wx.createSelectorQuery().select('#heart_rate_bg')
      .fields({ id: true,node: true, size: true })
      .exec((res) => {
        if(!res[0]) return;
        const canvas = res[0].node;
        // let dpr = 750/canvas.width;
        let {width,height} = canvas;
        // that.canvas = canvas; 
        const ctx = canvas.getContext('2d');
        // ctx.clearRect(0, 0, 100000, 100000)
        // ctx.setLineDash([3,5],3)
        // ctx.strokeStyle = '#000';
        
        
        ctx.beginPath()
        ctx.lineWidth = 0.1
        ctx.moveTo(0, height/3);
        ctx.lineTo(width, height/3);
        ctx.closePath();
        ctx.stroke();


        ctx.beginPath()
        ctx.lineWidth = 1;
        ctx.moveTo(0, height*2/3);
        ctx.lineTo(width, height*2/3);
        ctx.closePath();
        ctx.stroke();
      })
class="heart_rate_bg" type="2d" canvas-id="heart_rate_bg" id="heart_rate_bg">canvas>