canvas绘制渐变矩形，安卓机不显示，ios正常

  let ctx = canvas.getContext("2d");
      var gradient=ctx.createLinearGradient(0,0,170,0);
gradient.addColorStop("0","magenta");
gradient.addColorStop("0.5","blue");
gradient.addColorStop("1.0","red");


// 用渐变进行填充
ctx.strokeStyle=gradient;
ctx.lineWidth=5;
ctx.strokeRect(20,20,150,100);