Typeahead - Showing conditionally the list

[abelardolg]

Please remember, the issues forum is NOT for support requests. It is for bugs and feature requests only.
Please read https://github.com/ng-bootstrap/ng-bootstrap/blob/master/CONTRIBUTING.md and search
existing issues (both open and closed) prior to opening any new issue and ensure you follow the instructions therein.

[x] Feature request

Bug description:

I would like to show conditionally the list when the typeahead gets the focus. It could be passed a flag to indicate if show, or not, the list of items on focus.

Link to minimally-working plunker that reproduces the issue:

You can fork a plunker from one of our demos and use it as a starting point.
Please note that we can not act on bug reports without a minimal reproduction scenario in plunker. Here is why:
https://github.com/ng-bootstrap/ng-bootstrap#you-think-youve-found-a-bug

Version of Angular, ng-bootstrap, and Bootstrap:

Angular: 5.2.10

ng-bootstrap: last version.

Bootstrap: 4.1.0

[pkozlowski-opensource]

• check the "Open on focus" example at https://ng-bootstrap.github.io/#/components/typeahead/examples
• we plan to make it easier in the future via NgbTypeahead: Open Function  #2318

Closing as a duplicate of #2318