代码： var res = wx.getSystemInfoSync(); const ctx = wx.createCanvasContext('myCanvas'); ctx.drawImage('../../resource/img/dlt.jpeg', 0, 0, res.windowWidth, res.windowHeight); ctx.draw(); wx.canvasToTempFilePath({ x: 100, y: 200, width: 50, height: 50, destWidth: 100, destHeight: 100, canvasId: 'myCanvas', success: function (res) { console.log(res.tempFilePath) }, fail: function (e) { console.log(e); } }) 图片可以画出来，但是保存的时候输出：fail canvas is empty，这是什么原因？