vuecanvas繪制矩形并解決由clearRec帶來(lái)的閃屏問(wèn)題

<!--臨時(shí)層-->
<canvas id="customPositionImg2" ref="table2"  ></canvas>
<!--顯示層 增加鼠標(biāo)按下，移動(dòng)，松開(kāi)事件-->
<canvas id="customPositionImg" ref="table"  @mousedown="mousedown" @mousemove="mousemove" @mouseup="mouseup" ></canvas>

//因?yàn)轫?xiàng)目是dialog展示自定義畫板，所以圖片展示就寫在了dialog打開(kāi)的鉤子里，如果需要直接復(fù)制
vue.nextTick里面的代碼就行
show () {
   vue.nextTick(_ => {
     let customCanvas =this.$refs.table;// canvas顯示層
     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; //原圖與展示圖片的寬高比
       that.customRheight = customCanvas.height / img.height;
       that.customcxt.drawImage(img, 0, 0, customCanvas.width, customCanvas.height); 
     };

   })

},

//鼠標(biāo)按下時(shí)執(zhí)行
mousedown(e){
  this.isMouseDownInCanvas = true;
  // 鼠標(biāo)按下時(shí)開(kāi)始位置與結(jié)束位置相同 防止鼠標(biāo)在畫完矩形后 點(diǎn)擊圖畫形成第二個(gè)圖形
  this.endX = e.offsetX;
  this.endY = e.offsetY;
  this.startX = e.offsetX;
  this.startY = e.offsetY;

},
//鼠標(biāo)移動(dòng)式時(shí)執(zhí)行
mousemove(e){
  if (this.isMouseDownInCanvas){ // 當(dāng)鼠標(biāo)有按下操作時(shí)執(zhí)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臨時(shí)層
      let tempCtx = tempCanvas.getContext('2d');
      tempCanvas.width = 1460; tempCanvas.height = 740; // 設(shè)置寬高
      // 清除臨時(shí)層指定區(qū)域的所有像素
      tempCtx.clearRect(0, 0, 1460, 740);
      // 重新展示圖片
      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"; //矩形框顏色
      this.customcxt.lineWidth="2"; //矩形框?qū)挾?      this.customcxt.strokeRect(this.startX,this.startY,wwidth,wheigth); //繪制矩形
    }else{
　　　　　　　　//鼠標(biāo)按下靜止時(shí)顯示矩形的大小。
      let wwidth3 = this.endX - this.startX;
      let wheigth3 = this.endY - this.startY;
      this.customcxt.strokeRect(this.startX,this.startY,wwidth3,wheigth3)
    }
  }
},
//鼠標(biāo)松開(kāi)時(shí)執(zhí)行
mouseup(e){
  this.isMouseDownInCanvas = false;
　　// 繪制最終的矩形框
  let wwidth = this.endX - this.startX;
  let wheigth = this.endY - this.startY;
  this.customcxt.strokeRect(this.startX,this.startY,wwidth,wheigth)
},