How to set button (+) on subgrid show/hide with statuscode

Dear all,

As subject, I have entity contract (some status such as: begin, review, done, finish) & invoice with relationship 1:N. 

So when I custom subgird invoice on contract form.

I want to hide button Add new (+) until status of contract is done. It'll show button Add new (+).

I suggest use ribbon but I don't have experience with it.

Could you help me this case?

Thanks,

Son Dang