onShow: function(){
    this.initClock();
    this.interval = setInterval(this.initClock, 40);
  },
  onUnload: function(){
    clearInterval(this.interval)
  },
  initClock(){
    const query = wx.createSelectorQuery()
    query.select('#myClock')
      .fields({ node: true, size: true })
      .exec((res) => {
        const canvas = res[0].node
        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        var ctx = canvas.getContext('2d')
        ctx.scale(dpr, dpr)
        this.drawClock(ctx)
      })
  },
 就一个非常简单的重复渲染模拟一个时钟的方法，开发者工具上没问题，但是在真机上，不重复渲染