeRect(this.startX,this.startY,wwidth,wheigth); //绘制矩形
    }else{
　　　　　　　　//鼠标按下静止时显示矩形的大小。
      let wwidth3 = this.endX - this.startX;
      let wheigth3 = this.endY - this.startY;
      this.customcxt.strokeRect(this.startX,this.startY,wwidth3,wheigth3)
    }
  }
},
//鼠标松开时执行
mouseup(e){
  this.isMouseDownInCanvas = false;
　　// 绘制最终的矩形框
  let wwidth = this.endX - this.startX;
  let wheigth = this.endY - this.startY;
  this.customcxt.strokeRect(this.startX,this.startY,wwidth,wheigth)
},