How to implement a multiple-level-drop-down menu in Shiny

[Liang Yang]

Hi, All

I am considering to implement a cascading dropdown menu, the children's menu items are dependent on their parents' selections. 

I am not sure what's the best approach to achieve this.

Can I implement this by manipulating the existing Shiny widgets or should I build my input objects (https://shiny.rstudio.com/articles/building-inputs.html) ? What is the effort to implement this?

Any comments are really appreciated. 

Thanks,

Liang