const ctx = canvas.getContext('2d') // 创建一个 2d context const screenWidth = wx.getSystemInfoSync().screenWidth const screenHeight = wx.getSystemInfoSync().screenHeight export default class Main{ constructor(){     this.drawing() } drawing(){     ctx.fillStyle = '#ffffff' // 矩形颜色     ctx.fillRect(0, 0, screenWidth, screenHeight) // 矩形左上角顶点为(0, 0)，右下角顶点为(100, 100)          wx.onTouchStart(function(e){         ctx.beginPath()         ctx.moveTo(100, 200);         ctx.lineTo(100, 300);         ctx.stroke()         ctx.closePath();     }) } }