html5 canvas 鼠标事件在浏览器中的区别

  if (ev.layerX || ev.layerX == 0) { // Firefox
    x = ev.layerX;
    y = ev.layerY;
  } else if (ev.offsetX || ev.offsetX == 0) { // Opera
    x = ev.offsetX;
    y = ev.offsetY;
  }