This event handler is called at the very beginning so that the initial screen contents can be set to be viewed.
在刚启动时调用这个事件处理函数，将初始屏幕内容设置为可见。
2
When one of these menu text items is clicked, then the corresponding onclick event handler makes the corresponding screen appear in the browser window.
当单击这些菜单文本项时，对应的 onclick 事件处理函数让对应的屏幕出现在浏览器窗口中。
3
The ViewHandler factory calls the corresponding view handler based on the screen name.