新手一枚，在学习canvas时碰到以下疑问，根据开发文档自己写demo的时候，在离屏canvas上画了一个红色的矩形，然后在上屏canvas上画一个黄色的矩形，然后把离屏canvas绘制到上屏canvas上，代码如下

var screenCanvas = wx.createCanvas() var offScreenCanvas = wx.createCanvas() var offContext = offScreenCanvas.getContext('2d') offContext.fillStyle = 'red' offContext.fillRect(0, 0, 100, 100) var screenContext = screenCanvas.getContext('2d')  screenContext.fillStyle = 'yellow'  screenContext.fillRect(50,50,100,100) screenContext.drawImage(offScreenCanvas, 0, 0)

保存以后，模拟器确实得到了预期的结果

然后我想把矩形换成图片，代码修改如下：

var screenCanvas = wx.createCanvas() var offScreenCanvas = wx.createCanvas() var offContext = offScreenCanvas.getContext('2d') var offImg = wx.createImage() offImg.onload = function(){   offContext.drawImage(offImg,0, 0) } offImg.src = 'images/enemy.png' var screenContext = screenCanvas.getContext('2d')  var screenImg = wx.createImage()  screenImg.onload = function(){    screenContext.drawImage(screenImg,50, 50)  }  screenImg.src = 'images/hero.png' screenContext.drawImage(offScreenCanvas, 0, 0)

可是结果只画了上屏的图片，即hero.png，而没有画出离屏的图片，即enemy.png，请教下是我哪里写得不对吗？