可以尝试的再次获取canvas实例，重新设置宽高，可以达到清空画布的效果  const query = wx.createSelectorQuery() query.select('#download')     .fields({ node: true, size: true })     .exec(async (result) => {   const canvas = result[0].node       const ctx = canvas.getContext('2d')       const windowW = wx.getSystemInfoSync().windowWidth;       const ratio = windowW / 750 * 2;       canvas.width = result[0].width * ratio;       canvas.height = result[0].height * ratio;       ctx.scale(ratio, ratio);     })