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 [代码]var[代码] [代码]screenCanvas = wx.createCanvas()[代码][代码]var[代码] [代码]offScreenCanvas = wx.createCanvas()[代码][代码]var[代码] [代码]offContext = offScreenCanvas.getContext([代码][代码]'2d'[代码][代码])[代码][代码]offContext.fillStyle = [代码][代码]'red'[代码][代码]offContext.fillRect(0, 0, 100, 100)[代码][代码]var[代码] [代码]screenContext = screenCanvas.getContext([代码][代码]'2d'[代码][代码])[代码][代码] [代码][代码]screenContext.fillStyle = [代码][代码]'yellow'[代码][代码] [代码][代码]screenContext.fillRect(50,50,100,100)[代码][代码]screenContext.drawImage(offScreenCanvas, 0, 0)[代码]保存以后，模拟器确实得到了预期的结果 [图片] 然后我想把矩形换成图片，代码修改如下： [代码]var[代码] [代码]screenCanvas = wx.createCanvas()[代码][代码]var[代码] [代码]offScreenCanvas = wx.createCanvas()[代码][代码]var[代码] [代码]offContext = offScreenCanvas.getContext([代码][代码]'2d'[代码][代码])[代码][代码]var[代码] [代码]offImg = wx.createImage()[代码][代码]offImg.onload = [代码][代码]function[代码][代码](){[代码][代码]  [代码][代码]offContext.drawImage(offImg,0, 0)[代码][代码]}[代码][代码]offImg.src = [代码][代码]'images/enemy.png'[代码][代码]var[代码] [代码]screenContext = screenCanvas.getContext([代码][代码]'2d'[代码][代码])[代码][代码] [代码][代码]var[代码] [代码]screenImg = wx.createImage()[代码][代码] [代码][代码]screenImg.onload = [代码][代码]function[代码][代码](){[代码][代码]   [代码][代码]screenContext.drawImage(screenImg,50, 50)[代码][代码] [代码][代码]}[代码][代码] [代码][代码]screenImg.src = [代码][代码]'images/hero.png'[代码][代码]screenContext.drawImage(offScreenCanvas, 0, 0)[代码]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？ [图片]