华为 MATE9 fillRect() 宽度或高度是负数时没有画

  onReady: function ()

  {

      var canvas=wx.createCanvasContext(‘chart’);

      canvas.fillStyle = ‘rgb(255,0,0)’;

      canvas.fillRect(0,0,50,50);

      canvas.fillStyle = ‘rgb(255,255,0)’;     

      canvas.fillRect(20,20,-20,-20);  //在华为 MATE9 真机没有画出来

      canvas.draw();

  },